Principle and Method of Thermocouple Temperature Compensation

Principle and Method of Thermocouple Temperature Compensation
Several common questions about cold end compensation:
1. The compensation wire compensates for the difference between the temperature at the connection of the temperature measuring element and the temperature in the control room;
2. The compensation circuit compensates for the difference between the control room temperature and the theoretical cold junction temperature that needs to be fixed;
3. The consequences of using thermocouples and compensating wires in reverse. For example:
(1) When the temperature at the connection between the thermocouple and the compensation wire is higher than the temperature in the control room, the compensation potential of the compensation wire is positive, which should be the sum of the thermoelectric potential generated by the thermocouple and the compensation potential generated by the compensation wire. Connecting it in reverse is equivalent to adding a negative value, which will cause the indication to be lower;
(2) When the temperature at the connection between the thermocouple and the compensation wire is lower than the temperature in the control room, the compensation potential of the compensation wire is negative. It should be the difference between the thermoelectric potential generated by the thermocouple and the compensation potential generated by the compensation wire. Connecting it in reverse is equivalent to subtracting a negative value, which will cause the indication to be higher;
(3) When the temperature at the connection between the thermocouple and the compensation wire is equal to the temperature in the control room, the compensation potential of the compensation wire is zero, which does not affect the measurement (i.e. displays room temperature).
Thermocouple temperature transmitters or cards all have temperature compensation circuits. Its function is based on the law of intermediate temperature of thermocouples - "The thermoelectric potential between the two contacts of the thermocouple circuit (at temperatures T and T0) is equal to the algebraic sum of the thermoelectric potential of the thermocouple at temperatures T and Tn and the thermoelectric potential at temperatures Tn and T0. Tn is called the intermediate temperature". Generate a thermoelectric potential at temperatures Tn and T0 at the cold junction of the thermocouple. To compensate for the uncertainty of the thermocouple cold junction temperature. It is essentially capable of generating a DC millivolt signal that varies with the temperature of the reference environment. When it is connected in series in the thermocouple measurement circuit for temperature measurement, the reference terminal temperature can be automatically compensated.
The fundamental reason for using a thermocouple cold junction compensation circuit is the uncertainty of the cold junction temperature. Although the calibration standard for thermocouples is based on zero degrees; Although the principle of temperature compensation is usually explained with the cold end at zero degrees as the reference. But in reality, if the cold end temperature can be fixed, only the difference between T (T, To) and T (T, Tn) needs to be calculated, and an opposite error can be artificially created during the indication process to offset the error caused by non-zero cold end temperature. You can obtain the correct measurement value.
So in temperature compensation, the cold end compensation circuit does not necessarily have to use zero degrees as the reference. At the same time, the cold end compensation circuit of thermocouples often has an adaptive range, beyond which the compensation requirements cannot be met. Starting from various factors such as adapting to the environment, reducing costs, simplifying circuits, and reducing volume, selecting a suitable, non-zero temperature point as the compensation reference for the cold end compensation circuit has become a common practice. The compensation reference of this cold end compensation circuit is called the compensation temperature of the compensation circuit. Every instrument or card with cold end compensation function has this indicator. If not marked, it defaults to 0 ℃, commonly seen in temperature indicators and temperature transmitters with cold end compensation.
What is cold end compensation
The thermoelectric potential generated by a thermocouple depends on the temperature at both ends. Only when the temperature at the cold end remains constant, is the output thermoelectric potential a single value function of the temperature at the measuring end (hot end). Moreover, the thermocouples and temperature display instruments widely used in engineering technology are calibrated based on the cold junction temperature of 0 ° C. Therefore, compensation for its cold end temperature is necessary to ensure the accuracy of thermocouple measurement. There are many compensation methods for the cold junction temperature of thermocouples. In industrial instruments and production sites, conventional compensation methods include cold junction temperature compensation method and compensation bridge method. Advanced compensation methods, such as intelligent compensation.