The Four Laws of K-type Thermocouples?

K-type thermocouples have a wide range of applications and follow certain laws in the process of performing their functions. Do you know what these laws are? What are the effects of these laws on K-type thermocouples?

1. Law of homogeneous conductor

Thermocouple wires are formed by welding two ends of the same homogeneous material (conductor or semiconductor) to form a closed circuit. Regardless of the conductor cross-section and temperature distribution, no contact potential will be generated, and the temperature difference potential will cancel out, resulting in a low total potential in the circuit.

It can be seen that thermocouples must be composed of two different homogeneous conductors or semiconductors. If the material of the hot electrode is uneven, an additional heating potential will be generated due to the existence of a temperature gradient.

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.

Application: According to the law of intermediate conductor, in the actual temperature measurement application of thermocouples, the form of hot end welding and cold end open circuit is often used. The cold end is connected to the display instrument through a connecting wire to form a temperature measurement system.

Some people are concerned that using copper wires to connect the cold end of the thermocouple to the instrument to read the mV value may cause additional measurement errors due to the contact potential generated at the connection between the wire and the thermocouple.

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.

Application: Due to the non-linear relationship between thermocouples E-T, when the cold end temperature is not 0 degrees Celsius, the actual thermoelectric potential E (t, t0) of the known circuit cannot be directly used to lookup the table to obtain the temperature value of the hot end; It is also not possible to use the actual thermoelectric potential E (t, t0) of the known circuit to directly look up the temperature value from the table, and then add the cold end temperature to determine the measured temperature value of the hot end. It needs to be corrected according to the law of intermediate temperature.

4. Reference electrode law

Using high-purity platinum wire as the standard electrode, assuming that the positive and negative electrodes of the nickel chromium nickel chromium thermocouple are paired with the standard electrode, their values add up to the value of this nickel chromium nickel chromium thermocouple.