Some related knowledge about thermocouples

The main factors affecting the stability of thermocouples are:

(l) Thermoelectric electrodes undergo oxidation during use, particularly selective oxidation and volatilization of certain elements.
(2) The deformation stress caused by the external force acting on a thermoelectric electrode.
(3) Thermoelectric electrodes experience grain growth at high temperatures.
(4) Contamination and corrosion of thermoelectric electrodes.

Why does the thermocouple input circuit need to have a cold junction temperature compensation function?
Answer: Because when measuring temperature with a thermocouple, it is always desirable for the cold end temperature to remain constant, but this is difficult to achieve in industrial production. Therefore, it is required that the input circuit of the transmitter has the function of a "cold end temperature compensator". That is to say, when the output of the thermocouple changes with the temperature of the cold end, the output of the bridge circuit should also change accordingly, and the magnitude of the change should be equal and the direction opposite. In this way, measurement errors caused by changes in cold end temperature can be automatically compensated for.

Why is compensation wire used for on-site temperature measurement?
Answer: The use of compensating wires not only moves the reference end of the thermocouple from a high temperature location to a relatively stable ambient temperature, but also saves a large amount of expensive metals and rare metals with stable performance; The use of compensating wires also facilitates installation and line laying; Replacing the thermoelectric electrode with a compensating wire with a thicker diameter and higher conductivity can reduce the resistance of the thermocouple circuit, facilitating the normal operation of the moving coil display instrument and automatic temperature control.

What is the model of compensation wire for commonly used thermocouples?
Answer: Models and names of commonly used compensating wires for thermocouple: The compensating wire models are divided into SC and SC according to the product variety KC,KX,EX,JX,TX.

(1) The first letter of the model corresponds to the scale mark of the thermocouple.

(2) The letter "X" represents an extended compensating wire (type).

(3) The letter "C" represents a compensating wire (type).
Note: SC type compensating wires can be equipped with R-type index thermocouples.

What should be noted when using thermocouple compensation wires?

Answer: (1) The compensation wire must be matched with the corresponding model of thermocouple, and the thermoelectric characteristics must be the same within the range of 0-100 ℃.
(2) When connecting compensation wires to thermocouples and instruments, the positive and negative poles should not be connected incorrectly, and the two pairs of connection points should be at the same temperature.
(3) The temperature at the connection point of the compensating wire and thermocouple shall not exceed the specified temperature range for use.
(4) The diameter of the compensating wire should be selected according to the different requirements of the equipped instrument.

What are the commonly used thermocouple cold junction temperature compensation methods?
Answer: There are five methods: cold end constant temperature method, calculation correction method, instrument mechanical low point adjustment method, bridge compensation method, and compensation wire method.