Introduction to the working principle of thermocouples
Thermocouple is a commonly used temperature measuring element in temperature
measuring instruments. It directly measures the temperature, converts the
temperature signal into a thermoelectromotive force signal, and converts it into the
temperature of the measured medium through an electrical instrument (secondary instrument).
The appearance of various thermocouples is often very different due to needs,
but their basic structure is roughly the same. They are usually composed of main parts
such as hot electrodes, insulating sleeve protection tubes and junction boxes,
and are usually combined with display instruments, recording instruments and
electronic adjustments. Used in conjunction with the device.
When there are two different conductors or semiconductors A and B forming a loop,
and their two ends are connected to each other, as long as the temperature at the
two junctions is different, the temperature at one end is T, which is called the working
end or the hot end, and the temperature at the other end is T0 , 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 the electromotive force
are related to the conductor material and the temperature of the two junctions.
This phenomenon is called the "thermoelectric effect", the loop composed of two
conductors is called a "thermocouple", these two conductors are called "thermoelectrodes",
and the electromotive force generated is called "thermoelectromotive force"
The thermoelectromotive force consists of two parts of electromotive force,
one part is the contact electromotive force of two conductors, and the other part is
the thermoelectromotive force of a single conductor. The size of the thermoelectromotive
force in the thermocouple loop is only related to the temperature of the conductor material
and the two junctions that make up the thermocouple, and has nothing to do with the shape
and size of the thermocouple. When the two electrode materials of the thermocouple are
fixed, the thermoelectromotive force is the two junction temperature t and t0.
The function is poor. That is, this relationship has been widely used in actual temperature
measurement. Because the cold junction t0 is constant, the thermoelectromotive force
generated by the thermocouple only changes with the temperature of the hot junction
(measurement end), that is, a certain thermoelectromotive force corresponds to a certain
temperature. We can achieve the purpose of temperature measurement as long as we
use the method of measuring thermoelectromotive force.
The basic principle of thermocouple temperature measurement is that two
different components of material conductors form a closed loop. When there
is a temperature gradient at both ends, there will be a current passing through
the loop. At this time, there is an electromotive force-thermoelectromotive force
between the two ends. It is the so-called Seebeck effect. Two homogeneous
conductors with different components are thermoelectrodes, the end with a
higher temperature is the working end, the end with a lower temperature is t
he free end, and the free end is usually at a certain constant temperature.
According to the functional relationship between thermoelectromotive force and
temperature, a thermocouple indexing table is made; the indexing table is obtained
when the free end temperature is 0℃, and different thermocouples have different indexing tables.
Contact Person: Mr. Ivan Zhu