ADI thermocouple measurement solution makes measurement more accurate and flexible

The principle of thermoelectric effect: as shown in Figure 1, two different colors are used to represent two different metal materials. The A and B terminals are used for temperature measurement ports in a normal temperature environment, which are called cold ends. Point C is the measured terminal. Due to the thermoelectric effect, the temperature between the A terminal and the C terminal, and the B terminal and the C terminal is different, so a potential difference will occur. And because of the difference between the two metal materials, the two potential differences are different. Finally, there is a potential difference between A and B. After measuring the potential difference between AB, the reference metal characteristic value and the cold junction temperature are checked and calibrated. Finally, the temperature value of the corresponding C terminal can be obtained by measuring the potential difference output by the AB terminal.

Thermocouple, thermoelectric effect and thermoelectric effect principle

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

In 1821, German scientist Thomas John Seebeck discovered the inverse effect of the thermal effect of electric current: that is, when different temperatures are applied to the two ends of a piece of wire, an electromotive force will be generated at both ends of the wire, and there will be an electric current in the wire after the loop is closed. flow past. This phenomenon is called the “thermoelectric effect”, also called the “Seebeck effect”.

The principle of thermoelectric effect: as shown in Figure 1, two different colors are used to represent two different metal materials. The A and B terminals are used for temperature measurement ports in a normal temperature environment, which are called cold ends. Point C is the measured terminal. Due to the thermoelectric effect, the temperature between the A terminal and the C terminal, and the B terminal and the C terminal is different, so a potential difference will occur. And because of the difference between the two metal materials, the two potential differences are different. Finally, there is a potential difference between A and B. After measuring the potential difference between AB, the reference metal characteristic value and the cold junction temperature are checked and calibrated. Finally, the temperature value of the corresponding C terminal can be obtained by measuring the potential difference output by the AB terminal.

figure 1

Thermocouple type

China standardized thermocouples have been produced in accordance with IEC international standards ON January 1, 1988, and eight standardized thermocouples S, R, B, K, J, T, N, and E are designated as China’s unified design thermocouples (as shown in the figure) 2).

figure 2

The metals used in S, R, and B thermocouples are relatively expensive, so the price is relatively high; the metals used in K, T, J, N, and E thermocouples are relatively cheap, so the relative price is relatively cheap. The following describes the temperature measurement range and advantages and disadvantages of these types of thermocouples:

S, R, B type thermocouple

The long-term maximum operating temperature of S, R-type and B-type thermocouples are respectively 1300℃ and 1600℃, and the short-term maximum operating temperature is 1600℃ and 1800℃ respectively. Advantages: It has the highest accuracy, best stability, wide temperature measurement zone, long service life, etc. It has good physical and chemical properties, thermoelectric potential stability and good oxidation resistance at high temperatures, and is suitable for oxidizing and inert atmospheres.

The S-type thermocouple has excellent comprehensive performance. The S-type thermocouple conforms to the international temperature standard and has been used as an interpolating instrument for the international temperature standard for a long time. Although “ITS-90” is stipulated that it will no longer be used as an interpolating instrument for the international temperature standard in the future, the International Temperature Advisory Committee (CCT) believes that the S-type thermocouple can still be used to approximate the international temperature standard.

The overall performance of the R-type thermocouple is equivalent to that of the S-type thermocouple. Type B thermocouple is similar to S and R, but it is not suitable for reducing atmosphere or atmosphere containing metal or non-metal vapor. But its obvious advantage is that it does not need to use compensation wire for compensation, because the thermoelectric potential is less than 3μV in the range of 0~50℃.

Disadvantages of T, R, and B thermocouples: This type of thermocouple has a low thermoelectric potential rate, low sensitivity, reduced mechanical strength at high temperatures, and is very sensitive to pollution, and precious metal materials are expensive.

K, N, E, J, T type thermocouple

The temperature measurement range and advantages and disadvantages are shown in Table 1:

Supplement: The N-type thermocouple overcomes two important shortcomings of the K-type thermocouple: the K-type thermocouple is between 300 and 500 ℃, and the thermoelectromotive force is unstable due to the short-range order of the nickel-chromium alloy crystal lattice; at 800 ℃ About, the thermoelectromotive force is unstable due to the preferential oxidation of nickel-chromium alloy.
Table 1

a. Wide temperature range: from low temperature to jet engine exhaust, thermocouples are suitable for most practical temperature ranges.Thermocouple measurement temperature range is

Thermocouple, thermoelectric effect and thermoelectric effect principle

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

In 1821, German scientist Thomas John Seebeck discovered the inverse effect of the thermal effect of electric current: that is, when different temperatures are applied to the two ends of a piece of wire, an electromotive force will be generated at both ends of the wire, and there will be an electric current in the wire after the loop is closed. flow past. This phenomenon is called the “thermoelectric effect”, also called the “Seebeck effect”.

The principle of thermoelectric effect: as shown in Figure 1, two different colors are used to represent two different metal materials. The A and B terminals are used for temperature measurement ports in a normal temperature environment, which are called cold ends. Point C is the measured terminal. Due to the thermoelectric effect, the temperature between the A terminal and the C terminal, and the B terminal and the C terminal is different, so a potential difference will occur. And because of the difference between the two metal materials, the two potential differences are different. Finally, there is a potential difference between A and B. After measuring the potential difference between AB, the reference metal characteristic value and the cold junction temperature are checked and calibrated. Finally, the temperature value of the corresponding C terminal can be obtained by measuring the potential difference output by the AB terminal.

figure 1

Thermocouple type

Chinese standardized thermocouples have been produced in accordance with IEC international standards on January 1, 1988, and eight standardized thermocouples S, R, B, K, J, T, N, and E are designated as China’s unified design thermocouples (as shown in the figure). 2).

figure 2

The metals used in S, R, and B thermocouples are relatively expensive, so the price is relatively high; the metals used in K, T, J, N, and E thermocouples are relatively cheap, so the relative price is relatively cheap. The following describes the temperature measurement range and advantages and disadvantages of these types of thermocouples:

S, R, B type thermocouple

The long-term maximum operating temperature of S, R-type and B-type thermocouples are respectively 1300℃ and 1600℃, and the short-term maximum operating temperature is 1600℃ and 1800℃ respectively. Advantages: It has the highest accuracy, best stability, wide temperature measurement zone, long service life, etc. It has good physical and chemical properties, thermoelectric potential stability and good oxidation resistance at high temperatures, and is suitable for oxidizing and inert atmospheres.

The S-type thermocouple has excellent comprehensive performance. The S-type thermocouple conforms to the international temperature standard. It has been used as an interpolating instrument for the international temperature standard for a long time. Although “ITS-90” is stipulated that it will no longer be used as an interpolating instrument for the international temperature standard in the future, the International Temperature Advisory Committee (CCT) believes that the S-type thermocouple can still be used to approximate the international temperature standard.

The overall performance of the R-type thermocouple is equivalent to that of the S-type thermocouple. Type B thermocouple is similar to S and R, but it is not suitable for reducing atmosphere or atmosphere containing metal or non-metal vapor. But its obvious advantage is that it does not need to use compensation wire for compensation, because the thermoelectric potential is less than 3μV in the range of 0~50℃.

Disadvantages of T, R, and B thermocouples: This type of thermocouple has a small thermoelectric potential rate, low sensitivity, reduced mechanical strength at high temperatures, and is very sensitive to pollution, and precious metal materials are expensive.

K, N, E, J, T type thermocouple

The temperature measurement range and advantages and disadvantages are shown in Table 1:

Supplement: The N-type thermocouple overcomes two important shortcomings of the K-type thermocouple: the K-type thermocouple is between 300 and 500 ℃, and the thermoelectromotive force is unstable due to the short-range order of the nickel-chromium alloy crystal lattice; at 800 ℃ About, the thermoelectromotive force is unstable due to the preferential oxidation of nickel-chromium alloy.
Table 1