How to understand the working principle of thermocouple temperature sensor

Thermocouple type temperature sensor has the characteristics of large range, low cost, fast response, good durability, etc., and is widely used in temperature measurement in industrial sites. The R-type thermocouple can measure high temperatures of more than 1,700 degrees (℃), and has a wide range of applications in high temperature measurement occasions. Today in this article, we will talk about the working principle of thermocouples.

What is a thermocouple?

A thermocouple is a sensor that connects one end of two different materials of metal and uses the thermoelectric effect to measure temperature. As shown below:

The thermoelectric effect is the physical basis of the thermocouple. What is the thermoelectric effect?

We know that when a voltage is applied to both ends of a piece of metal wire, current will flow through the metal wire and generate heat. This phenomenon is called the thermal effect of current.

In 1821, German scientist Thomas John Seebeck (seebeck) discovered the inverse effect of the current thermal effect: that is, when different temperatures are applied to the two ends of a piece of wire, the two ends of the wire will generate an electromotive force. Electric current will flow through the wire. This phenomenon is called the thermoelectric effect, also known as the Seebeck effect.

The following is a diagram of the thermoelectric effect:

The black line in the figure represents the metal wire, T1, T2 and T3 represent different temperature differences on the metal wire, and T3 is greater than T1.

Suppose the temperature difference between T3 and T1 is ΔT, then the voltage across the wire: V31=S(T)xΔT; where S(T) is called the Seebeck coefficient.

Since the two ends of a metal wire can produce a voltage difference at different temperatures, why use two metals in a thermocouple?

The answer is: If the probe of the voltmeter uses the same metal as the metal wire to be tested, theoretically the voltmeter will not measure any voltage. Because this is equivalent to extending the wire, and there is no temperature difference between the ends of the wire after the extension, so there will be no voltage difference.

Different metals have different Seebeck coefficients. The measured voltage is equal to the integral of the difference between the Seebeck coefficient functions of the two materials. This is the reason why the thermocouple uses two different metals.

According to the different types and contents of the two metals, thermocouples can be divided into different types.

GB/T 16839 divides thermocouples into the following categories:

The connecting end of the two metals in the thermocouple is called the measuring end, also called the hot end; the corresponding end is called the cold end. The cold end is used as the reference end, and the ice water temperature (0°C) was used as a reference in the early days. Calculate the temperature of the hot end by taking the cold junction as a reference through the difference in the measured voltage.

With the development of technology, the cold junction of the thermocouple does not have to be 0°C. Some PLC thermocouple modules on the market, such as the SM1231 Thermocouple of S7-1200, can use cold junction compensation technology to calculate the temperature of the measurement terminal based on the measured voltage value. The schematic diagram is as follows:

Note: The cold junction is used as the reference junction and the temperature should remain basically unchanged.

Well, the working principle of the thermocouple is introduced here first.