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This post was last edited by Xidian Hua Electronics on 2016-3-23 at 10:23. Summary: This article focuses on the 8 most commonly used temperature instruments in the chemical industry, covering their working principles, installation requirements, issues to be considered during selection and use, as well as their structure. It provides a detailed overview of these 8 common temperature instruments to offer theoretical and practical guidance for those working in this field Working principle of bimetallic thermometers: The working principle of bimetallic thermometers relies on two metals with different coefficients of thermal expansion. To enhance temperature sensing sensitivity, the metal strips are usually shaped into a spiral form. When the temperature changes, the various layers of metal expand or contract to different degrees, causing the spiral to tighten or loosen. Since one end of the helical coil is fixed while the other end is connected to a pointer that can rotate freely, when the bimetallic strip detects a change in temperature, the pointer can indicate the temperature on a circular scale. The temperature measurement range of such instruments is generally between -80°C and +500°C, with an allowable error of around 1.5% of the full scale range. Categories: Ordinary bimetallic thermometers, shock-resistant bimetallic thermometers, electric-contact bimetallic thermometers. Based on the direction of connection between the pointer disc of the bimetallic thermometer and the protective tube, bimetallic thermometers can be divided into four types: axial type, radial type, 135° type, and universal type. ①Axial bimetallic thermometer: The dial is connected vertically to the protective tube. ②Radial bimetallic thermometer: The dial is connected parallel to the protective tube. ③135° orientation bimetallic thermometer: the dial is connected to the protective tube at a 135° angle. ④Universal bimetallic thermometer: The angle between the dial and the protective tube can be adjusted arbitrarily. Selection and Use: When choosing a bimetallic thermometer, it is necessary to take into full account the actual application environment and requirements, such as dial diameter, accuracy class, mounting method, type of medium being measured, and environmental hazards. In addition, factors such as cost-performance and maintenance workload also need to be taken into consideration. In addition, the following points should be noted when using bimetallic thermometers: A. The length of the protective tube of the bimetallic thermometer that is submerged in the medium being measured must be greater than the length of the temperature-sensing element; generally, this immersion length should be over 100 mm, and for a measurement range of 0–50°C, it should be over 150 mm, in order to ensure accurate measurements. B. Various types of bimetallic thermometers should not be used to measure the temperature of a medium in an open container; thermometers with electrical contacts should not be used in control circuits in environments with significant vibrations. C. When storing, using, installing, and transporting bimetallic thermometers, care should be taken to avoid damaging the protective tube; the tube must not be bent or deformed, and the thermometer must not be used as a wrench. D. Thermometers should be regularly inspected under normal conditions of use. It is generally advisable to do it every six months. Electrical contact thermometers must not be used in conditions of severe vibration, to avoid affecting the reliability of the contacts. E. The temperature at which the instrument operates regularly should preferably be between 1/3 and 2/3 of the scale range. Principle of operation of pressure thermometers: The principle of pressure thermometers is based on the relationship between the saturated vapor pressure of a evaporating liquid and temperature within a closed temperature-measuring system, which is utilized for temperature measurement. When the temperature sensor detects a change in temperature, saturated vapor within the sealed system generates corresponding pressure, which causes a change in the curvature of the elastic element and results in displacement at its free end; this displacement is then amplified by a gear mechanism to produce an indication value. Composition and classification: A pressure-type thermometer consists of a sensitive element, namely a bulb, a pressure-transmitting capillary, and a Bourdon tube gauge. If the system is filled with a gas such as nitrogen, it is called a gas-filled pressure thermometer; its temperature measurement range can reach up to 500°C. The relationship between pressure and temperature is approximately linear, but the bulb has a large volume and high thermal inertia. If filled with liquids such as xylene or methanol and the thermosack is smaller, the temperature measurement ranges are –40°C to 200°C and –40°C to 170°C respectively. If a liquid with a low boiling point is used, its saturated vapor pressure should change with the temperature being measured; for example, propane can be used for temperatures ranging from 50°C to 200°C. However, since there is a nonlinear relationship between saturated vapor pressure and saturated vapor temperature, the thermometer scale is uneven. Feature: The heating pack must be fully immersed in the medium being tested ； The length of the capillary shall not exceed 60m ； The instrument has low precision, but it is easy to use and resistant to vibration. Working principle of resistive thermometers: The temperature measurement principle of thermal resistors relies on the fact that the resistance value of conductors or semiconductors changes with temperature, and this property is used to measure temperature or parameters related to temperature. The resistance value of the vast majority of metals changes with temperature; the higher the temperature, the greater the resistance, that is, they have a positive resistance temperature coefficient. Most semiconductor materials have a negative temperature coefficient of resistance; that is, the higher the temperature, the lower the resistance. Requirements for constituent materials: 1. Stable chemical and physical properties within the temperature measurement range ； 2. Good reproducibility ； 3. High resistance temperature coefficient to achieve high sensitivity ； 4. High resistivity allows for the creation of components with small volumes ； 5. The resistance-temperature characteristic should be as linear as possible ； 6. Low price. Common thermistor elements: Common thermistor elements include platinum thermistors, copper thermistors, and semiconductor thermistors. Platinum resistance thermometers are made by winding high-purity platinum wire. They offer advantages such as high temperature measurement accuracy, stable performance, good reproducibility, and resistance to oxidation; therefore, they are widely used in calibration, laboratories, and industrial applications. However, it is prone to being contaminated by a reducing atmosphere at high temperatures, which makes the platinum wire brittle and alters its resistance-temperature characteristics; therefore, it needs to be protected by a sleeve in order to be used. The purity of the platinum wire is key to determining the accuracy of the thermometer. The higher the purity of the platinum wire, the greater its stability, the better its reproducibility, and the higher its temperature measurement accuracy. Copper thermistors have a resistance value that is nearly linearly related to temperature; they also have a high temperature coefficient of resistance. Moreover, they are inexpensive, which is why they are often used in situations where high measurement accuracy is not required. However, it is easily oxidized in an atmosphere above 100°C; therefore, it is mostly used for measuring temperatures in the range of -50 to 150°C. Advantages of semiconductor thermistors: they have a large negative resistance temperature coefficient, resulting in high sensitivity. It has a high electrical resistivity, allowing it to be made into a resistor element with a small volume but a high resistance value. This gives it low thermal inertia and the ability to measure point temperature or dynamic temperature. Disadvantages: Semiconductor thermistors of the same type exhibit large variability in their resistance-temperature characteristics, severe non-linearity, and unstable performance; as a result, they have poor interchangeability and low accuracy. RTD connection methods: Two-wire system: This is a method in which one wire is connected to each end of the RTD to transmit the resistance signal. This wiring method is quite simple; however, since there is inevitably a lead resistance R associated with the connecting wires, and the magnitude of R depends on the material and length of the wires, this method is only suitable for applications requiring relatively low measurement accuracy. Three-wire system: In this method, one wire is connected to one end of the RTD, while two wires are connected to the other end. This method is typically used in conjunction with a bridge circuit, effectively minimizing the impact of lead resistance. It is the most commonly used method in industrial process control. Four-wire system: The method in which two wires are connected to each end at the base of the thermoresistor is called the four-wire system. Two of these wires supply a constant current I to the thermoresistor, converting R into a voltage signal U; this signal U is then transmitted to the secondary instrument through the other two wires. It can be seen that this type of lead configuration completely eliminates the impact of lead resistance, and is mainly used for high-precision temperature measurement. Installation requirements: When installing thermal resistors, care should be taken to ensure accurate temperature measurement, safety and reliability, as well as ease of maintenance, without affecting the operation of the equipment or production processes. When selecting the installation location and insertion depth of the thermal resistor, the following points should be taken into account: 1. To ensure adequate heat exchange between the measuring end of the thermal resistor and the medium being measured, the location of the measurement point should be chosen carefully; it is advisable to avoid installing the thermal resistor near valves, elbows, as well as in dead corners of pipes and equipment. 2. Thermal resistors with protective sleeves incur heat transfer and heat dissipation losses. To minimize measurement errors, thermocouples and thermal resistors should have sufficient insertion depth: 1) For thermal resistors used to measure the temperature of fluid at the center of a pipe, their measuring ends should generally be inserted into the center of the pipe (whether installed vertically or at an angle). If the pipe diameter of the fluid being measured is 200 millimeters, the insertion depth of the thermoresistor should be set at 100 millimeters ； 2) For temperature measurement of high-temperature, high-pressure, and high-speed fluids (such as main steam temperature), in order to reduce the resistance exerted by the protective sleeve on the fluid and prevent the sleeve from breaking under the influence of the fluid, a shallow insertion method for the protective tube or a heat-shielded thermistor can be employed. For shallowly inserted thermal resistance protection sleeves, the depth of insertion into the main steam pipeline should be no less than 75 mm ； The standard insertion depth for shrink-fit thermoresistors is 100 mm. 3) If it is necessary to measure the temperature of the smoke in the flue, even though the flue diameter is 4m, a heat resistor with an insertion depth of 1m will suffice. 4) When the insertion depth of the measuring element exceeds 1 m, it should be installed as vertically as possible, or support frames and protective sleeves should be added. Thermocouple thermometer. Working principle: Two conductors made of different materials (known as thermocouple wires or thermo-elements) are joined at both ends to form a circuit. When the temperatures at the junctions differ, an electromotive force is generated in the circuit. This phenomenon is called the thermoelectric effect, and this electromotive force is referred to as the thermoelectromotive force. Thermocouples use this principle to measure temperature; the end that is directly used to measure the temperature of the medium is called the working end (also known as the measuring end), while the other end is called the cold end (also known as the compensation end) ； The cold end is connected to a display instrument or accompanying instrument, which indicates the thermoelectric potential generated by the thermocouple. Installation requirements: Firstly, the thermocouples and thermal resistors should be installed as vertically as possible to prevent the protective sleeves from deforming at high temperatures. However, in the presence of flow, they must be inserted in the direction of the fluid being measured, so as to ensure adequate contact between the temperature-sensing elements and the fluid and thus maintain measurement accuracy. Additionally, thermocouples and thermal resistors should be installed in pipes with protective coatings as much as possible to prevent heat loss. Secondly, when thermocouples and thermal resistance sensors are installed in negative-pressure pipelines, it is essential to ensure good sealing at the measurement point in order to prevent outside cold air from entering and causing the readings to be too low. When thermocouples and thermal resistance sensors are installed outdoors, the cover of their wiring boxes should face upward while the wire entry ports should face downward, in order to prevent rain or dust from entering the wiring boxes and damaging the wires inside them, which could affect their measurement accuracy. The wiring at all connections of thermocouples and thermal resistance thermometers should be checked regularly. In particular, thermocouple thermometers are prone to having their connections break due to the high hardness of the material used in their compensation wires, which can cause circuit interruptions; therefore, it is important to ensure proper wiring, to avoid disturbing the connections too much, and to conduct regular checks in order to obtain accurate temperature readings. When installing a thermocouple, it should be placed as close as possible to the temperature control point to be measured. To prevent heat from escaping along the thermocouple or to avoid the protective tube affecting the temperature being measured, the thermocouple should be immersed in the fluid being measured to a depth of at least 10 times its diameter. When measuring the temperature of a solid, the thermocouple should be pressed against or in close contact with the material. To minimize the heat conduction error, the temperature gradient near the junction should be reduced. When using a thermocouple to measure the temperature of gas in a pipe, if the wall temperature of the pipe is significantly higher or lower, the thermocouple will absorb or emit heat to it, thereby significantly altering the temperature being measured. At this time, a radiation shield can be used to bring its temperature close to that of the gas, by employing a so-called shielded thermocouple. The temperature measurement point selected should be representative; for example, when measuring the temperature of fluid in a pipe, the sensing end of the thermocouple should be located at the point in the pipe where the flow velocity is highest. Generally, the end of the thermocouple’s protective sheath should extend beyond the flow centerline. Principle of operation of glass-tube liquid thermometers: Glass liquid thermometers operate based on the principle of thermal expansion and contraction. When the temperature changes, the volume of the liquid inside the glass bulb expands or contracts, thereby altering the height of the liquid column in the capillary tube. This change can then be read from the scale to indicate the temperature variation. The resolution of the temperature gauge’s scale is related to its sensitivity; the higher the sensitivity, the greater the resolution of the gauge’s scale. To increase the sensitivity of a thermometer, one can increase the volume of the thermometric liquid or decrease the diameter of the capillary tube. However, increasing the volume of the temperature-measuring fluid makes it difficult to reach thermal equilibrium with the substance being measured, resulting in significant lag errors, and it also tends to cause deformation of the spherical portion ； Reducing the capillary diameter makes it difficult to achieve uniform processing of the capillary, resulting in uneven rise of the liquid column and affecting the accuracy of the measurements. Therefore, an appropriate sensitivity should be chosen. Furthermore, the sensitivity of the thermometer is also related to the difference between the thermal expansion coefficients of the temperature-measuring fluid and glass, and is proportional to it. Generally, liquids with a high coefficient of thermal expansion are chosen as temperature-measuring fluids, while the coefficient of thermal expansion of glass should be as low as possible. Commonly used temperature-measuring liquids include mercury and alcohol. Main causes of errors: (1) Permanent zero drift; (2) Temporary deformation of the spherical part; (3) Pressure changes; (4) Inaccurate scales; (5) Incorrect reading methods; (6) Thermal hysteresis effect; (7) Special reasons for errors in alcohol thermometers; (8) Special reasons for errors in maximum temperature thermometers. Working principle of temperature transmitters: A temperature current transmitter converts the signal from a temperature sensor into a current signal, which is then connected to secondary instruments to display the corresponding temperature. Temperature transmitters use thermocouples or thermal resistors as temperature sensing elements. The signals generated by these sensing elements are sent to the transmitter module, where they undergo various circuit processes such as voltage stabilization and filtering, operational amplification, nonlinear correction, V/I conversion, constant current regulation, and reverse protection. As a result, a 4–20mA current signal that is linearly related to temperature is generated and output. Installation requirements: 1. Before installation, check that all accessories are present and that the fasteners are tight; tighten the antenna as well. 2. During installation, handle it gently; do not knock or drop it. Once the antenna is tightened, it will function normally. 3. After installation, when power is applied, unauthorized personnel must not open the front cover. In case an operator makes a mistake, saving any data is strictly prohibited; simply turn off the power and then restart the device. The main causes of errors: When the temperature of the measured medium rises or falls, there is no change in the transmitter’s output. This situation is mostly due to sealing issues with the temperature transmitter; it could be that the transmitter isn’t properly sealed, or a small hole may have been created in the sensor during welding. In such cases, replacing the transmitter housing is usually necessary to resolve the problem. The output signal is unstable; this is due to the instability of the temperature source itself, which is a source of uneven temperatures. If it is the instrument that shows instability, then it is because the instrument lacks sufficient resistance to interference. The output error of the transmitter is relatively large. There are many possible causes for this: it could be that the wrong resistance wire was used in the temperature transmitter, resulting in an incorrect measurement range; it’s also possible that the transmitter wasn’t properly calibrated at the time of manufacture. Working principle of the temperature switch: A temperature switch is a type of switch that uses a bimetallic strip as the temperature sensing element. When the electrical appliance is operating normally, the bimetallic strip remains in its natural state, with the contacts in either a closed or open position. When the temperature rises to the set threshold, the bimetallic element heats up, generating internal stress that causes it to act quickly, thereby opening or closing the contacts and cutting off or connecting the circuit, thus providing thermal protection. When the gradient drops to the reset temperature, the contacts automatically close/open, restoring normal operation. Installation requirements: 1. When using a contact-type temperature sensor, the metal cover should be pressed tightly against the installation surface of the device to be monitored. To ensure proper temperature sensing, thermal conductive silicone grease or another similar thermal conductive material should be applied to the temperature-sensing surface. 2. During installation, the top of the cover must not be crushed, loosened, or deformed, to avoid affecting performance. 3. Liquid must not penetrate inside the temperature controller; the casing must not develop cracks, and the shape of the external terminals must not be altered arbitrarily. 4. When the product is used in a circuit with a current not exceeding 5A, wires with a copper core cross-section of 0.5–1 mm² should be selected for connection ； For circuits with a current not exceeding 10A, wires with a copper core cross-section of 0.75–1.5 mm² should be used for connections. 5. The products should be stored in a well-ventilated, clean, dry warehouse with no corrosive gases, where the relative humidity is below 90% and the ambient temperature is below 40°C. Principle of operation of optical, radiation thermometers: A radiation thermometer is designed and manufactured based on the functional relationship between an object’s radiant energy across the entire wavelength range and its temperature. It uses a radiation sensor as the primary instrument and an electronic potentiometer as the secondary instrument. It belongs to the category of lens-focused sensors and features an aluminum alloy casing; the front part of this casing contains the objective lens. Inside the casing there is a thermopile along with a light shield for compensation. On the light shield located near the thermopile, there is a adjustment plate whose function is to regulate the amount of radiant energy reaching the thermopile, thereby ensuring consistent calibration values. An eyepiece is mounted on the removable rear cover, allowing for the observation of the image of the object being measured. A radiation thermometer focuses the radiant energy from the object being measured using a lens onto a thermosensitive element; this element converts the radiant energy into electrical parameters. The thermoelectric potential is then determined using the known relationship between thermoelectric potential and object temperature, and an instrument measures this potential to display the temperature value. This temperature value needs to be corrected using the object’s total radiative emissivity coefficient, or else a platinum-rhodium 10-platinum thermocouple can be inserted directly into a high-temperature salt bath furnace and used along with a DC potentiometer to measure the temperature. The resulting value can then be compared with the temperature displayed by the instrument in order to calibrate the accuracy of the thermometer’s temperature readings. Installation requirements: The instrument is designed for fixed installation. The temperature sensor can operate in an environment with a temperature ranging from 10 to 80°C. When the ambient temperature exceeds 80°C, or when there is water vapor or smoke in the air, auxiliary devices such as water cooling and ventilation can be used to lower the ambient temperature and clear the smoke from the measurement channel, thereby reducing measurement errors. Thermistor auxiliary devices are available in light and heavy types. The heavy-duty version is used in harsh environments; to prevent the flame or high-temperature furnace gases from the furnace being measured from bursting out through the measurement channel and damaging the instrument, a flame protection device has been installed. This device can activate automatically in case of danger, protecting the instrument and issuing an alarm signal. Working principle of non-contact infrared thermometers: A non-contact infrared thermometer (hereinafter referred to as “thermometer”) can determine the surface temperature by measuring the infrared energy emitted by the surface of the target. The contactless infrared thermometer features an ultra-low power intelligent design. The ultra-low power consumption design ensures that the product can operate for longer periods of time, eliminating the need for frequent battery replacements and the hassle of running out of power while in use. Smart design helps users test more conveniently and capture the true values of the objects being tested more quickly; meanwhile, the instrument can intelligently choose between battery power or USB connection for power supply.