What are the three basic laws of thermocouples?

The basic laws of thermocouples are not three, but four:

1. Law of homogeneous conductor

A closed circuit is formed by welding two ends of the same homogeneous material (conductor or semiconductor), regardless of the cross-sectional area of the conductor and the temperature distribution. There will be no contact potential, and the temperature difference potential will cancel out, resulting in a low total potential in the circuit.

2. Law of intermediate conductor

The intermediate conductor (third conductor) is connected to the thermocouple circuit. As long as the temperature at both ends of the intermediate conductor is the same, the introduction of the intermediate conductor has no effect on the total potential of the thermocouple circuit. This is the law of intermediate conductor.

3. Law of intermediate temperature

The thermoelectric potential between the two contacts of a 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 at temperatures Tn and T0. Tn refers to the intermediate temperature.

4. Reference electrode law

This law is only studied and paid attention to by professionals, and is generally not well understood by those involved in production and use. Simply put, it is: using high-purity platinum wire as the standard electrode, assuming that the positive and negative electrodes of a nickel chromium thermocouple are paired with the standard electrode, the sum of their values is equal to the value of this nickel chromium nickel chromium thermocouple.

Additional Information:

When two different conductors or semiconductors A and B form a circuit, and their two ends are connected to each other, as long as the temperature at the two nodes is different, with one end having a temperature of T, it is called the working end or hot end, and the other end having a temperature of T0, it is called the free end (also known as the reference end) or cold end, an electromotive force will be generated in the circuit, and the direction and magnitude of this electromotive force are related to the material of the conductor and the temperature of the two nodes.

Calculation method for thermocouple cold junction compensation:

1. From millivolts to temperature: measure the cold junction temperature, convert it to the corresponding millivolt value, add it to the millivolt value of the thermocouple, and convert it to temperature;

2. From temperature to millivolts: Measure the actual temperature and cold end temperature, convert them into millivolts, subtract them to obtain the millivolt value, and then obtain the temperature.

Temperature is one of the important parameters that need to be measured and controlled in industrial production processes. Thermocouples are widely used in temperature measurement, with many advantages such as simple structure, convenient manufacturing, wide measurement range, high accuracy, low inertia, and easy remote transmission of output signals.

In addition, as thermocouples are active sensors that do not require an external power source for measurement, they are very convenient to use and are often used to measure the temperature of gases or liquids in furnaces and pipelines, as well as the surface temperature of solids.