Temperature measurement conditions of thermocouple

Thermocouple is a temperature sensing element and a primary instrument. Thermocouple directly measures temperature. A closed loop composed of two conductors of different materials. Due to different materials, different electron densities produce electron diffusion, and after stable equilibrium, an electric potential is generated. When there is a gradient temperature at both ends, a current will be generated in the loop, generating a thermoelectric potential. The greater the temperature difference, the greater the current. After measuring the thermoelectric potential, the temperature value can be known. Thermocouple is actually an energy converter that can convert thermal energy into electrical energy.
Technical advantages of thermocouples: Thermocouples have a wide temperature measurement range and relatively stable performance; high measurement accuracy, the thermocouple is in direct contact with the measured object and is not affected by the intermediate medium; fast thermal response time, the thermocouple is flexible to temperature changes; large measurement range, the thermocouple can continuously measure temperature from -40~+ 1600℃; the thermocouple has reliable performance and good mechanical strength. Long service life and easy installation.

The thermocouple must be composed of two conductor (or semiconductor) materials with different properties but meeting certain requirements to form a loop. There must be a temperature difference between the measuring end and the reference end of the thermocouple.

Weld two conductors or semiconductors A and B of different materials together to form a closed loop. 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 the two, so a current of a certain size is formed in the loop. This phenomenon is called the thermoelectric effect. Thermocouples work by using this effect.

Three basic laws of thermocouples

1. Law of homogeneous conductors
When the two ends of the same homogeneous material (conductor or semiconductor) are welded together to form a closed loop, no matter how the conductor cross-section is or how the temperature is distributed, no contact potential will be generated, the temperature difference potential will cancel each other out, and the total potential in the loop will be zero.
It can be seen that thermocouples must be composed of two different homogeneous conductors or semiconductors. If the thermocouple material is not uniform, additional thermoelectric potential will be generated due to the existence of a temperature gradient.
2. Intermediate conductor law
When an intermediate conductor (third conductor) is connected to the thermocouple loop, as long as the temperatures at both ends of the intermediate conductor are the same, the introduction of the intermediate conductor has no effect on the total potential of the thermocouple loop. This is the intermediate conductor law.
Application: According to the intermediate conductor law, in the actual temperature measurement application of thermocouples, the hot end is often welded and the cold end is open. The cold end is connected to the display instrument through the connecting wire to form a temperature measurement system.
Some people are worried that the contact potential generated at the connection between the wire and the thermocouple when the copper wire is connected to the cold end of the thermocouple to read the mV value will cause additional errors in the measurement. According to this law, there is no such error!
3. Intermediate temperature law
The thermoelectric potential between the two junctions of the thermocouple loop (temperature T, T0) is equal to the algebraic sum of the thermoelectric potential of the thermocouple at temperatures T, Tn and the thermoelectric potential at temperatures Tn, T0. Tn is called the intermediate temperature.
Application: Since the thermocouple E-T usually has a nonlinear relationship, when the cold end temperature is not 0 degrees Celsius, the actual thermoelectric potential E (t, t0) of the known circuit cannot be used to directly look up the table to obtain the hot end temperature value; nor can the actual thermoelectric potential E (t, t0) of the known circuit be used to directly look up the temperature value obtained by the table, and then add the cold end temperature to determine the measured temperature value of the hot end. Correction must be made according to the intermediate temperature law. Beginners often do not make corrections according to the intermediate temperature law!