Why thermocouple can measure high temperature?

Thermocouple temperature sensor has the characteristics of large range, low cost, fast response speed and good durability, and is widely used in temperature measurement in industrial sites. R-type thermocouples can measure high temperatures of more than 1700 degrees (°C), and are widely used in high temperature measurement occasions. In today's article, let's talk about how thermocouples work.

What is a thermocouple sensor?

A thermocouple is a sensor that connects two ends of metals of different materials and uses the thermoelectric effect to measure temperature.

Thermoelectric effect is the physical basis of thermocouple, what is thermoelectric effect?

We know that when a voltage is applied across a length of wire, current flows through the wire and heats up. This phenomenon is called the thermal effect of current.

In 1821, German scientist Thomas Johann Seebeck discovered the inverse effect of the thermal effect of current: that is, when different temperatures are applied to both ends of a piece of wire, electromotive force will be generated at both ends of the wire, and after the circuit is closed, the metal A current will flow through the wire. This phenomenon is called the thermoelectric effect, also known as the Seebeck effect.

Temperature thermocouple basic principle

A thermocouple is a temperature sensing element and a primary instrument. It directly measures the temperature, converts the temperature signal into a thermoelectromotive force signal, and then converts it into the temperature of the measured medium through an electrical instrument (secondary instrument).

   The basic principle of thermocouple temperature measurement is that two different material conductors form a closed loop. When there is a temperature gradient at both ends, a current will flow through the loop. At this time, there is an electromotive force between the two ends - thermoelectromotive force. This is the so-called Seebeck effect.

   Homogeneous conductors with two different compositions are hot electrodes, the higher temperature end is the working end, the lower temperature end is the free end, and the free end is usually at a constant temperature. According to the functional relationship between thermoelectromotive force and temperature, a thermocouple graduation table is made; the graduation table is obtained under the condition that the temperature of the free end is at 0°C, and different thermocouples have different graduation tables.

   When the third metal material is connected to the thermocouple loop, as long as the temperature of the two junctions of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it will not be affected by the third metal being connected to the loop. Therefore, when the thermocouple is measuring temperature, the measuring instrument can be connected, and the temperature of the measured medium can be known after measuring the thermoelectromotive force. A thermocouple welds two conductors or semiconductors A and B of different materials together to form a closed loop.

   When there is a temperature difference between the two sticking points 1 and 2 of conductors A and B, an electromotive force is generated between the two, thus forming a large current in the loop, which is called the thermoelectric effect. Thermocouples use this effect to work.

   Two conductors of different compositions (called thermocouple wires or thermodes) are connected at both ends into a loop. When the temperature of the junction is different, an electromotive force will be generated in the loop. This phenomenon is called the thermoelectric effect. The electromotive force is called thermoelectric potential. Thermocouples use this principle to measure temperature. One end that is directly used to measure the temperature of the medium is called the working end (also known as the measuring end), and the other end is called the cold end (also known as the compensation end); the cold end and the display The meter or supporting meter is connected, and the display meter will indicate the thermoelectric potential generated by the thermocouple.

High temperature thermocouple notes

   A thermocouple is actually an energy converter, which converts thermal energy into electrical energy, and uses the generated thermoelectric potential to measure the temperature. For the thermoelectric potential of the thermocouple, the following issues should be paid attention to:

   1: The thermoelectric potential of a thermocouple is the difference of the temperature function across the thermocouple, not the function of the temperature difference across the thermocouple;

   2: The size of the thermoelectric potential generated by the thermocouple, when the material of the thermocouple is uniform, has nothing to do with the length and diameter of the thermocouple, but only the composition of the thermocouple material and the temperature difference between the two ends;

3: When the material composition of the two thermocouple wires of the thermocouple is determined, the size of the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; if the temperature of the cold end of the thermocouple remains constant, the thermoelectric potential of the thermocouple is only A single-valued function of the working side temperature.