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Instrumentation Basic Training Materials Table of Contents Chapter 1 Chemical Process Measurement Instruments 3 Introduction 3 1. Measurement Process and Measurement Errors 3 2. Quality Specifications of Measurement Instruments 3 3. Classification and Composition of Measurement Instruments 5 Pressure Measurement 6 1. Concept of Pressure 6 2. Pressure Measurement Instruments 7 Level Measurement 14 1. Differential Pressure Level Transmitters 15 2. Float Level Gauges 18 3. Radar Level Gauges 19 4. Float Flip-Plate Level Gauges 20 Temperature Measurement 20 1. Expansion Thermometers 20 2. Pressure Thermometers 21 3. Thermocouples 21 4. Thermal Resistance Thermometers 28 5. Electric Temperature Transmitters 31 6. Installation of Temperature Sensing Elements 32 Flow Measurement 33 1. Differential Pressure Flow Meters 34 2. Rotameter Flow Meters 39 3. Elliptical Gear Flow Meters 40 4. Turbine Flow Meters 41 5. Electromagnetic Flow Meters 42 6. Mass Flow Meters 43 7. Vortex Flow Meters 45 Chapter 2 Control Valves 47 1. Types of Control Valves 47 2. Pneumatic Diaphragm Direct-Acting Two-Seat Control Valves 47 3. Three-Way Valves 48 4. Angle Valves 48 5. Butterfly Valves 48 6. Eccentric Rotating Valves 48 7. Sleeve Valves 49 8. Ball Valves (Globe Valves) 49 9. Self-Acting Control Valves 49 10. Pneumatic Long-Stroke Valves 50 11. Diaphragm Valves 50 12. Valve Body Separated Valves 50 13. Valves with Electric Actuators 50 Chapter 3 Basics of Process Control 51 1. Overview and Characteristics of Process Control 51 2. Composition of Process Control Systems 53 3. Two Representations of Process Control Systems 55 4. Main Types of Process Control Systems 58 5. Performance Metrics and Requirements for Process Control Systems 60 Chapter 1 Chemical Process Measurement Instruments Introduction In chemical production, in order to facilitate better operational control and automatic regulation, it is necessary to automatically monitor four key parameters: pressure, liquid level, temperature, and flow rate. Instruments used to measure these parameters are called chemical process measuring instruments. I. Measurement Process and Measurement Error 1. Measurement Process The measurement process refers to the use of experimental methods to determine the value of a certain quantity. To measure the length of a wire, one simply needs to use a meter stick to compare it with the wire, and thus the length of the wire can be determined. This measurement method is called the direct measurement method. There are also indirect measurement methods and combined measurement methods. If a potentiometer is used to measure temperature, that is, the temperature is first converted into a potential (or the resistance is changed), the potential value (or resistance value) is measured, and then it is converted back into temperature, this belongs to the indirect measurement method. When direct measurements and indirect measurements are combined for measurement, it is called the combined measurement method. To measure the weight of the gas mixture in a particular container, it is necessary to determine its volume using direct measurement methods, as well as to find the concentration of each component in the mixture using indirect measurement methods; only then can the weight of the gas mixture be calculated. 2. Measurement error: The purpose of measurement is to accurately reflect the objective reality, that is, to determine the “true value” of the parameter. However, there is a \"true value\" that cannot be measured no matter how much effort is put in (including efforts in terms of principles, measurement methods, instruments, etc.), and one can only strive to get as close as possible to this true value; in other words, there is always a certain difference between the measured value and the true value. This difference is the measurement error. Measurement errors can be divided into three categories based on their causes. ⑴ Systematic error (also known as regular error): an error whose magnitude and direction remain unchanged. The reasons for such errors are mainly defects in the instrument itself, the observer’s biases or prejudices, and changes in single environmental factors. This type of error is easy to eliminate and correct in measurements, as it is regular. ⑵ Negligence error: This type of error occurs due to the carelessness of the measurer during the measurement process. It is relatively easy to notice that it can be avoided. ⑶ Random error: It is the type of error that occurs when repeated tests are carried out under the same conditions, with each test yielding different results. This type of error is caused by random factors, and it is difficult to detect and correct. There are usually two ways to express measurement error: absolute error and relative error. a. Absolute error: The difference between the measured value and the true value; that is, absolute error Δ = Measured value – True value x0.
b. Relative error: The ratio of the absolute error to the true value; that is, relative error δ = Absolute error Δ / True value x0.

II. Quality indicators of measuring instruments
The quality of an instrument can be assessed using its quality indicators, with the following being common ones. 1. The accuracy of a measuring instrument (also known as precision) In measurements, the error caused by the instrument is referred to as the instrument’s error, which can be expressed either in absolute terms or in relative terms: Absolute error Δ = x – x0; Relative error δ = Δ / x0. Here, x represents the reading given by the measuring instrument ； x0: The reading of the standard instrument. The absolute error of the instrument varies at different points within the measurement range; the term absolute error usually refers to the maximum value of this error. To evaluate the accuracy of a measuring instrument, it is not sufficient to consider only the absolute and relative errors, as the precision of the instrument depends not only on the absolute error but also on its scale range. For example, with two instruments having different measurement ranges, if their absolute errors are equal, the instrument with the larger measurement range has higher precision than one with a smaller measurement range. Therefore, to facilitate comparison between instruments, relative percent error is actually used to measure the accuracy of the instruments. The relative percent error, also known as the relative reference error or equivalent error, is calculated as δ = (x – x0) / (upper limit of the scale – lower limit of the scale) × 100%. In fact, it is by using this relative percent error and removing the % sign that the accuracy grade of an instrument can be determined. If the relative percentage error is 0. 5%, so by removing the % sign, the precision level is 0. Level 5. 2. Constancy (drift) of measuring instruments: Under unchanged external conditions, when using the same instrument to measure a certain parameter value in both forward and reverse directions, it is found that the readings obtained by the instrument for that same parameter value are not identical. The percentage of the difference between the maximum positive and negative indication values of an instrument, divided by the range of the instrument’s scale, is called the instrument’s error margin: Error margin = △’max / (upper limit of scale – lower limit of scale) × 100%. Here, △’max represents the difference between the positive and negative indication values. The degradation is caused by friction between the various components and the elastic hysteresis of the elastic elements. 3. Sensitivity and sensitivity limit of measuring instruments. Sensitivity: It is the ratio of the change in the instrument’s output (displacement) △a to the change in the parameter being measured that causes this change: Sensitivity = △a/△x. Sensitivity limit: It is the smallest change in the parameter being measured that can cause the instrument’s pointer to move. The sensitivity limit should be less than half of the allowable error. 4. Response time of measuring instruments: To evaluate the quality of an instrument, in addition to the three static characteristics mentioned above, its dynamic characteristics, that is, its response time, also need to be taken into consideration. The response time of a measuring instrument is the time elapsed from the occurrence of a step change in the parameter being measured until the pointer of the instrument stabilizes in its position. There are two representation methods: (1) When the input signal changes by a certain amount suddenly, the output signal will gradually change from its original value to the new steady-state value. The output signal of the instrument (i.e., the indicated value), the time it takes for it to change from its initial value to 63.2% of the new steady-state value, is known as the response time. (2) The reaction time is expressed as the time taken to change to 95% of the new steady-state value. III. Classification and Composition of Measuring Instruments Any device used to directly or indirectly compare the parameter being measured with the unit of measurement is referred to as a measuring instrument. There are several methods for classifying measuring instruments: 1. Classification by the energy source used by the instrument. (1) Electric instruments: These instruments use electricity as their energy source; it is convenient to transmit signals between them, making them suitable for long-distance transmission and centralized control ； Easy to use with computers ； In recent years, electric meters have also become fireproof and explosion-proof, which facilitates their safer use. However, electric meters generally have a more complex structure ； It is susceptible to environmental influences such as temperature, humidity, electromagnetic fields, and radiation. (2) Pneumatic instruments: Pneumatic instruments have a relatively simple structure and are intuitive ； The work is relatively reliable ； It has a strong ability to resist interference from environmental factors such as temperature, humidity, electromagnetic fields, and radiation ； Fireproof and explosion-proof ； The price is relatively cheap. However, pneumatic instrument signals have a slow transmission speed and short transmission range; pipeline installation and maintenance are inconvenient, making it unsuitable for centralized display and control over long distances and large areas ； It is relatively difficult to use with a computer. 2. Classification based on the process of information acquisition, transmission, reflection, and processing: (1) Detection instruments: The main function of detection instruments is to acquire information and perform appropriate conversions on it. During the production process, measuring instruments are primarily used to detect certain process parameters such as temperature, pressure, flow rate, level, as well as the composition and physical properties of the materials. The values of these measured parameters are converted proportionally into electrical signals (voltage, current, frequency, etc.) or pneumatic signals. (2) Indicating instruments: The function of indicating instruments is to display the information obtained by sensing instruments, including various analog and digital electric and pneumatic indicators, recorders, accumulators, as well as industrial TVs and image displays. (3) Central control devices: include various patrol detectors, patrol controllers, program controllers, data processors, electronic computers, as well as instrument control panels and operation consoles, etc. (4) Control instruments: Control instruments can perform various operations on input signals as needed, such as amplification, integration, differentiation, etc. Control instruments include various electric and pneumatic controllers, as well as microprocessors used to replace analog control instruments. (5) Actuator: The actuator can receive the output signal from control instruments or commands directly from operators, in order to operate or control the production process. Actuators include various pneumatic, electric, and hydraulic actuators as well as control valves. 3. Classification by the configuration of the instrument: (1) Benchtop instruments: These instruments are characterized by the fact that the various functions such as measurement, display, and control are integrated within a single instrument housing, forming one unified unit. This type of instrument is suitable for on-site monitoring and control, but it cannot enable the centralized display and control of multiple parameters. (2) Modular instrument panels: The various components involved in parameter measurement, as well as transmission, display, and control, are each developed into independently functional modular instruments (referred to simply as modules, such as transmission modules, display modules, control modules, etc.). These units are interconnected through standardized signals, and various control systems can be easily formed by combining them as needed, offering excellent versatility and flexibility. In chemical production, there are two types of combined instrument units: electric combined instrument units and pneumatic combined instrument units. Domestic electric combined instrument panels are designated by the pinyin initials of the characters meaning \"electric\", \"single\", and \"combination\", and are abbreviated as DDZ instruments ； Similarly, the pneumatic unit combination instrument is abbreviated as QDZ instrument. Pressure measurement: In industrial production processes, especially in industries such as chemicals and petroleum refining, pressure is one of the important operational parameters. Measurements of pressure and vacuum are encountered frequently, including high pressures that are much higher than atmospheric pressure, ultra-high pressures, as well as vacuums that are much lower than atmospheric pressure. If the pressure is not within the required range, it will not only affect production efficiency and reduce product quality, but sometimes it can also lead to serious production accidents. Furthermore, the significance of pressure measurement is not limited to itself; the measurement of some other parameters, such as level and flow rate, is often carried out by measuring pressure or differential pressure. In other words, by determining the pressure or differential pressure, it is possible to determine the level or flow rate. I. The concept of pressure Pressure is a force that acts vertically and evenly on a unit area. It is expressed as follows: P = F/S, where P represents pressure, in Pa ； F: Vertical force, N ； S: Area under stress, in cm2. Pressure can also be expressed as the equivalent height of a liquid column, that is: P = F/S = S•ρ•h/S = ρ•h. Here, h is the height of the liquid column, in cm ； ρ: Density of the liquid, in g/cm2. The units of pressure include the following: 1. Physical atmosphere (also known as standard atmosphere). Internationally, it is defined as the gravitational pressure of an air column at sea level at a latitude of 45°, with a temperature of 0°C and an area of 1 cm2; this value corresponds to one physical atmosphere, or one standard atmosphere. 2. Engineering atmosphere: One engineering atmosphere refers to a uniform, vertical pressure of 1 kg per square centimeter of area. 3. Pascal (Pa) – Pascal is a unit in the International System of Units (SI). 1 Pa is the pressure resulting from a force of 1 N acting vertically and evenly on an area of 1 m2. 4. Millimeter of water column and millimeter of mercury are two pressure units commonly used to represent low pressures. It is equivalent to the gravitational pressure exerted by 1 mm of water or mercury vertically acting on a base area. The condition is that the acceleration due to gravity g=980. 665 cm/s2, at a temperature of 0°C (mercury) or 4°C (water). In pressure measurement, there are often distinctions between gauge pressure, absolute pressure, negative pressure, and vacuum level. The relationship among them is as follows: Absolute pressure: Pressure measured with the absolute pressure zero point as a reference, i.e., pressure above that zero point. Positive pressure: Pressure that is higher than atmospheric pressure, measured with atmospheric pressure as a reference. Negative pressure (vacuum): A pressure that is below atmospheric pressure, measured with respect to atmospheric pressure. Differential pressure: The difference between two pressures. Gauge pressure: Pressure that is greater than or less than atmospheric pressure, measured with atmospheric pressure as a reference. II. Pressure measuring instruments 1. Elastic pressure gauges Elastic pressure gauges are measuring instruments that utilize various types of elastic elements; they work on the principle that, under the pressure of the medium being measured, these elastic elements undergo elastic deformation as a result of being compressed. This instrument boasts advantages such as a simple structure, reliable operation, clear readings, durability, low cost, a wide measurement range, and sufficient accuracy. By adding additional devices such as recording mechanisms, electrical conversion units, control elements, etc., it is possible to achieve pressure recording, remote transmission, signal alarms, automatic control, and more. Elastic pressure gauges can be used to measure pressures in the range of several hundred pascals to several thousand megapascals, which is why they are the most widely used type of pressure measuring instrument in industry. (1) Elastic elements are simple and reliable pressure-sensitive components. It is not only the pressure-sensing element of elastic pressure gauges, but it is also often used as a basic component in pneumatic combined instrument panels. Commonly used elastic elements include helical springs, diaphragms, bellows, and corrugated tubes; their structures are shown in the figure: ① Helical spring-type elastic element. The pressure measurement range of helical spring-type elastic elements is quite wide, allowing them to measure pressures as high as 1000 MPa. A single-loop spring tube is a metal tube bent into an arc, with a cross-section that is oblate or elliptical, as shown in Figure (a). To increase the displacement of the free end, a multi-turn helical spring can be used, as shown in Figure (b). ② Film-type elastic elements can be further divided into diaphragms and bellows, etc., depending on their structure. Its pressure measurement range is lower than that of the Bourdon tube type. Figure (c) shows a diaphragm-type elastic element, which is an elastic diaphragm made of metallic or non-metallic materials (available in flat and corrugated forms) that can deform under pressure. Sometimes, two metal diaphragms can also be welded together along their perimeters to form a thin-walled box filled with a liquid (such as silicone oil), which is called a diaphragm box, as shown in figure (d). ③ Bellows-type elastic element: A bellows-type elastic element is a thin-walled metal cylinder with corrugations around its perimeter, as shown in Figure 3–5(e). Such elastic elements are easy to deform and can achieve large displacements; they are commonly used for measuring micro- and low pressures (generally not exceeding 1 Mpa). (2) Bourdon tube pressure gauge ① Structure: ② Working principle: When the pressure to be measured is applied, the cross-section of the Bourdon tube tends to expand from oval to circular, causing the entire tube to straighten slightly (the angle of rotation for a typical pressure gauge is 5–20°), which in turn forces the free end to expand upward to the right. The pressure value is displayed on the dial through gear transmission. 2. Electrical pressure gauge: An electrical pressure gauge is a device that can convert pressure into electrical signals for transmission and display. This type of gauge has a wide measurement range, capable of measuring pressures from 7×10-5 Pa to 5×102 MPa, with an allowable error as low as 0. 2%. Since signals can be transmitted over long distances, automatic pressure control and alarm functions can be implemented in industrial production processes, and it can also be used in conjunction with industrial control computers. An electrical pressure gauge generally consists of a pressure sensor, a measurement circuit, and a signal processing unit. Common signal processing devices include indicators, recorders, controllers, microprocessors, etc. The function of a pressure sensor is to detect pressure signals and convert them into electrical signals for output. When the electrical signals generated can be further transformed into standard signals, such a pressure sensor is also referred to as a pressure transmitter. A standard signal is one whose form and numerical range of a physical quantity comply with international standards. For example, direct current of 4–20 mA, air pressure of 0. 02 one 0. 1 Mpa is the commonly used standard signal at present. (1) Hall plate pressure sensor: The Hall plate pressure sensor is based on the Hall effect; it uses a Hall element to convert the displacement of an elastic element caused by pressure into a Hall voltage, thereby enabling the measurement of pressure. A Hall plate is a thin sheet made of a semiconductor material such as germanium, as shown in the figure: a constant magnetic field with intensity B is applied in the Z-axis direction of the Hall plate, while an external electric field is applied in the Y-axis direction (by connecting it to a DC regulated power supply). A constant current then flows in the Y-axis direction. As electrons move through the Hall element, they are deflected due to electromagnetic forces, causing an accumulation of electrons on one end face of the element and a surplus of positive charges on the other end face. As a result, a potential difference is created in the X-direction of the Hall element; this potential difference is known as the Hall voltage. This physical phenomenon is referred to as the “Hall effect”. The magnitude of the Hall potential is related to factors such as the semiconductor material, the current flowing through it (commonly referred to as the control current), the magnetic flux density, and the geometric dimensions of the Hall element. It can be expressed by the following formula: UH = RHBI, where UH represents the Hall potential ； RH: Hall constant, which is related to the material of the Hall element and its geometry ； B: Magnetic induction intensity ； I: Current passing through. From the above equation, it can be seen that the Hall potential is proportional to the magnetic flux density and the current. Increasing the B and I values can increase the Hall potential UH. But both have certain limits: generally, I is between 3 and 20 mA, B is around a few thousand gauss, and the resulting Hall voltage UH is on the order of several tens of millivolts. If a Hall element is selected and the current is kept constant, then in a non-uniform magnetic field, the magnetic flux density experienced by the Hall element varies depending on its position; this results in a Hall voltage that is proportional to the displacement, thereby achieving a linear conversion between displacement and voltage. (2) Strain gauge pressure sensor A strain gauge pressure sensor is constructed based on the principle of resistive strain. Resistive strain gauges are divided into two types: metal strain gauges (metal wires or metal foils) and semiconductor strain gauges. The pressure under test causes strain in the strain gauge. When a strain gauge experiences compressive strain, its resistance decreases ； When a strain gauge experiences tensile strain, its resistance increases. The change in the resistance value of the strain gauge results in a corresponding millivolt-level voltage output through a bridge circuit; this voltage is then displayed using a millivolt meter or other recording instrument to indicate the pressure being measured, thereby forming a strain gauge pressure gauge. Strain gauges r1 and r2, together with two fixed resistors r3 and r4, form a bridge circuit, as shown in the figure. Changes in the resistance values of r1 and r2 cause the bridge to lose balance, resulting in an unbalanced voltage ΔU that serves as the output signal of the sensor. When a DC regulated power supply with a maximum voltage of 10V is used for the bridge, a maximum output value of ΔU of 5mV can be obtained. The pressure to be measured by the sensor can reach 25 Mpa. Since the natural frequency of the sensor is above 25,000 Hz, it exhibits good dynamic performance. Suitable for pressure measurement in rapidly changing conditions. The nonlinearity and hysteresis errors of the sensor are less than 1% of the rated pressure. (3) Piezoresistive pressure sensor: A piezoresistive pressure sensor is constructed by utilizing the piezoresistive effect of single-crystal silicon. Its working principle is as follows: A single-crystal silicon wafer is used as the elastic element; through integrated circuit techniques, equivalent resistors are diffused in specific directions within the single-crystal silicon membrane, and these resistors are connected in a bridge configuration. The single-crystal silicon wafer is then placed inside the sensor chamber. When the pressure changes, the single crystal silicon undergoes strain, causing the strain resistor directly attached to it to exhibit a change that is proportional to the measured pressure; a corresponding voltage output signal is then generated by the bridge circuit. Piezoresistive pressure sensors feature high precision, reliable operation, high frequency response, low hysteresis, small size, light weight, and a simple structure. They can operate in harsh environmental conditions and facilitate digital display. Piezoresistive pressure sensors can be used not only to measure pressure, but with slight modifications, they can also be used to measure parameters such as differential pressure, height, velocity, and acceleration. (4) Torque-balanced pressure transmitter: The torque-balanced pressure transmitter is a typical self-balancing measuring instrument. It utilizes the principle of negative feedback to overcome the adverse effects of factors such as component materials and manufacturing processes, thereby granting the instrument advantages such as high measurement accuracy (usually at 0.5 grade), stable and reliable operation, good linearity, and a small dead zone. The DDZ—Ⅲ type electric torque-balanced pressure transmitter is used as an example below. The DDZ—Ⅲ series is powered by 24V DC, outputs 4–20mA (DC), operates on a two-wire system, and is intrinsically safe and explosion-proof. Through derivation, the relationship between the output current and the measured pressure can be obtained: I0=Kpθ, where K is the conversion ratio coefficient; once the structure and electromagnetic properties of the transmitter are determined, K becomes a constant. The above equation shows that, once the angle θ of the vector mechanism is determined, the output current I0 of the transmitter is proportional to the input pressure p. As shown in the figure above, by adjusting the range-setting screw 5, it is possible to change the angle θ of the vector mechanism; this allows for a continuous adjustment of the transmission ratio between the two levers, and thus enables the adjustment of the transmitter’s range. Typically, the vector angle θ can be adjusted between 4° and 15°; as a result, tgθ changes by about a factor of 4, and consequently the corresponding range can also change by a factor of 4. Adjusting the tension of spring 12 allows for the adjustment of the zero point. If the pressure-sensing elastic element of the above-mentioned pressure transducer is slightly modified, it can be used to continuously measure differential pressure or absolute pressure, with the working principle remaining essentially the same. (5) Capacitive pressure transmitters: First introduced to the market in the early 1970s by the United States, these are open-loop sensing instruments. They boast a series of advantages such as simple structure, high overload capacity, good reliability, high measurement accuracy, small size, light weight, and ease of use. Today, they have become the most popular type of pressure and differential pressure transmitters, with their output signal being a standard 4–20mA (DC) current signal. A capacitive pressure transmitter first converts pressure changes into changes in capacitance, and then performs the measurement. The schematic diagram of the capacitive differential pressure transmitter is shown on the left: the outer sides of the symmetrically shaped stainless steel bases are processed into annular corrugated grooves, and a corrugated diaphragm is welded to them. The structure of the capacitive differential pressure transmitter enables effective protection of the measuring diaphragm. When the differential pressure becomes too high and exceeds the allowable measurement range, the measuring diaphragm presses smoothly against the concave glass surface, which prevents it from being damaged. It also has good recovery characteristics after being overloaded, thereby **improving its overload tolerance. Compared to the torque-balancing type, the capacitive type does not have a lever mechanism, resulting in a compact size, good sealing and vibration resistance, as well as improved measurement accuracy that can reach 0. Level 2. 3. Intelligent pressure transmitters: Intelligent pressure or differential pressure transmitters are smart sensing instruments that are created by adding a microprocessor circuit to conventional pressure or differential pressure sensors. For example, by combining a temperature-compensated capacitive sensor with a microprocessor, an accuracy of 0 can be achieved. A pressure or differential pressure transmitter of grade 1, with a range ratio of 100:1. The time constant can be adjusted between 0 and 36 seconds. Using a handheld communicator, it is possible to set the operating parameters of field transmitters within a range of 1500 meters, adjust their measurement ranges, and upload information data to the transmitters. Intelligent transmitters are characterized by their ability to enable remote communication. Using a handheld communicator, it is possible to select and calibrate various operating parameters of the field transmitter ； It has high precision and is easy to use and maintain. By developing various programs, the transmitter is equipped with multiple functions such as self-correction, self-compensation, self-diagnosis, and error condition alerting. Thus, the accuracy of the transmitter is improved, and the adjustment, calibration, and maintenance processes are simplified. It enables direct communication between the transmitter, the computer, and the control system. 4. Selection and installation of pressure gauges (1) Selection of pressure gauges ① Selection of instrument type: The choice of instrument type must meet the requirements of the production process. For example, whether remote transmission, automatic recording, or alarms are required ； Do the physicochemical properties of the medium being tested (such as corrosivity, temperature, viscosity, degree of contamination, flammability, and explosiveness) impose any special requirements on the measuring instruments? ； Whether on-site environmental conditions (such as high temperatures, electromagnetic fields, vibrations, and installation conditions at the site) impose any special requirements on the type of instrument, etc. ②Determination of the instrument’s measurement range: The measurement range of an instrument refers to the range within which it can measure the quantity being measured with the specified accuracy. It is determined based on the magnitude of the parameters that need to be measured during operation. When measuring pressure, in order to extend the service life of the instrument and prevent the elastic components from being damaged due to excessive stress, the upper limit of the pressure gauge should be higher than the maximum pressure that may occur during production. According to the \"Technical Regulations for Chemical Process Automation Control Design\": when measuring stable pressure, the maximum operating pressure should not exceed 2/3 of the upper measurement limit ； When measuring pulsating pressure, the maximum operating pressure should not exceed 1/2 of the upper measurement limit ； When measuring the maximum pressure, the maximum operating pressure should not exceed 3/5 of the upper measurement limit. To ensure the accuracy of the measurement values, the pressure value being measured should not be too close to the lower limit of the instrument; in other words, the range of the instrument should not be set too wide. It is advisable that the minimum value of the pressure to be measured be no less than 1/3 of the instrument’s full scale. After calculating the upper and lower limits of the instrument based on the maximum and minimum values of the parameters being measured, rounding should be performed in accordance with the regulations or standards set by the **responsible authority. Therefore, when selecting the scale limits for instruments, it is also necessary to use the values specified in the relevant regulations or standards (which can generally be found in the corresponding product catalogs). ③Selection of instrument accuracy class: The accuracy of an instrument is determined based on the maximum allowable measurement error in the manufacturing process. For example: the outlet pressure range of a reciprocating compressor is 25–28 MPa, and the measurement error must not exceed 1 MPa. The process requires on-site monitoring as well as alarm functions for high and low limits. Try to select a pressure gauge correctly, indicating the model, accuracy, and measurement range. Solution: Due to the large pressure fluctuations at the outlet of reciprocating compressors, the upper limit value for the instrument is set to P1 = Pmax × 2 = 28 × 2 = 56 MPa. Based on on-site observations and the need for high and low limit alarms, a Y-150 type electric contact pressure gauge was selected, with a measurement range of 0–60 MPa. Since 25/60 > 1/3, the minimum value of the measured pressure is not lower than 1/3 of the full scale, which is acceptable. Furthermore, based on the requirements for measurement error, the allowable error can be calculated as 1/60×100% = 1. 67%, so the accuracy level is 1. A gauge of grade 5 is more than sufficient to meet the error requirements. It can be determined that the selected pressure gauge is the Y-150 type electric contact pressure gauge, with a measurement range of 0 to 60 MPa. The precision grade is 1. Level 5. (2) Installation of the pressure gauge The proper installation of the pressure gauge directly affects the accuracy of the measurement results as well as its service life. ①Selection of pressure measurement points: The selected pressure measurement points should be able to reflect the true magnitude of the pressure being measured. To this end, the following points must be noted: a It should be selected in the section of the pipeline where the medium flows in a straight line, and not at pipe bends, forks, dead ends, or other areas prone to vortex formation. b When measuring the pressure of the flowing medium, the pressure measurement point should be perpendicular to the direction of flow. The inner end surface of the pressure measurement tube should be flush with the inner wall at the connection point to the production equipment; there should be no protrusions or burrs. When measuring liquid pressure, the pressure sampling point should be located at the lower part of the pipe to prevent gas from accumulating in the pressure conduit; when measuring gas pressure, the pressure sampling point should be located at the upper part of the pipe to prevent liquid from accumulating in the pressure conduit. ②Layout of pressure conduits: The diameter of tube A should be appropriate, with an inner diameter generally ranging from 6 to 10 mm. Its length should be as short as possible, with a maximum length of 50 m, in order to reduce delays in pressure indication. When the pressure guide tube is installed horizontally, it should have a slope of 1:10 to 1:20. To facilitate the discharge of the liquid (or gas) accumulated within it. c When the medium is prone to condensation or freezing, insulation and heating pipelines must be installed. A shut-off valve should be installed between the pressure tapping point and the pressure gauge, for use when servicing the pressure gauge. ③Installation of the pressure gauge: The pressure gauge should be installed in a location where it is easy to observe and maintain. b The installation location should be chosen to avoid the effects of vibration and high temperatures. When measuring steam pressure, a condensate tube should be installed to prevent high-temperature steam from coming into direct contact with the pressure sensing element, as shown in Figure (a) below ； For pressure measurement in environments with corrosive media, an isolation tank filled with a neutral medium should be used. Figure (b) shows the two scenarios where the density ρ2 of the medium to be measured is greater than or less than the density ρ1 of the isolation fluid. At the connection points of the c-pressure gauge, appropriate materials should be selected as sealing gaskets, depending on the level of pressure to be measured and the properties of the medium, in order to prevent leaks. When the pressure being measured is low, and the pressure gauge and the pressure sampling point are not at the same height, the measurement error resulting from this height difference should be corrected using Δp=±Hρg. For safety reasons, in addition to choosing a pressure gauge with vent holes for measuring high pressures, its casing should be installed facing the wall or in a location where no one passes by, to prevent accidents. Level measurement: The height of the liquid medium in a container is referred to as the liquid level, while the height of the solid or granular material accumulated in a container is called the material level. The instrument used to measure liquid level is called a level gauge ； The instrument used to measure the material level is called a level gauge ； A device used to measure the interface between two liquid media with different densities is called an interfacemeter. These three types of instruments are collectively referred to as level instruments. There are generally two purposes for measuring liquid levels: one is to achieve high accuracy in the absolute value of the level measurement, in order to determine the quantity of raw materials, auxiliary materials, semi-finished products, or finished products in a container or storage facility ； Another approach is to require high accuracy in the relative values of level measurement, so as to be able to quickly and accurately reflect the relative changes in the material at a specific level, thereby enabling continuous control of the production process – that is, by using level measuring instruments for monitoring and control. Requirements for level gauges in industrial production: The main aspects include accuracy, range, cost-effectiveness, as well as safety and reliability, among which safety and reliability are of primary importance. Based on their working principle, level measurement instruments mainly fall into the following categories: (1) Direct-reading level instruments: These include glass tube level gauges, glass plate level gauges, and similar devices. (2) Differential pressure level gauges: These can be further divided into pressure-type level gauges and differential pressure type level gauges, and they operate on the principle that a liquid column or a pile of material exerts pressure on a specific point. (3) Buoyant level gauges: They operate on the principle that the height of a float changes as the liquid level changes, or that the buoyant force exerted by the liquid on a float (or float tube) submerged in the liquid changes with the liquid level. It can be further divided into types with a float equipped with a steel wire rope or steel strip, types with a float and a lever, and barrel-type types. (4) Electromagnetic level gauges: They convert changes in liquid level into changes in electrical quantities, and the level is determined by measuring these electrical changes. It can be divided into resistive (i.e., electrode-type), capacitive, and inductive level gauges, etc. There are also level gauges that operate using the magnetostrictive effect. (5) Nuclear radiation level gauge: It operates on the principle that the intensity of nuclear radiation decreases as it passes through a material layer, depending on the thickness of that layer. Gamma rays are currently the most widely used. (6) Acoustic property meters: As changes in liquid level cause variations in acoustic impedance, as well as differences in the obstruction of sound waves and their reflection distances, by detecting these changes it is possible to determine the liquid level. Therefore, acoustic level gauges can be classified into acoustic interruption type, reflection type, and damping type based on their working principles. (7) Optical level gauge: It operates on the principle of the obstruction and reflection of light waves by the liquid level; the light source it uses can be a regular incandescent lamp or a laser, among others. In addition, there are other types of level gauges as well. Below, the differential pressure level gauge will be discussed in detail, while a brief overview of several other types of level measurement instruments is provided. I. Differential Pressure Level Transmitter 1. Working Principle The differential pressure level transmitter operates on the principle that when the liquid level in a container changes, the static pressure generated by the liquid level also changes accordingly, as shown in the diagram: One end of the differential pressure transmitter is connected to the liquid phase, while the other end is connected to the gas phase. Assume that the space above the container is filled with dry gas at a pressure of p. When the container under test is open and the pressure in the gas phase is equal to atmospheric pressure, it is not necessary to use a remote signal; a pressure gauge can be installed at the bottom of the container, as shown in the figure on the right. Since the pressure p is proportional to the liquid level H, the pressure gauge can be calibrated directly according to the liquid level. 2. Zero-point shift issue (1) No shift: Generally, there is the following relationship between the differential pressure Δp and the liquid level height H: Δp = Hρg/a. This represents the typical case of \"no shift\". When H=0, the pressures acting on the positive and negative chambers are equal. (2) Negative migration: In practical applications, the relationship between H and Δp is often not that simple. As shown in the figure, to prevent liquids and gases inside the container from entering the transmitter and causing pipeline blockage or corrosion, and to maintain a constant liquid column level in the negative pressure chamber, isolation tanks filled with isolation fluid are installed between the transmitter, the negative pressure chamber, and the pressure sampling points. If the density of the medium being measured is ρ1 and the density of the isolation fluid is ρ2 (usually ρ2 > ρ1), then the pressures in the positive and negative pressure chambers are respectively p1 = h1ρ2g + Hρ1 + p0, and p2 = h2ρ2g + p0. The pressure difference between the positive and negative pressure chambers is: p1 – p2 = h1ρ2g + Hρ1 – h2ρ2g. In other words, Δp = Hρ1 – (h2 – h1)ρ2g. Here, Δp represents the pressure difference between the positive and negative pressure chambers of the transmitter ； H: Measured liquid level height ； h1: Height from the isolation tube level in the positive pressure chamber to the transmitter ； h2: The height from the isolation tube level in the positive pressure chamber to the transmitter. By comparing equations a and e, it can be seen that the pressure difference is reduced by the term (h2–h1)ρ2g at this point. In other words, when H=0, Δp = (h2–h1)ρ2g; compared to the case with no migration, this corresponds to an additional pressure in the negative-pressure chamber, whose value is fixed at (h2–h1)ρ2g. Assuming a DDZ-Ⅲ type differential pressure transmitter is used, its output is a current signal in the range of 4–20 mA. In the case of no migration, H=0 and Δp=0; at this time the transmitter’s output I0 is 4mA ； H=Hmax, Δp =Δpmax; at this point, the transmitter’s output I0 is 20mA. However, during migration, as indicated by equation e, due to the presence of a fixed differential pressure, when H=0, the transmitter’s input is less than 0, and its output must be less than 4 mA. When H=Hmax, the transmitter’s input is less than Δpmax, and its output must be less than 20 mA. In order for the output of the instrument to accurately reflect the liquid level value, that is, to ensure that the zero point and full scale of the liquid level correspond to the upper and lower limits of the transmitter’s output, it is necessary to counteract the effect of the constant pressure difference Δp = (h2 – h1)ρ2g. This ensures that when H = 0, the transmitter’s output remains at 4 mA, and when H = Hmax, the transmitter’s output is 20 mA. . This goal can be achieved by using zero-point shift, that is, by adjusting the shift spring on the instrument to counteract the effect of the fixed pressure difference Δp=-(h2–h1)ρ2g. The function of the shift spring here is, in essence, to change the zero point of the transmitter. Both migration and zero adjustment are used to align the output starting point of the transmitter with the starting point of the measured quantity; however, the amount of zero adjustment is usually small, while the amount of zero migration is larger. (3) Positive migration: Due to differences in working conditions, positive migration can occur, as shown in the figure. When H=0, an additional pressure of hρg is present in the positive pressure chamber; in other words, when H=0, Δp = hρg, and at this point the transmitter’s output is greater than 4mA. . Migration simultaneously changes the upper and lower limits of the measurement range, which is equivalent to a translation of the measurement range; it does not change the size of the range. For example, if a differential pressure transmitter has a measurement range of 0 to 5000 Pa, then as the pressure difference changes from 0 to 5000 Pa, the transmitter’s output will change from 4 mA to 20 mA; this is the case with no drift, as shown in curve a. When there is migration, a constant pressure difference of (h2 – h1)ρ2g = 2000 Pa is assumed. When H = 0, according to equation e, Δp = -(h2 – h1)ρ2g = -2000 Pa, and in this case the transmitter’s output should be 4 mA. When H is at its maximum value, Δp = Hρ1 – (h2 – h1)ρ2g = 5000 – 2000 = 3000 Pa, and at this point the transmitter’s output should be 20 mA. As shown in curve b of the figure. . In other words, as Δp varies from -2000 Pa to 3000 Pa, the transmitter’s output should vary from 4 mA to 20 mA. It maintains its original range of 5000 Pa unchanged, merely shifting by a fixed pressure difference in the negative direction. This phenomenon is known as negative transfer. During positive migration, an additional pressure of hρg is present in the positive pressure chamber; in other words, when H=0, Δp = hρg. At this point, the transmitter output is greater than 4 mA. By plotting the relationship between the transmitter output and the input pressure difference at this condition, we obtain a curve similar to curve c in the diagram. 3. Using a flanged differential pressure transmitter to measure liquid level: To address the problems of corrosion and blockage of the pressure transfer pipes when measuring the level of corrosive liquids, liquids containing crystalline particles, or liquids with high viscosity that tend to solidify, a flanged differential pressure transmitter equipped with an isolation diaphragm box at the inlet of the pressure transfer pipe should be used, as shown in the figure on the right. The measuring head 1 (metal diaphragm box), as a sensitive element, is connected to the measurement chamber of transmitter 3 via capillary 2. A closed system consisting of a diaphragm box, capillaries, and a measurement chamber is filled with silicone oil as a pressure-transmitting medium, preventing the medium being measured from entering the capillaries and the transmitter and thus avoiding blockages. Flanged differential pressure transmitters are further divided into single-flange and double-flange types based on their structural design. A differential pressure transmitter that requires only one flange to connect the pipeline between the container and the transmitter is known as a single-flange differential pressure transmitter. For a closed container whose upper end is isolated from the atmosphere, since the pressure in the upper space is usually different from atmospheric pressure, two flanges must be used to transmit the pressures of the liquid phase and the gas phase to the differential pressure transmitter respectively; as shown in the figure above, this is what a double-flange differential pressure transmitter is. II. Float-type level gauges 1. Application areas Float level gauges can perform continuous measurement and provide local as well as remote indication; they are suitable for use in applications with various specific gravities and operating pressures, as well as in vacuum systems for measuring interfaces and liquid levels. However, their range is relatively small (usually less than 2 meters). It is not suitable for applications with large liquid level ranges, highly corrosive media, high temperatures, high viscosity, or materials that are prone to solidification. 2. Working Principle and Structure The transmitter consists of three parts: detection, conversion, and transmission. The detection section consists of a buoy chamber, a buoy, and a connecting rod ； The conversion section consists of levers, supports, and sensors. The transmitter section consists of an amplifier, voltage and current conversion circuits, and the indicator gauge housing. When the buoy is submerged in a liquid, it experiences an upward buoyant force according to Archimedes’ principle; as a result, its own weight is counteracted. The volume of the buoy that is submerged in the liquid, along with the density of the medium, are proportional to the buoyant force. The formula for this is: F = π·H·r, where F represents the buoyant force, D is the diameter of the buoy, H is the height of the medium, and r is the density of the medium. When the liquid level rises and the float loses its own weight, the lever force at the support point changes. The electrical signal generated by the sensor is converted, via an operational amplifier, into a 4–20 mA DC standard signal that is linearly related to the measured liquid level; this signal is then transmitted to the control room for centralized control and recording, thereby enabling automatic control of the process flow. The transmitter is equipped with a 100% on-site indicator, with 0% at 4mA and 100% at 20mA. Due to the different densities of the media, the gauge is equipped with range and zero adjustment functions. Generally, differential pressure transmitters feature high measurement accuracy, fast response times, a wide range, the ability to perform continuous measurements and provide remote indication. Moreover, there is a linear relationship between the measured differential pressure and the output signal, which makes them widely used. III. Radar Level Gauge The radar level gauge operates on a transmit-reflect-receive principle. The antenna of the radar level gauge emits electromagnetic waves, which are reflected by the surface of the substance being measured before being detected by the antenna again. The time it takes for these electromagnetic waves to travel from emission to reception is proportional to the distance to the liquid surface; the formula for this relationship is as follows: D = CT/2, where D represents the distance from the radar level gauge to the liquid surface, C represents the speed of light, and T represents the time it takes for the electromagnetic waves to travel. By recording the time taken by the pulse waves, and since the speed of propagation of the electromagnetic waves is constant, it is possible to calculate the distance from the liquid surface to the radar antenna, thereby determining the level of the liquid. In practical use, radar level gauges come in two types: frequency-modulated continuous wave and pulse wave types. Level gauges that use frequency-modulated continuous wave technology consume a lot of power, require a four-wire system, and have complex electronic circuits. Level gauges that utilize radar pulse wave technology have low power consumption; they can be powered by 24V DC using a two-wire system, are easy to make intrinsically safe, offer high precision, and have a wider range of applications. IV. Floating Ball and Flip-Plate Type: The column-type magnetic floating ball level gauge uses a magnetic floating ball as the measuring element; through a magnetic system, it transmits the level of the liquid medium in a pressurized or open container to an indicator, thereby serving as a level or boundary measurement instrument. The level gauge is multi-functional, capable of providing alarms for excessive levels above or below the set point, as well as enabling limit control or interlock functions. The flap level gauge has advantages such as a simple structure, comprehensive detection functions, intuitive and prominent readings, and a wide measurement range. It is particularly suitable for applications with large measurement ranges, high corrosivity, as well as those involving flammable and explosive substances. Flip-top level gauges come in three structural types: standard, explosion-proof, and intrinsically safe. Temperature measurement: Temperature is a physical quantity that indicates the degree of hotness or coldness of an object, and it is one of the most common and important operational parameters in various industrial processes and scientific experiments. Temperature cannot be measured directly; it can only be determined indirectly through heat exchange between objects of different temperatures, as well as by taking advantage of the fact that certain physical properties of objects change depending on their temperature. In contact temperature measurement, a certain object is chosen to come into contact with the object whose temperature is to be measured, allowing heat exchange to occur; when the two reach thermal equilibrium, the temperature of the chosen object becomes equal to that of the object being measured. Thus, by measuring a certain physical property of the object under consideration (such as the volume of a liquid or the electric charge of a conductor), it is possible to determine the temperature value of that object quantitatively. In non-contact temperature measurement, the principle of thermal radiation is used for remote temperature sensing. The temperature measurement range is very wide; some take place at low temperatures close to absolute zero, while others are conducted at high temperatures of several thousand degrees. Such a wide measurement range requires various different temperature measurement methods and instruments. Based on the measurement range used, temperature instruments that measure temperatures above 6000°C are commonly referred to as pyrometers, while those that measure temperatures below 6000°C are called thermometers. Classified by purpose, they can be divided into standard instruments and practical instruments. Classified by working principle, they are divided into five categories: expansion thermometers, pressure thermometers, thermocouple thermometers, thermal resistance thermometers, and radiation pyrometers. Based on the measurement method, they can be divided into two main categories: contact and non-contact. In the former case, the temperature-sensing element is in direct contact with the medium being measured, which allows for sufficient heat exchange between the medium and the sensing element to achieve temperature measurement ； In the latter case, the temperature-sensing element does not come into contact with the medium being measured; heat exchange is achieved through radiation or convection in order to determine the temperature. I. Expansion thermometers: Expansion thermometers are designed based on the property that an object’s volume increases when it is heated. Glass tube thermometers belong to the category of liquid expansion thermometers, while bimetallic thermometers fall under the category of solid expansion thermometers. The temperature-sensing element in a bimetallic thermometer is made by welding two metal sheets with different coefficients of linear expansion together. When a bimetallic strip is heated, it bends due to the different expansion lengths of the two metals. As shown in the figure: II. Pressure-type thermometers are instruments that use the change in pressure with temperature to measure temperature. It is based on the principle that when a liquid, gas, or saturated vapor of a low-boiling-point liquid in a closed system is heated, its volume expands or the pressure changes; a pressure gauge is used to measure this change, thereby determining the temperature. The structure of a pressure-type thermometer is shown in the figure: it consists mainly of three parts. (1) Thermocouple: It is a component that comes into direct contact with the medium being measured in order to detect temperature changes; therefore, it requires high strength, a low coefficient of expansion, high thermal conductivity, and corrosion resistance. Depending on the working substance filled and the medium being measured, the thermowell can be made of copper alloy, steel, or stainless steel. (2) Capillary: It is a seamless circular tube drawn from materials such as copper or steel, used to transmit pressure changes. Its outer diameter is 1. 2–5 mm, with an inner diameter of 0. 15 to 0. 5mm。 The more slender its diameter and the longer its length, the more severe the lag in pressure transmission becomes. In other words, the thermometer is less responsive to the temperature being measured. However, at the same length, the finer the capillary, the higher the precision of the instrument. Capillaries are prone to damage and breaking. Therefore, it must be protected. For capillaries that are not bent frequently, a metal hose can be used as a protective sleeve. (3) Bourdon tube (or coiled spring tube): It is the elastic element used in ordinary pressure gauges. III. Thermocouples Thermocouple thermometers are temperature measuring instruments based on the thermoelectric effect. It has a wide measurement range, a simple structure, and is easy to use. It provides accurate and reliable temperature measurement, facilitates the transmission of signals, as well as automatic recording and centralized control; therefore, it is widely used in chemical production. 1. Thermoelectric phenomenon and temperature measurement principle: Take two metal wires of different materials, A and B, and weld their ends together, thus forming a closed circuit. If one end of it is heated, so that the temperature t at contact 1 becomes higher than the temperature t0 at contact 2, then a thermoelectric potential is generated in this closed circuit, as shown in Figure (a). If a DC millivoltmeter is connected in series in this circuit (by disconnecting metal B and connecting it to the millivoltmeter, or by disconnecting at the junction t0 between the two metal wires), as shown in Figures (b) and (c), a voltage reading can be observed on the millivoltmeter. This phenomenon is known as the thermoelectric effect. Why is a thermoelectric potential generated? Two different metals have different free electron densities. In other words, the number of free electrons per unit volume is different in the two metals. Assume that the free electron density in metal A is greater than that in metal B. According to classical electron theory, since metal A has a higher electron density, it also has a higher pressure. When two metals come into contact, at the junction between them, more electrons diffuse from metal A to metal B than from B to A. Once the free electrons cross the contact surface, metal A becomes positively charged due to the loss of electrons, while metal B becomes negatively charged due to the gain of electrons. As a result of this migration, an electric double layer is formed on both sides of the contact surface between the two metals. The electric field of this double layer points from A to B, and its function is to prevent further diffusion of free electrons. Diffusion motion is caused by the imbalance in electron density; as a result of this diffusion, an electrostatic field is generated. The presence of this electrostatic field in turn acts as a resistance to the diffusion motion. When diffusion progresses to a certain extent, the effect of the pressure difference counteracts that of the electrostatic field, leading to a temporary equilibrium between diffusion and back-diffusion. Figure (a) above shows that electron flows of opposite directions and different magnitudes occur at the contact surface between the two metals, causing an excess of electrons to gradually accumulate in metal B, and resulting in a gradually increasing electrostatic field and potential difference eAB from A to B. Figure (b) shows the situation when the electron flow reaches dynamic equilibrium. The contact potential difference at this time. It depends only on the materials of the two metals and the temperature at the contact points; the higher the temperature, the more active the free electrons in the metals, and thus more free electrons migrate from A to B. This leads to an increase in the electric field strength at the contact surface, and as a result, the contact electromotive force also increases. Due to the magnitude of this potential, which, once the thermocouple material is determined, depends only on temperature, it is referred to as the thermoelectromotive force, denoted as eAB(t). Footnote A denotes the positive metal electrode, while footnote B denotes the negative metal electrode. If the order of the subscripts is changed to BA, then the sign in front of e must also change accordingly; that is, eAB(t) = -eBA(t). If the other end of the conductor is also closed to form a closed circuit, two thermoelectromotive forces in opposite directions are generated at the two junctions, as shown in the figure: Figure (a) illustrates that the temperatures at the junctions of the two metals are different; assuming t > t0, the difference in temperatures between the two metals results in two thermoelectromotive forces, eAB(t) and eAB(t0), of unequal magnitudes but in opposite directions. It must be noted that for the same metal A (or B), due to the different temperatures at its two ends, the free electrons possess different kinetic energies, which in turn generates a corresponding electromotive force; this electromotive force is known as the thermoelectromotive force. However, since the thermoelectric potential due to temperature difference is much smaller than the contact thermoelectric potential, it is often ignored. In this way, figure (b) can be used as the equivalent circuit of figure (a), with R1 and R2 representing the equivalent resistances of the thermocouple wires. The total thermoelectric potential E(t, t0) in this closed loop should be: E(t, t0) = eAB(t) – eAB(t0) or E(t, t0) = eAB(t) + eBA(t0). The thermoelectric potential E(t, t0) is equal to the algebraic sum of the thermoelectric potentials at the two junctions of the thermocouple. When materials A and B are fixed, if the temperature at one end, t0, remains constant, then eAB(t0) is a constant. Then the thermoelectromotive force E(t, t0) becomes a function of temperature t alone, independent of the length and diameter of the thermocouple. In this way, by measuring the magnitude of the thermoelectric potential, it is possible to determine the temperature at the measurement point. This is the principle behind using the thermoelectric effect for temperature measurement. 2. The issue of introducing a third wire: When using thermocouples to measure temperature, it is necessary to use certain instruments to measure the value of the thermoelectric potential, as shown in the diagram. Since the measuring instruments are often located far away from the point where temperature is being measured, it is required to use a connecting wire C. This introduces a third wire into the thermocouple circuit formed by AB, and the connection of this third wire creates new junction points, such as point 3 and point 4 in figure (a), and point 2 and point 3 in figure (b). Will the introduction of this third wire affect the thermoelectric potential of the thermocouple? Let’s first analyze the circuit shown in Figure (a). The temperatures at points 3 and 4 are the same (equal to t1); therefore, the total thermoelectromotive force Et is equal to… It can be seen that the total thermoelectromotive force is the same as when no third wire is connected. Let’s analyze circuit (b) again. In this circuit, the temperatures at points 2 and 3 are equal and equal to t0. Therefore, the total thermoelectrical potential Et of the circuit is equal to… According to the principle of energy conservation, in a closed loop made up of various metals, regardless of their different materials, as long as the temperatures at all connection points are equal, the total potential within that closed loop is zero. . If wires of metals A, B, and C are connected to form a closed circuit, with all junctions having the same temperature (all equal to t0), then the total thermoelectrical potential in the circuit is zero. That is, the visible value is the same as the thermoelectric potential when no third wire is connected. This shows that introducing a second type of metal wire into the thermocouple circuit has no effect on the value of the thermoelectric potential generated by the original thermocouple. . However, it is necessary to ensure that the temperatures at both ends of the lead are the same. . Similarly, if more types of wires are connected in series in the circuit, as long as the temperatures at both ends of the connecting wires are the same, it does not affect the value of the thermoelectric potential generated by the thermocouple. 3. Thermoelectric electrode materials: Industrial requirements for thermoelectric electrode materials are as follows: ① The thermoelectromotive force generated per increase of 1 °C in temperature should be high, and this thermoelectromotive force should have a linear relationship with temperature as much as possible ； ② It must have high physical stability, that is, its thermoelectric properties remain unchanged over time within the temperature measurement range, to ensure the accuracy of measurements taken by the thermometer used in conjunction with it ； ③ It must have high chemical stability, that is, it should not be oxidized or corroded at high temperatures ； ④ The material structure should be uniform and possess toughness. Easy to process into threads ； ⑤Good reproducibility (the property whereby thermocouples made from materials with the same components exhibit identical thermoelectric characteristics is known as reproducibility). However, it is difficult to fully meet the above requirements. Currently, only a few types of thermoelectric electrode materials are recognized as good worldwide. These materials have been carefully selected and standardized, and they are used in various temperature ranges to achieve satisfactory measurement results. 4. Types of commonly used thermocouples: The most commonly used (standardized) thermocouples in industry are as follows: ① Platinirhodium 30–platinirhodium 6 thermocouple (also known as the double platinirhodium thermocouple). In this type of thermocouple (with a calibration grade of B), platinirhodium 30 wire serves as the positive electrode, while platinirhodium 6 wire functions as the negative electrode. Its measurement range is from 300 to 1600°C, with the ability to measure up to 1800°C for short periods of time. Its thermoelectric properties are more stable at high temperatures, making it suitable for use in oxidative and neutral media. However, it generates a low thermoelectric potential and is expensive. The thermoelectromotive force is extremely small at low temperatures; therefore, when the cold junction temperature is within the range below 40°C, a correction for the cold junction temperature is generally not required. ② Platinum-rhodium 10–platinum thermocouple: In the platinum-rhodium 10–platinum thermocouple (with a calibration grade of S), the platinum-rhodium 10 wire serves as the positive pole, while the pure platinum wire functions as the negative pole ； The measurement range is -20 to 1300°C; under favorable operating conditions, short-term measurements up to 1600°C are possible℃ ； Suitable for use in oxidative or neutral media. Its advantage is high temperature resistance and low susceptibility to oxidation ； It has good chemical stability ； It features high measurement accuracy and can be used for precise temperature measurements as well as as a reference thermocouple. ③ Nickel-chromium—nickel-silicon (nickel-chromium—nickel-aluminum) thermocouples: In these thermocouples, whose calibration type is K, nickel-chromium serves as the positive electrode, while nickel-silicon (nickel-aluminum) functions as the negative electrode. The measurement range is from -50 to 1000°C; short-term measurements up to 1200°C are possible. They can be used in oxidative and neutral media. In low-temperature ranges below 500°C, they can also be utilized for measurements in reducing media. Such thermocouples have a high electromotive force, good linearity, a wide temperature measurement range, and low cost, which is why they are widely used. The thermoelectric properties of nickel-chromium–nickel-aluminum thermocouples are almost identical to those of nickel-chromium–nickel-silicon thermocouples. However, nickel-aluminum alloys are prone to oxidation and degradation at high temperatures, causing changes in their thermoelectric properties. Nickel-silicon alloys outperform nickel-aluminum alloys in terms of oxidation resistance and thermoelectrical potential stability. At present, nickel-chromium-nickel-silicon thermocouples have basically replaced nickel-chromium-nickel-aluminum thermocouples in our country. ④ Nickel-chromium-copper thermocouple: In this thermocouple (with the designation XK), nickel-chromium serves as the positive electrode, while copper serves as the negative electrode ； It is suitable for use in reducing or neutral media, with a measurement range of -50 to 600°C; short-term measurements up to 800°C are possible. This type of thermocouple has a high electromotive force, about twice that of nickel-chromium/nickel-silicon thermocouples, and it is relatively inexpensive. Its drawback is that the upper temperature measurement limit is not high. It cannot adapt in many cases. Furthermore, copper alloys are prone to oxidation and deterioration, and it is difficult to achieve a uniform wire diameter due to the hard texture of the material. Such thermocouples will be phased out internationally. In China, nickel-chromium–copper-nickel (grade E) thermocouples are used to replace these thermocouples. The one-to-one relationship between the thermoelectric potential of various thermocouples and temperature can be found in standard data tables, which are known as thermocouple calibration tables. In addition, there are many thermocouples used for various special purposes. Such as infrared receiving thermocouples ； Tungsten-rhenium thermocouple for high-temperature measurements at 2000℃ ； Nickel-chromium-gold-iron thermocouples for ultra-low temperature measurements ； Non-metallic thermocouples, etc. 3. Structure of thermocouples: Depending on their purpose and installation location, the shapes of various thermocouples vary greatly. Classified by structure type, there are four types: ordinary type, armored type, surface type, and rapid type. (1) Ordinary thermocouples mainly consist of key components such as thermal electrodes, insulating tubes, protective sleeves, and terminal boxes. As shown in the figure: the thermal electrodes are the two thermocouple wires that make up a thermocouple. The diameter of the thermoelectric electrode is determined by factors such as the cost of the material, mechanical strength, electrical conductivity, as well as the application and measurement range of the thermocouple. The thermoelectrodes for precious metals mostly use a diameter of 0. 3-0. 65mm filaments; the diameter of ordinary metal electrode wires is generally 0. 5 minus 3. 2mm。 Its length depends on the installation conditions and insertion depth, generally ranging from 350 to 2000 mm. Insulating tubes (also known as insulators) are used to prevent short-circuiting between two hot electrodes. The choice of material depends on the operating temperature range, and the structural types typically include single-hole tubes, double-hole tubes, and four-hole tubes. The protective sleeve is placed over the thermal electrode and the insulator, and its function is to protect the thermal electrode from chemical corrosion and mechanical damage. The selection of the protective sleeve material is generally determined by factors such as the temperature measurement range, insertion depth, and temperature measurement time constant. The requirements for the material of the protective sleeve are: high temperature resistance, corrosion resistance, the ability to withstand sudden temperature changes, good airtightness, and a high thermal conductivity. Its structure generally comes in two types: threaded and flanged. The junction box is used to connect the heating electrode and the compensation wire. It is usually made of aluminum alloy and is generally divided into two types: standard and sealed. (2) The armored thermocouple is formed by composite drawing of a metal sleeve, insulating material (magnesium oxide powder), and thermocouple wire together, after which the end wires are welded into a smooth spherical structure. . On the working end, there are three types: exposed type, shell-connected type, and insulated type. Its outer diameter ranges from 1–8 mn, and can even be as low as 0. 2mm, with a length of up to 50m. Armored thermocouples offer advantages such as fast response time, ease of use, bendability, good airtightness, resistance to shock, and high-pressure tolerance; they are therefore a structure that is widely used and increasingly being promoted at present. (3) Surface-type thermocouples: The commonly used design is the thin-film thermocouple, in which two electrode materials are vapor-deposited onto an insulating substrate using vacuum coating techniques. It is a special type of thermocouple designed specifically for measuring the temperature at the surface of objects. Its characteristics include an extremely fast response time and very low thermal inertia. (4) Rapid thermocouple: It is a specialized thermocouple used for measuring high-temperature molten materials. The entire thermocouple element is very small in size, and it is known as an expendable thermocouple. When selecting a thermocouple, three aspects need to be considered: the material of the thermal electrodes ； Structure, materials, and pressure resistance of the protective sleeve ； Insertion depth of the protective sleeve. 4. Selection of compensation wires: In practical applications, since the working end (hot end) of the thermocouple is very close to the cold end, and the cold end is exposed to the environment, it is susceptible to fluctuations in the surrounding temperature; as a result, it is difficult to maintain a constant temperature at the cold end. To keep the temperature at the cold end of the thermocouple constant, it is certainly possible to make the thermocouple very long so as to place the cold end far away from the working end. However, this approach requires the use of a large amount of expensive metal material, which is not economical. The solution to this problem is to use a special wire to extend the cold end of the thermocouple, as shown in the figure: this special wire is called a \"compensation wire\". Compensation wires are made of two metal materials with different properties; within a certain temperature range (0–100°C), they possess the same thermoelectric characteristics as the thermocouple to which they are connected, and their materials are inexpensive metals. Different thermocouples use different compensation wires as well; for thermocouples made of inexpensive metals such as nickel-chromium and constantan, the material itself can be used as the compensation wire. When using thermocouple compensation wires, it is important to ensure that the models match, that the polarity is correct, and that the temperature at the connection point between the thermocouple and the compensation wire does not exceed 100°C. . 5. Compensation for the cold junction temperature: By using compensation wires, the cold junction of the thermocouple is moved from areas with higher and more unstable temperatures to a cooler and more stable operating environment; however, the temperature of the cold junction is still not 0°C. The temperature-thermoelectric potential relationship curves of various thermocouples commonly used in industry are obtained with the cold junction temperature maintained at 0°C, and the instruments used in conjunction with them are also calibrated based on this relationship curve. Since the temperature in the control room is often above 0°C and not constant, the thermoelectric potential generated by the thermocouple will inevitably be low. Moreover, the measurement value also changes with the cold-end temperature, which leads to errors in the measurement results. Therefore, when using thermocouples for temperature measurement, accurate results can be obtained only by keeping the cold junction temperature at 0°C or by applying certain corrections. Doing this is known as cold junction temperature compensation for thermocouples. Generally, the following methods are used: ① The method of maintaining the cold-end temperature at 0°C. The method of keeping the cold-end temperature at 0°C is shown in the figure: The two cold ends of the thermocouple are inserted into test tubes filled with insulating oil, which are then placed in a container containing a mixture of ice and water. This method is mostly used in laboratories. ② Cold-end temperature correction method: In actual production, the cold-end temperature is often not 0°C, but rather some temperature t1, which leads to measurement errors. Therefore, the cold-end temperature must be corrected. For example, if the actual temperature of a certain device is t and the temperature of its cold end is t1, then the thermoelectrical potential measured at this point is E(t, t1). To obtain the actual temperature t, a correction can be applied using the following formula. It can be seen that the method for correcting the cold-end temperature involves adding the measured thermoelectric potential E(t, t1) to the thermoelectric potential E(t1, 0) of the thermocouple when the hot end is at room temperature t1 and the cold end is at 0°C; this gives the thermoelectric potential E(t, 0) at the actual temperature. Example: Use a nickel-chromium/copper-nickel thermocouple to measure the temperature of a heating furnace. The measured thermoelectric potential is E(t, t1) = 66982 μV, and the temperature at the cold end is t1 = 30°C. Determine the actual temperature of the object being measured. Since the relationship between the thermoelectric potential generated by a thermocouple and temperature is nonlinear (with varying degrees of nonlinearity among different types of thermocouples), when the temperature at the free end is not zero, adding the temperature corresponding to the measured thermoelectric potential to the temperature at the free end does not equal the actual temperature being measured. It should be noted that correcting the cold-end temperature using computational methods refers to the effect on temperature measurement when the cold-end temperature is a constant value. This method is only suitable for laboratory or temporary temperature measurement, and is clearly not practical for continuous monitoring. ③ Zero adjustment method for instruments: Generally, when an instrument is not in use, its pointer should point at the zero position (mechanical zero). When a thermocouple is used as the temperature sensing element, in order to ensure that the reading shown during temperature measurement is not too low, the instrument pointer can be adjusted in advance to a value corresponding to room temperature (this is because the compensation wire is connected all the way to the input terminal of the display instrument; the room temperature at the input terminals of the instrument is thus the cold junction temperature of the thermocouple). This method is relatively simple, which is why it is also often used in industry. It must be clearly stated that this method can only be used in situations where the temperature measurement requirements are not very high, as room temperature also changes frequently. ④ The compensation bridge method utilizes the potential generated by an unbalanced bridge to compensate for the changes in thermoelectric potential caused by variations in the cold junction temperature, as shown in Figure 3–64. The unbalanced bridge (also known as a compensation bridge or cold junction temperature compensator) consists of four bridge arms: R1, R2, R3 (wound from manganin wire) and Rt (wound from copper wire), along with a voltage stabilizing source; it is connected in series within the thermocouple measurement circuit. In order for the cold end of the thermocouple and the resistor Rt to experience the same temperature, Rt must be placed alongside the cold end of the thermocouple. The bridge is usually in equilibrium at 20°C, that is, R1=R2=R3=Rt. At this time, the potentials at the diagonal points a and b are equal, i.e., Uab=0, and the bridge has no effect on the instrument’s reading. When the surrounding temperature is above 20°C, the thermoelectric potential of the thermocouple decreases due to the rise in the temperature of its cold end. Meanwhile, the resistance values of R1, R2, and R3 in the bridge do not change with temperature, whereas the copper resistance Rt increases as the temperature rises. As a result, the bridge loses its balance; the potential at point a becomes higher than that at point b, generating an unbalanced voltage Uab between the diagonal points a and b. This voltage is then added to the thermoelectric potential generated by the thermocouple, and together they are sent to the measuring instrument. By appropriately selecting the values of the bridge arm resistances and currents, the unbalanced voltage Uab generated by the bridge can exactly compensate for the change in thermoelectric potential caused by variations in the cold junction temperature, allowing the instrument to indicate the correct temperature. IV. Thermal resistance thermometers and thermocouple thermometers: In these types of thermometers, the element that senses temperature is the thermocouple, and they are generally suitable for measuring higher temperatures of 500°C and above. For medium and low temperatures below 500°C, using thermocouples for measurement is not necessarily appropriate. Firstly, in the medium and low temperature ranges, the thermoelectric potential generated by the thermocouples is very small, which places high demands on the amplifiers of the potentiometer as well as on the measures taken to reduce interference, and makes instrument maintenance difficult. Secondly, in lower temperature ranges, the relative errors caused by changes in the cold junction temperature and environmental temperature become quite significant, and it is not easy to achieve full compensation for these errors. Therefore, in medium and low temperature ranges, it is generally more appropriate to use thermistor thermometers for temperature measurement. A thermistor thermometer consists of a thermistor (the temperature-sensing element), a display instrument (an unbalanced bridge or a balanced bridge), and connecting wires. As shown in the figure:. It is worth noting that the connection wires use a three-wire configuration. 1. Temperature measurement principle: A thermistor thermometer utilizes the property that the resistance value of a metal conductor changes as temperature changes, in order to measure temperature. Its resistance value versus temperature is given by the following formula: It can be seen that changes in temperature lead to changes in the resistance of metal conductors. In this way, by measuring the change in resistance value, it is possible to achieve temperature measurement. Thermistor thermometers are suitable for measuring the temperature of liquids, gases, vapors, and solid surfaces within the range of -200 to 500°C. Like thermocouple thermometers, they also offer advantages such as remote transmission, automatic recording, and the ability to perform multi-point measurements. Additionally, the thermal resistor has a large output signal, ensuring accurate measurement. 2. Thermistors commonly used in industry: The materials used for thermistors are generally required to have a high resistivity and a large temperature coefficient of resistance; low heat capacity; and stable physical and chemical properties as well as good reproducibility over the entire temperature measurement range ； The relationship between resistance value and temperature should ideally be linear. . However, it is actually difficult to find a thermistor material that fully meets the above requirements. Depending on the specific circumstances, the most widely used thermistor materials at present are platinum and copper. (1) Platinum resistors (new model WZP, old model WZB): Metal platinum is easy to purify, and its physical and chemical properties remain very stable in oxidative media, even at high temperatures. However, in reducing media, especially at high temperatures, it is easily contaminated. It makes the platinum wire brittle and alters its resistance-temperature relationship. Therefore, special attention must be paid to protection. In the temperature range of 0 to 650 °C, the relationship between the platinum resistor and temperature is as follows: To determine the Rt–t relationship, it is first necessary to determine the value of R0; different values of R0 result in different Rt–t relationships. This Rt—t relationship is called a scale table, and is represented by a scale number. The purity of platinum is often expressed as R100/R0, where R100 represents the resistance value of the platinum resistor at the boiling point of water; the higher the purity, the larger this ratio becomes. For the platinum resistor, which serves as the reference instrument, the R100/R0 ratio must be no less than 1. 3925. In general industrial applications, the requirement for the purity of the platinum wire in platinum resistance thermometers is that R100/R0 must be no less than 1. 385. There are two types of platinum resistors used in industry; one has R0 = 10Ω, and its designation is Pt10. Another type has R0 = 100Ω, and its designation is Pt100 (2). Copper resistors (WZC is the new model, while WZG is the old model) are easy to process and purify, and are inexpensive; they have a large resistance temperature coefficient, and the resistance varies linearly with temperature ； It exhibits excellent stability within a temperature range of -50 to +150°C. . Its drawback is that it is prone to oxidation when the temperature exceeds 150°C, and loses its good linear characteristics after oxidation ； Furthermore, due to copper’s low resistivity (usually 0. 017Ω•mm2/m); in order to achieve a certain resistance value, the copper resistive wire must be thin. The length also needs to be longer, which results in a larger copper resistive element and reduced mechanical strength. Within the range of -50 to +150°C, the relationship between copper resistance and temperature is linear. That is, there are two types of copper resistors used in industry: one has R0=50Ω, and its designation is Cu50. The other option is Ro=100Ω, with the designation Cu100; its resistance ratio is R100/R0=1. 428. 3. Structure of thermal resistors (1) Ordinary thermal resistors: The structural types of thermal resistors include ordinary thermal resistors, armored thermal resistors, and thin-film thermal resistors. It is mainly composed of key components such as a resistive element, a protective sleeve, and a junction box. Among them, the protective sleeve and junction box are basically the same as those of the thermocouple. The resistance wire is wound (using a double-wire inductive-free winding method) on a support of a certain shape, and this entire assembly is referred to as the resistor body. (2) Encased thermoresistor: The resistance element is pre-formed by drawing and combined with insulating material and a protective sleeve. This type of thermistor is small in size, highly resistant to deformation, bendable, has low thermal inertia, and a long service life. (3) Thin-film thermistor: It is one in which a thermistor material is directly vapor-deposited onto an insulating substrate using vacuum coating. This type of thermistor has a very small size, low thermal inertia, and high sensitivity. V. Electric temperature transmitter: The DBW type temperature (temperature difference) transmitter is a key component among the electric unitized detection and control instruments in the DDZ—Ⅲ series. It is used in conjunction with various types of thermocouples and thermal resistors to convert temperature or the temperature difference between two points into standardized signals of 4–20mA and 1–5V ； It can also be used in conjunction with various transmitters that provide millivolt outputs, to convert them into unified output signals of 4–20mA and 1–5V. . Then, it works in conjunction with the display unit and the control unit to display temperature, temperature differences, and various other parameters. It enables a comparison between DDZ-Ⅲ type temperature transmitters and DDZ-Ⅱ type temperature transmitters, and it has the following main features. (1) Safety spark-type explosion-proof measures are employed in the circuit, thereby enabling the measurement of temperature or millivolt signals in hazardous environments. (2) Linearization mechanisms are employed in thermocouple and thermal resistor temperature transmitters, thereby establishing a linear relationship between the transmitter’s output signal and the temperature being measured. (3) In the circuit, the use of integrated circuits enables the transmitter to possess various technical advantages such as good reliability and stability. A temperature transmitter is a rack-mounted instrument installed in the control room; there are three types of them, namely thermocouple temperature transmitters, thermal resistance temperature transmitters, and DC millivolt transmitters. 1. Thermocouple temperature transmitter: The thermocouple temperature transmitter is used in conjunction with thermocouples to convert temperature into standard signals of 4–20 mA and 1–5 V. It consists of an input bridge circuit, an amplification circuit, and a feedback circuit. . (1) Input bridge: The figure below shows the input circuit of a thermocouple temperature transmitter. It resembles a bridge in structure, which is why it is often referred to as an input bridge. Its function is to provide cold-junction temperature compensation and adjust the zero point. (2) Feedback circuit: In the DDZ-Ⅲ type temperature transmitter, in order to ensure that the output signal of the transmitter is linearly related to the temperature being measured, thereby facilitating display and control – and especially enabling seamless integration with computers – a linearization circuit is incorporated into the feedback loop of the temperature transmitter to correct the non-linearity of the thermocouple. Since the thermoelectric potential generated by thermocouples is very small, it is not suitable for correction within the input circuit; instead, a nonlinear feedback circuit is used for such correction, as shown in the figure: (3) Amplification circuit. Because the value of the thermoelectric potential produced by thermocouples is quite low, usually only in the range of dozens or a few dozen millivolts, it needs to be amplified through multiple stages in order to be converted into a high-level output. 2. Thermal resistor temperature transduction: A thermal resistor temperature transmitter is used in conjunction with thermal resistors to convert temperature into standard signals of 4–20mA and 1–5V. The structure of a thermoresistive temperature transmitter can also be generally divided into three main parts: the input bridge, the amplification circuit, and the feedback circuit. Compared to thermocouple temperature transmitters, the amplification circuit is common to both types; it is only the input bridge and the feedback circuit that differ. VI. Installation of the temperature sensing element 1. Requirements for installing the temperature sensing element (1) When measuring the temperature of a pipeline, it is necessary to ensure that the temperature sensing element is in full contact with the fluid in order to reduce measurement errors. Therefore, it is required that the temperature sensing element be inserted in the direction opposite to the flow of the medium being measured, at least perpendicularly (at 90°) to it, and must not be aligned in the same direction as the flow of the medium. As shown in the figure: (2) The temperature sensing point of the temperature sensing element should be located at the point in the pipe where the flow velocity is highest. For a ship, the ends of the thermocouples, platinum resistors, and copper resistance protection sleeves should extend 5–10 mm, 50–70 mm, and 25–30 mm beyond the centerline of the flow stream, respectively. . (3) The temperature sensing element should have sufficient insertion depth to reduce measurement errors. To this end, the temperature sensing element should be installed at an angle or at a elbow, as shown in the figure. : (4) If the process pipeline is too small (with a diameter of less than 80 mm), an expansion tube should be installed at the location where the temperature sensing element is placed, as shown in Figure a. (5) The cover of the wiring box for thermocouples and thermal resistors should face upward, in order to prevent rainwater or other liquids and dirt from entering the wiring box and affecting the measurements, as shown in Figure b. (6) The temperature sensing element should be inserted into pipes or equipment with insulation to prevent heat loss. (7) When the temperature sensing element is installed in a negative-pressure pipeline, its sealing must be ensured to prevent outside cold air from entering and causing the reading to drop. 2. Wiring requirements: (1) Use compensation wires for thermocouples that match the specified types. Make sure that the positive and negative terminals of the thermocouple are connected to the corresponding positive terminals of the compensation wires, and avoid any mistakes in wiring. (2) The line resistance of the thermal resistor must meet the requirements of the associated secondary instruments. (3) To protect the connection wires and compensation wires from external mechanical damage, they should be placed inside steel pipes or along channel plates. (4) Joints in the wires should be avoided as much as possible. Good insulation is required; it is prohibited to share the same conduit with AC power lines to avoid induction. (5) Avoid crossing power cables. (6) Compensation wires shall not have intermediate joints; otherwise, junction boxes shall be installed. Additionally, it is best to lay it separately from other wires. Flow measurement: In chemical and petroleum refining processes, in order to carry out production operations and control effectively, it is often necessary to measure the flow rate of various media (liquids, gases, vapors, etc.) involved in these processes, so as to provide a basis for such operations and control. At the same time, for economic accounting, it is often necessary to know the total amount of medium that flows within a certain period of time (such as a shift, a day, etc.). Therefore, the medium flow rate is an important parameter necessary to control the production process for achieving high quality and high yield as well as safe production, and for conducting economic calculations. Instantaneous flow rate: refers to the amount of fluid that passes through a certain cross-section of a pipe per unit of time. Total amount (accumulated amount): The sum of the fluid flow rates that pass through a pipe over a certain period of time, that is, the cumulative value of the instantaneous flow rate over that time period. Flow rate and total amount can be expressed in terms of mass or volume. The mass of fluid that flows per unit time is called mass flow rate, commonly denoted by the symbol M. The value expressed in terms of volume is called volumetric flow rate, commonly denoted by the symbol Q. If the density of the fluid is ρ, then the relationship between volumetric flow rate and mass flow rate is: M = Qρ or Q = M/ρ. If time is denoted by t, then the relationships between flow rate and total amount are: QT = ∫0^t Q dt and MT = ∫0^t M dt. Instruments used to measure fluid flow rates are generally called flow meters ； Instruments used to measure the total volume of a fluid are often called meters. However, the two are not completely separate; by equipping a flow meter with an accumulation mechanism, it is also possible to read the total amount. Common units of flow rate include tons per hour (t/h), kilograms per hour (kg/h), kilograms per second (kg/s), cubic meters per hour (m3/h), liters per hour (L/h), and liters per minute (L/min), among others. There are currently many classification methods for flow measurement, and this book presents only one general classification method, as described below: 1. Velocity-type flow meters – These are instruments that measure the flow velocity of a fluid within a pipe as a basis for calculating the flow rate; examples include differential pressure flow meters, rotameters, electromagnetic flow meters, turbine flow meters, and weir flow meters. 2. Positive displacement flow meter: A type of instrument that calculates flow rate by using the number of fixed volumes of fluid discharged per unit of time as a measurement basis. Such as gear flow meters, piston flow meters, etc. 3. Mass flow meter: A type of flow meter that measures the mass M of fluid flowing through it. Mass flow meters are divided into direct-type and indirect-type. A direct-type mass flow meter measures mass flow directly. An indirect mass flow meter determines the mass flow rate by calculating it from density and volumetric flow rate. Mass flow meters have the advantage that their measurement accuracy is not affected by changes in the fluid’s temperature, pressure, viscosity, etc., and they represent a type of flow measurement device that is continuously evolving. I. Differential pressure flow meters Differential pressure (also known as throttling) flow meters are based on the principle of throttling in fluid flow; they measure flow rate by utilizing the pressure difference that occurs when fluid passes through a throttling device. It is one of the most mature and commonly used methods for measuring flow rate in current production. It usually consists of a throttling device that can convert the flow rate to be measured into a pressure difference signal, a differential pressure gauge that can convert this pressure difference into the corresponding flow rate value, and a display instrument. In panel meters, the pressure difference signal generated by the throttling device is often converted into a corresponding standard signal (electrical or pneumatic) using a differential pressure transmitter, for display, recording, or control purposes. 1. Throttling phenomenon and the basic equation of flow rate (1) Throttling phenomenon: When a fluid flows through a pipe equipped with a throttling device, the phenomenon in which there is a difference in the static pressure of the fluid at the pipe walls before and after the throttling device is known as the throttling phenomenon. The throttling device consists of a throttling element and a pressure tapping device. The throttling element is a component that causes local constriction in the fluid flowing through a pipe; the most commonly used one is the orifice plate, followed by nozzles, Venturi tubes, etc. The orifice plate is used below as an example to illustrate the throttling phenomenon. The energy of a flowing fluid exists in two forms: static pressure energy and kinetic energy. Under certain conditions, these two forms of energy can be converted into one another. However, according to the law of conservation of energy, the sum of the static pressure energy and kinetic energy possessed by a fluid, along with the energy loss due to overcoming flow resistance, remains constant in the absence of any external energy input. Differential pressure flowmeters, due to their long history of use, have accumulated extensive practical experience and comprehensive experimental data. Therefore, the most commonly used throttling devices at home and abroad, such as orifice plates, nozzles, and Venturi tubes, have been standardized and are referred to as \"standard throttling devices\". The specific contents of standardization include the structure, dimensions, machining requirements, pressure sampling methods, and operating conditions of the throttling device. For example: Standard orifice plates have detailed specifications regarding size and tolerances, as well as surface finish. As shown in the figure on the right: here, d/D should be between 0.2 and 0.8 ； The minimum pore size should be no less than 12.5 mm ； The thickness h of the straight-hole portion = (0.005 – 0.02)D ； Total thickness H