How to wire thermocouples and what are the connection laws?

Thermocouples are a type of temperature sensor that we often use in our daily lives. However, some users have certain doubts about whether the cold end temperature compensation wire of the thermocouple is connected to the DCS when using it.

There are two laws of conductor connection for thermocouples, which are:

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 distribution of temperature. There will be no contact potential, and the temperature difference potential will cancel out, resulting in a total potential of zero 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 thermoelectric potential will be generated due to the existence of temperature gradients.

The Law of Intermediate Conductors:

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.

According to the law of intermediate conductor, in practical temperature measurement applications of thermocouples, the hot end is often welded and the cold end is open circuited. The cold end is connected to the display instrument through a connecting wire to form a temperature measurement system.

If there is no temperature transmitter in the middle and the DCS card is a thermocouple, then the compensation wire must be connected to the card; If there is a temperature transmitter, then the compensation wire is connected to the temperature transmitter, and the 4-20mA signal is transmitted from the temperature transmitter to the DCS

1. Thermocouples use compensating wires with nodes in between, but corresponding compensating cables must be used to directly connect to the DCS thermocouple module, such as the LLMUX card in HONEYWELL TPS. Try to minimize nodes to reduce interference. In addition, the corresponding thermocouple model and compensation method need to be selected in the DCS configuration.