thermocouple

Because the actual temperature we need to know is measured relative to zero, and the principle of operation of thermocouples allows for the measurement only of the temperature under operating conditions relative to the ambient temperature; a compensation value corresponding to the difference between the ambient temperature and zero must be added in

The thermoelectric potential values (in mV) in the thermocouple calibration table are based on the cold end of the thermocouple being at 0°C; I understand this is to establish a standard for easier practical use.

From the working principle of thermocouples, it is known that the thermoelectric potential of a thermocouple is the difference between a function of the temperature at the measuring end and the temperature at the cold end (which should more accurately be referred to as the reference end). The thermoelectric potential values in the thermocouple calibration table that we use are all given at a cold junction temperature of 0 degrees. If the cold junction temperature of the thermocouple is not 0 degrees but some other value, then without making any adjustments, even if the thermoelectric potential value is measured, it cannot be used directly from the calibration table; as a result, the exact temperature at the measuring end cannot be determined. To know the exact temperature, temperature compensation at the cold end is necessary. It should be said that thermocouples do not measure temperature based on the cold end being at zero degrees; rather, their calibration tables are established with the reference end at zero degrees.

Thank you so much, I understand. It means that as long as we have a thermometer with a cold end at a certain temperature, we can measure the desired temperature directly without any further processing, since the cold end is already at that temperature? I’m asking, does the secondary meter display thermoelectric potential or temperature value?

Thank you for your explanation; I basically understand now!

Secondary meters display temperature values and generally come equipped with a temperature compensation circuit.

Any temperature difference generates a potential difference, but there is not a linear relationship between the potential difference resulting from a temperature difference and the temperature of the cold end. Therefore, only the potential difference curve obtained at 0 degrees in the cold section is defined as the standard curve for use as a reference value.

How is this temperature compensation implemented on-site? Is it visible clearly, or is it within a circuit in the junction box? Is temperature compensation done during the design and manufacturing of the instrument, or do we need to carry out some corresponding adjustments during on-site installation and wiring?

Since the principle of operation of a thermocouple is based on the potential difference between its two ends, it is necessary to maintain a consistent temperature at the cold end in order to obtain accurate temperature readings. Therefore, a compensation wire made of the same material must be used to connect to the transmitter, so as to keep the temperature constant