Reference temperature for thermocouples

Thermocouple cold junction temperature, also known as cold junction reference temperature, cold junction temperature, or reference temperature. As a thermocouple itself, it is a component that responds to temperature differences, and its millivolt value is only related to the temperature difference between the cold and hot ends. If one end is 100 ℃ and the other end is 20 ℃, then the millivolt value generated by the thermocouple itself only corresponds to 80 ℃. When used for temperature measurement, such as measuring an object at 100 ℃ in an environment of 20 ℃, and obtaining a millivolt value corresponding to 80 ℃, the temperature of the measured object can be obtained by adding 20 ℃ of the environment. This 20 ℃ (ambient temperature) is the reference temperature for the cold end. The vast majority of temperature measurement surfaces can automatically detect the cold end temperature, and automatically add it, which is called automatic cold end compensation. But he becomes redundant when checking the watch, so automatic cold end compensation should be closed or manually corrected when checking the watch.
The thermoelectric potential of a thermocouple is related to the material of the thermoelectric electrode and the temperature of the two contacts. The division table of thermocouples and the temperature gauge calibrated according to the division table are both based on the condition that the temperature at the reference end of the thermocouple is equal to 0 ℃, so we must comply with this condition when using them. If the reference temperature tn is not equal to 0 ℃, although the measured temperature t remains constant, the thermoelectric potential E (t, tn) will also change with the change of the reference temperature tn.
For example, we insert a nickel chromium nickel silicon thermocouple into a 600 ℃ tubular electric furnace, when the reference end temperature of the thermocouple is 0 ℃; Its output thermoelectric potential is 24.91 millivolts; If the reference temperature is 30 ℃, the thermoelectric potential output by the thermocouple drops to 23.74 millivolts, which is the measurement error introduced when the reference temperature is not equal to 0 ℃. If the reference temperature changes, the introduced measurement error will be a variable. It can be seen that when the reference end temperature is not equal to 0 ℃, it has a very important impact on the accuracy of the measured temperature.
When using thermocouples for temperature measurement, it is difficult to maintain the reference end temperature at 0 ℃, and it is generally only necessary for precise measurements in the laboratory. In general engineering measurements, the reference temperature is mostly at room temperature or fluctuating temperature range. At this point, in order to measure the actual temperature, measures such as correction or compensation must be taken.