Common knowledge in instrument automation

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Basic Concepts of Instruments I. Measurement, Measurement Error, Direct Measurement, and Indirect Measurement 1. What is measurement error? Answer: The difference between the measured value and the true value is called measurement error. 2. What are direct measurement and indirect measurement? Answer: Direct measurement refers to the situation where the parameter being measured is directly compared with a certain standard value. Indirect measurement involves substituting the directly measured data into a specific formula to calculate the value of the parameter being measured. 3. What is measurement? Answer: It is the process of converting and transmitting the signal of the parameter to be measured, and comparing it with the corresponding unit of measurement; this is what is called measurement. II. Instrument error, drift, sensitivity, and sensitivity limit. 1. What are the allowable error of a instrument, its accuracy, and its accuracy class? Answer: The allowable error refers to the maximum percentage error that an instrument is permitted to have under specified normal conditions. The accuracy of a gauge refers to the value obtained by removing the percent sign (%) from the allowable error. The accuracy grade is a series of standard percentage values that are **uniformly specified**. 2. What is instrument drift? Answer: It refers to the difference between the readings obtained when using the same instrument to measure a certain parameter in both forward and reverse directions, under constant external conditions. 3. What are the sensitivity and detection limit of a measuring instrument? Answer: Sensitivity is used to indicate the degree to which a measuring instrument is responsive to changes in the parameter being measured. Sensitivity is expressed as the ratio of the change in the reading indicated by the instrument to the change in the parameter being measured that causes this change. The sensitivity limit refers to the minimum change in the parameter being measured that causes a visible change in the instrument’s reading. 4. What is the dynamic error of a measurement system? Answer: It refers to the difference between the instrument reading and the actual value of the parameter being measured, when the information related to that parameter is in a state of change within the measurement system. III. Pressure, engineering atmosphere, physical atmosphere, gauge pressure, absolute pressure. 1. What is pressure? Answer: Pressure is a force that acts vertically and evenly on a unit area. 2. What is engineering atmospheric pressure? Answer: Engineering atmospheric pressure is a unit commonly used in industry; it represents the pressure generated by 1 KG of force acting vertically and evenly on an area of 1 square centimeter, and it is expressed in kgf/cm2. 3. What is physical atmospheric pressure? Answer: Physical atmospheric pressure is the force generated by a mercury column 760 mm high, with a density of 13.5951 grams per cubic meter and a gravitational acceleration of 980.665 centimeters per second squared, acting on a surface. IV. The role of transmitters in automatic detection and control systems is to convert various process parameters, such as physical quantities like pressure, differential pressure, temperature, flow rate, liquid level, and composition, into corresponding standard signals. These signals are then sent to indicator recorders, calculators, and controllers for display, recording, and control purposes. Classified by the parameters being measured, transmitters mainly include differential pressure transmitters, pressure transmitters, temperature transmitters, flow rate transmitters, etc. Composition: It usually consists of an input conversion section, an amplifier, and a feedback section. The input conversion section includes sensitive components whose function is to sense the parameter being measured and convert it into an intermediate analog value. The intermediate quantity can be physical quantities such as voltage, current, displacement, and force. The feedback section converts the transmitter’s output signal into a feedback signal. The amplifier amplifies the difference between the intermediate analog signal and the feedback signal, and converts it into a standard output signal. Measurement principles and operation of commonly used instruments 1. Measurement principle and operation of thermal resistors. Principle: It utilizes the temperature measurement principle based on the linear relationship between the resistance value of a thermistor and temperature within a certain temperature range. Common thermal resistors include: platinum resistors, copper resistors, manganese resistors, and carbon resistors. Operation: When measuring the transmission resistance signal, thermoresistors use a three-wire system; this is done to reduce measurement errors caused by the resistance of the wires during transmission. When checking its resistance value, pay attention to which two wires are common wires. 2. Principle and operation of thermocouple measurement. Principle: When two different metals are connected at one end, a potential difference is established between their other ends, and within a certain range, this potential difference has a linear relationship with temperature. Operation: A two-wire system is used; during inspection, its resistance value should be very low. 3. Measurement principle and operation of pressure transmitters. Principle: Thin-film metal strain measuring element. Operation: When making connections, pay attention to the positive and negative poles. When in use, the manual valve should be opened slowly to avoid sudden shocks to the diaphragm box. Check for any leaks at the instrument connections to prevent measurement errors. When removing it, the manual valve should be closed first; once it is confirmed that the valve is closed, the gauge should be removed slowly, while shaking the transmitter to allow any remaining gas to escape. 4. Working principle of differential pressure level gauges. Principle: The diaphragm of the silicone-oil-filled measuring box is connected through two coupling pins; the differential pressure generates a force that causes the torsion rod to deform. This torsion rod is placed in front of film strain resistors arranged in a Wheatstone bridge, which converts the pressure difference into a balanced voltage signal. The electronic amplifier converts the signals from the measurement chamber into a 4–20 mA DC two-wire output signal, and it also provides a Wheatstone bridge. Operation: When making connections, pay attention to the positive and negative poles. When in use, first open the balance valve of the transmitter, close the manual valves in the high-pressure and low-pressure chambers, open the manual valves on the pressure lead lines of these chambers, slowly open the manual valves in the high-pressure and low-pressure chambers, and then close the balance manual valve. Precautions for using explosion-proof instruments. 1. Check whether the instrument enclosure has an EX mark, and verify that the explosion-proof marking complies with the regulations regarding hazardous substances at the site. 2. Intrinsic safety transmitters must be equipped with a safety barrier in order to be used in hazardous environments. 3. The instrument enclosure must have proper grounding. 4. In hazardous situations, the power must be turned off first before the cover can be opened. 5. The specifications for the incoming cable of the intrinsically safe transmitter are specified by the safety barrier with joint certification.