How to calculate the cold end compensation of thermocouple temperature measurement

The reason why beginners enter this line of thought is that they do not fully understand the "intermediate temperature law".

The intermediate temperature law means: The thermoelectric potential between the two junctions (temperature T, T0) in the thermocouple circuit is equal to the algebraic sum of the thermoelectric potential of the two nodes of the thermocouple at temperatures T and Tn and the thermoelectric potential at temperatures Tn and T0. Tn is called the intermediate temperature.

Application of the intermediate temperature law: 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. It is even more impossible to use the actual thermoelectric potential E (t, t0) of the known circuit to directly look up the table to obtain the temperature value, and then add the cold end temperature value to determine the measured temperature value of the hot end, but it needs to be corrected according to the intermediate temperature law. How is the cold end compensation of thermocouple temperature measurement calculated?

Thermocouple temperature measurement, cold end compensation calculation method, summarized in one sentence: "First sum, then look up the table to get the temperature".

Principle of cold-end compensation of thermocouple in temperature transmitter

Thermocouple is a commonly used temperature measuring element in temperature measuring instruments. It directly measures temperature and converts temperature signal into thermoelectric potential signal, which is converted into the temperature of the measured medium through electrical instrument (secondary instrument). The appearance of various thermocouples is often very different due to needs, but their basic structure is roughly the same. They are usually composed of main parts such as hot electrode, insulating sleeve protection tube and junction box, and are used in conjunction with display instrument, recording instrument and electronic regulator through compensation wire.

When measuring temperature, the temperature of the cold end (the measuring end is the hot end, and the end connected to the measuring circuit through the lead is called the cold end) is required to remain unchanged, and the size of its thermoelectric potential is in a certain proportional relationship with the measured temperature. If the (ambient) temperature of the cold end changes during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures at the cold end to compensate for the influence caused by the change of cold end temperature is called cold end compensation of thermocouple.

The cold end compensation of thermocouple usually adopts a bridge composed of thermal resistors in series at the cold end. The three arms of the bridge are standard resistors, and the other arm is composed of a (copper) thermal resistor. When the cold end temperature changes (for example, increases), the thermoelectric potential generated by the thermocouple will also change (decrease), and at this time the resistance of the thermal resistor in the series bridge will also change and cause the voltage at both ends of the bridge to change (increase). If the parameters are well selected and the wiring is correct, the voltage generated by the bridge is exactly equal to the amount by which the thermoelectric potential changes with temperature, and the total output voltage (potential) of the entire thermocouple measurement circuit truly reflects the measured temperature value. This is the cold end compensation principle of the thermocouple of the temperature transmitter.
Thermocouple thermometer is a widely used thermometer at present. Thermocouple thermometer is a temperature electrical measuring instrument, which usually consists of three parts: thermocouple, thermocouple cold end temperature compensation device (or element) and display instrument, and the three are connected by wires.