Working Principle of Thermocouple Thermometer

When two different conductors contact to form a circuit, an electric potential will be generated in the circuit. The magnitude of this electric potential is directly related to the temperature difference between the two contacts. This phenomenon is called the thermoelectric effect. The temperature sensing element made using the thermoelectric effect is a thermocouple, and the thermometer composed of a thermocouple as a temperature sensing element is a thermocouple thermometer.

In classical electronic theory, thermoelectric potential consists of two parts: temperature difference potential and contact potential.
Thermoelectric potential is caused by the temperature difference between the two ends of a homogeneous conductor. The contact potential is the potential formed when two different conductors A and B come into contact, due to the different free electron densities between the two, resulting in electron diffusion at the contact point. The contact potential is not only a function of temperature t, but its contribution to the thermoelectric potential is also much greater than the thermoelectric potential.
The thermoelectric potential generated by the thermocouple due to temperature change can be measured, and the temperature value causing the thermoelectric potential can be known based on the functional relationship between the thermoelectric potential and the temperature change. Working principle of thermocouple thermometer

What is cold end compensation

The thermoelectric potential generated by the thermocouple depends on the temperature of its two ends. Only when the cold end temperature remains constant, the thermoelectric potential output is a single-valued function of the temperature of the measuring end (hot end). Moreover, the thermocouple scale widely used in engineering technology and the scale of the temperature display instrument engraved according to the scale are made based on the cold end temperature of 0°C. Therefore, its cold end temperature must be compensated to ensure the measurement accuracy of the thermocouple.

There are many ways to compensate the cold end temperature of the thermocouple. In industrial instruments and production sites, conventional compensation methods include cold end temperature compensation method and compensation bridge method. More advanced compensation methods, such as intelligent compensation method.

Thermocouple temperature compensation principle and method

Once it is sorted out, you can easily understand several common problems about cold end compensation:

1. The compensation wire compensates for the difference between the temperature at the connection point of the temperature measuring element and the temperature of the control room;

2. The compensation circuit compensates for the difference between the temperature of the control room and the theoretical cold end temperature that needs to be fixed;

3. The consequences of using the thermocouple and the compensation wire in reverse. For example:

When the temperature at the connection point of the thermocouple and the compensation wire is higher than the temperature of the control room, the compensation potential of the compensation wire is positive, which should be the thermoelectric potential generated by the thermocouple plus the compensation potential generated by the compensation wire. If it is connected in reverse, it is equivalent to adding a negative value, which will make the indication low;

When the temperature at the connection point of the thermocouple and the compensation wire is lower than the temperature of the control room, the compensation potential of the compensation wire is negative, which should be the thermoelectric potential generated by the thermocouple minus the compensation potential generated by the compensation wire. If it is connected in reverse, it is equivalent to subtracting a negative value, which will make the indication high;

When the temperature at the connection point of the thermocouple and the compensation wire is equal to the temperature of the control room, the compensation potential of the compensation wire is zero, which has no effect on the measurement (that is, the room temperature is displayed).

Thermocouple temperature transmitters or cards all have temperature compensation circuits. Their function is to compensate for the uncertainty of the thermocouple cold end temperature based on the thermocouple intermediate temperature law - "The thermoelectric potential between the two junctions of the thermocouple circuit (temperature T, T0) is equal to the algebraic sum of the thermoelectric potential of the thermocouple at temperatures T, Tn and the thermoelectric potential at temperatures Tn, T0. Tn is called the intermediate temperature". A thermoelectric potential at the cold end temperature of the thermocouple is generated at a temperature of Tn, T0. In essence, it can generate a DC millivolt signal that changes with the ambient temperature of the reference end. When it is connected in series in the thermocouple measurement circuit for temperature measurement, the reference end temperature can be automatically compensated.

The fundamental reason for using the thermocouple cold end compensation circuit is the uncertainty of the cold end temperature. Although the graduation standard of the thermocouple is based on zero degrees; although the cold end is usually used as a reference to explain the temperature compensation principle. But in reality, if the cold end temperature can be fixed, it is only necessary to calculate the difference between T(T,To) and T(T,Tn), and artificially create an opposite error during the indication process to offset the error caused by the non-zero cold end temperature. Then the correct measurement value can be obtained.

Therefore, the cold end compensation circuit does not necessarily have to use zero degrees as the reference in temperature compensation. At the same time, the cold end compensation circuit of the thermocouple often has an adaptation range, and it cannot meet the compensation requirements beyond this range. From the perspective of adapting to the environment, reducing costs, simplifying circuits, reducing volume and other factors, it has become a common practice to select a suitable, non-zero temperature point as the compensation reference of the cold end compensation circuit. 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 it is not marked, it defaults to 0℃, which is common in temperature indicators and temperature transmitters with cold end compensation.