To understand the thermocouple temperature compensation problem, must start from the principle of thermocouple, now only talk about the related thermocouple total thermoelectric potential and middle temperature closed loop rule.
The former shows: for selected thermocouple, the reference side at constant temperature, the overall thermoelectric emfs as a single value function of measuring the temperature.
That certain thermoelectric potential corresponds to a certain temperature, and the thermocouple indexing table, reference junction are all zero.
But at the scene of the application, reference end temperature differ in thousands ways, can't all constant at 0 degrees, it will generate measurement error, this is the reason why thermocouple for temperature compensation.
In the practical application often called the reference end of the thermocouple cold end.
Thermocouple cold junction compensation method is 1, the ice bath is often used in laboratory, the reference end temperature in constant 0 degrees, but high cost, difficult to do.
2, the cold end temperature correction is often used in the request is not high, when the cold end can't constant temperature of 0 degrees, it need to modify indicating value of the instrument.
Easy to do but the error is bigger.
3, compensation bridge method is less used alone, is the use of electric potential generated by unbalanced bridge to compensation thermocouple cold junction temperature change caused by the thermoelectric potential value.
Compensation bridge has a separate product, also have to do within the instrument.
4 this is a common method, compensation wire method, which extended the thermocouple cold end temperature leads to a stable place (
Usually a control room)
And then adjusted by artificial cold end temperature, namely the instrument zero to room temperature, or by the instrument for automatic compensation circuit.
For precious metal thermocouple extended the thermocouple is impossible, because the price is not high, just use the thermoelectric properties of base metals for extended conductor, middle temperature rule is applied the theoretical foundation of the compensating conductor.
Compensation conductor does not automatically compensate the change of the thermocouple cold end temperature, only the thermocouple cold end temperature leads to a stable place, compensation by artificial and instrument.
Therefore compensation conductor should be called thermocouple extension, so as not to cause the wrong understanding to the person.