Differences and principles between thermocouples and thermal resistance temperature measuring instruments

(1) The difference between thermocouples and thermal resistance temperature measuring instruments

The difference between thermocouple and thermal resistor

Although they are both contact temperature measuring instruments, their temperature measurement ranges are different.

Thermocouples are used in environments with higher temperatures, because they output a very small thermoelectric potential in medium and low temperature areas. When the potential is small, anti-interference measures and secondary meters are required, otherwise the measurement will be inaccurate. Also, in higher temperature In low temperature areas, the relative errors caused by changes in cold end temperature and changes in ambient temperature are very prominent and are not easy to be fully compensated.

Thermal resistors are used in medium and low temperature environments. Generally, the temperature measurement range of thermal resistors is 200~500℃, and even lower temperatures can be measured (for example, carbon resistors can measure low temperatures of about 1K). Platinum thermal resistor Pt100 is now used normally. (There are also Pt50, and copper resistors are also used in industry, but the temperature measurement range is small, between -50 and 150°C. In some special occasions, there are also indium resistors, manganese resistors, etc.).

Thermometry principle

The basic principle of thermocouple measuring temperature is the thermoelectric effect. The secondary meter is a voltmeter or an electronic potentiometer is used to improve accuracy.

Thermal resistance works based on the characteristic that the resistance value of conductors and semiconductors changes with temperature. The secondary meter is an unbalanced bridge.

On-site fault diagnosis at work

Thermocouple: A thermocouple has positive and negative poles, and the compensation wire also has positive and negative poles. First, ensure that the connection and configuration are correct. During operation, common fault phenomena include short circuit, open circuit, poor contact (can be judged by a multimeter) and deterioration (identified according to the surface color). During inspection, keep the thermocouple and secondary meter separated.

Thermal resistance: nothing more than short circuit and open circuit. A multimeter can be used to judge that if a short circuit is suspected during operation, just remove a wire from the resistor end. If the display instrument reaches the maximum value, the thermal resistor is short-circuited; if the display instrument returns to zero, the wire is short-circuited. When ensuring normal connection and configuration, if the meter value displays low or unstable, the protection tube may have water in it.

Thermocouple and resistance signals enter the PLC system. If the instrument is open-circuited, the PLC data will return to zero; if the instrument is short-circuited, the PLC data will overflow; if the instrument signal is subject to electromagnetic interference, the PLC data will be unstable or keep overflowing.

Thermocouple and RTD selection

The graduation numbers of thermocouples mainly include S, R, B, N, K, E, J, T, etc. Among them, S, R, and B belong to precious metal thermocouples, and N, K, E, J, and T belong to cheap metal thermocouples. The T graduation number is characterized by the highest accuracy level among all low-cost metal thermocouples, and is usually used to measure temperatures below 300°C. Thermistor is the most commonly used temperature detector in medium and low temperature areas. Its main features are high measurement accuracy and stable performance. Among them, platinum thermal resistance has the highest measurement accuracy. It is not only widely used in industrial temperature measurement, but also made into a standard benchmark instrument. Thermal resistors are mostly made of pure metal materials. Currently, platinum and copper are the most widely used. In addition, thermal resistors have begun to be made of materials such as nickel, manganese and rhodium.

(2) Installation of thermocouples and thermal resistance temperature measuring instruments. General provisions for the installation of instrument equipment.

1. The installation location of on-site installation instruments should comply with the following regulations: ★ Sufficient light, convenient operation and maintenance; should not be installed in places that are subject to vibration, moisture, susceptible to mechanical damage, interference from strong magnetic fields, high temperatures, drastic temperature changes, and corrosive Gas place. ★The height of the center of the instrument from the ground should be 1.2 to 1.5 meters. ★The locally installed display instrument should be installed at a location where it is convenient to observe the value indicated by the instrument when manually operating the valve. 2. Before installation, the instrument should have a complete appearance and complete accessories, and its model, specifications and materials should be checked according to the design regulations. 3. The instrument should not be knocked or vibrated when installed. It should be firm and straight after installation. 4. Instruments that require degreasing according to the design must pass the degreasing inspection before installation. 5. Instruments installed directly on the process pipeline should be installed after the process pipeline is purged and before the pressure test. When it must be installed at the same time as the process pipeline, the instrument should be removed during the purge of the process pipeline. The direction of the arrow on the instrument housing should be consistent with the flow direction of the medium being measured. When the instrument is connected to the process pipeline, the axis of the flange on the instrument should be consistent with the axis of the process pipeline, and the force should be even when fixed. 6. After the instruments installed directly on the process equipment or pipelines are installed, they should be pressure tested together with the process system. 7. The inlet of the junction box on instruments and electrical equipment should not face upward to prevent oil, water and dust from entering the box. When unavoidable, sealing measures should be taken. 8. The text and terminal numbers on the signs of instruments and electrical equipment should be written correctly and clearly. The wiring of instruments and electrical equipment should comply with the following regulations: ★The wiring should be calibrated and marked before wiring. ★The wire core should not be damaged when stripping off the insulation layer. ★The ends of multi-strand wire cores should be tinned or wire lugs should be used. When using lugs, the connection between the wires and the lugs should be crimped or welded, and the connections should be even and firm with good conductivity. ★Non-corrosive flux should be used when soldering. ★The connection between the cable (wire) and the terminal should be firmly fixed with appropriate margin. ★The wiring should be correct and the arrangement should be neat and beautiful. ★When instruments and electrical equipment are susceptible to vibration, spring washers should be added to the terminal blocks. ★The line compensation resistor should be firmly installed and easy to disassemble and assemble. The allowable error of its resistance value is ±0.1 ohm.

Installation method of thermocouple and thermal resistance temperature meter

1. The installation position of the temperature source component should be selected in a place where the medium temperature changes sensitively and is representative. It should not be located near resistance components such as valves, in dead corners of the medium flow stream, or in places with large vibrations. 2. The thermocouple source components should be installed away from strong magnetic fields. 3. The installation of the temperature source component on the process pipeline should comply with the following regulations: ★When installed vertically with the process pipeline, the axis of the source component should intersect perpendicularly with the axis of the process pipeline.

★When installed obliquely with the process pipeline, it should be against the flow direction of the medium, and the axis of the source component should intersect with the axis of the process pipeline.

4. Temperature measuring components installed on dusty process pipelines should be protected from wear and tear. 5. When the thermocouple or thermal resistor is installed in a place susceptible to strong impact from the medium being measured, and when the insertion depth is greater than 1 meter when installed horizontally or the measured temperature is greater than 700°C, anti-bending measures should be taken. 6. The temperature-sensing surface of the surface thermometer should be in close contact with the surface to be measured and firmly fixed. 7. Install the thermometer on the elbow tube. During installation, the axis of the thermometer must coincide with the center line of the straight section of the elbow tube.

8. When using a thermocouple to measure furnace temperature, avoid direct contact between the temperature measuring component and the flame, and it should not be too close or installed next to the furnace door. The junction box should not touch the furnace wall to avoid excessive temperature at the cold end of the thermocouple.

9. When using thermocouples and thermal resistors to measure temperature, the introduction of interference signals should be prevented, and the opening of the junction box should be directed downward to prevent water vapor, dust, etc. from entering and affecting the measurement.

10. If the process pipeline is too small, an expansion pipe can be installed where the temperature measurement component is installed. Thermocouples, thermal resistors, and bimetal thermometers are installed on pipes with DN<80mm. 11. When installing the thermocouple, it should be placed as close as possible to the temperature control point to be measured. In order to prevent heat from being transferred along the thermocouple or preventing the protective tube from affecting the measured temperature, 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 held against or in close contact with the material. To minimize thermal conduction errors, the temperature gradient near the junction should be reduced.