Overview and principle of WRN thermocouple

Overview of Thermocouples

Thermocouple is a temperature sensing element and a primary instrument. It directly measures temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium through electrical instruments (secondary instruments). The basic principle of thermocouple temperature measurement is that two conductors with different compositions form a closed circuit. When there is a temperature gradient at both ends, current will flow through the circuit, and there will be an electromotive force - thermoelectric force - between the two ends, which is called the Seebeck effect. Two homogeneous conductors with different compositions are thermoelectric electrodes, with the high-temperature end as the working end and the low-temperature end as the free end, which is usually at a constant temperature. Create a thermocouple calibration table based on the functional relationship between thermoelectric potential and temperature; The calibration table is obtained under the condition of a free end temperature of 0 ℃, and different thermocouples have different calibration tables.

When a third metal material is connected to a thermocouple circuit, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged and will not be affected by the connection of the third metal to the circuit. Therefore, when using thermocouples for temperature measurement, a measuring instrument can be connected to measure the thermoelectric potential, and the temperature of the measured medium can be determined.

working principle

Two conductors with different compositions (called thermocouple wires or thermoelectric electrodes) are connected at both ends to form a circuit. When the temperature of the junction is different, an electromotive force is generated in the circuit, which is called thermoelectric effect, and this electromotive force is called thermoelectric potential. Thermocouples use this principle for temperature measurement, where the end directly used to measure the temperature of the medium is called the working end (also known as the measuring end), and the other end is called the cold end (also known as the compensating end); Connect the cold end to the display instrument or matching instrument, and the display instrument will indicate the thermoelectric potential generated by the thermocouple.

Thermocouples are actually energy converters that convert thermal energy into electrical energy and measure temperature using the generated thermoelectric potential. Regarding the thermoelectric potential of thermocouples, the following issues should be noted:

1: The thermoelectric potential of a thermocouple is the difference in temperature function between the two ends of the working end of the thermocouple, rather than the temperature difference between the cold end and the working end of the thermocouple;

2: The magnitude of the thermoelectric potential generated by a thermocouple is not related to the length and diameter of the thermocouple when the material is uniform, but only to the composition of the thermocouple material and the temperature difference at both ends;

3: After the composition of the two thermocouple wires is determined, the magnitude of the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; If the temperature of the cold end of the thermocouple remains constant, the thermoelectric potential of the thermocouple is a single value function of the working end temperature. Weld two different materials of conductors or semiconductors a and b together to form a closed circuit, as shown in the figure. When there is a temperature difference between the two attachment points 1 and 2 of conductors a and b, an electromotive force is generated between them, resulting in a current of a certain magnitude in the circuit. This phenomenon is called thermoelectric effect. Thermocouples work by utilizing this effect

characteristic

◆ Simple assembly and easy replacement

◆ Spring type temperature sensing element with good seismic performance

◆ Large measurement range

High mechanical strength and good pressure resistance

◆ High temperature resistance up to 2400 degrees Celsius

Thermocouple selection

The installation requirements for thermocouples require attention to the installation of thermocouples and thermal resistors, which should be conducive to accurate temperature measurement, safe and easy maintenance, and not affect equipment operation and production operations To meet the above requirements, attention should be paid to the following points when selecting the installation location and insertion depth of thermocouples and thermal resistors: 1. In order to ensure sufficient heat exchange between the measuring end of thermocouples and thermal resistors and the measured medium, the position of the measuring point should be selected reasonably, and thermocouples or thermal resistors should be avoided as much as possible near dead corners of valves, elbows, pipelines, and equipment 2. Thermocouples and thermistors with protective sleeves have heat transfer and dissipation losses. In order to reduce measurement errors, thermocouples and thermistors should have sufficient insertion depth: (1) For thermocouples that measure the fluid temperature at the center of pipelines, their measuring ends should generally be inserted into the center of the pipeline (vertically or obliquely installed) If the diameter of the tested fluid pipeline is 200 millimeters, the insertion depth of the thermocouple or thermistor should be selected as 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 of the protective sleeve to the fluid and prevent the protective sleeve from breaking under the action of the fluid, shallow insertion of the protective tube or the use of a hot sleeve thermocouple can be adopted The shallow insertion type thermocouple protective sleeve should be inserted into the main steam pipeline to a depth of not less than 75mm; The standard insertion depth for hot sleeve thermocouples is 100mm; (3) If it is necessary to measure the temperature of the flue gas inside the flue, even though the diameter of the flue is 4m, a thermocouple or thermistor can be inserted at a depth of 1m (4) When the insertion depth of the measuring element exceeds 1m, it should be installed vertically as much as possible, or support frames and protective sleeves should be added