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Application Principle Of Thermocouple
Jan 22, 2018

Application Principle of Thermocouple

Application Principle of Thermocouple

Thermocouples are one of the most commonly used temperature sensing devices in the industry. The advantages are:

① high measurement accuracy. Because the thermocouple directly with the measured object contact, not subject to the impact of intermediate media.

② measuring a wide range. Commonly used thermocouple from -50 ~ +1600 ℃ can be continued measurement, some special thermocouple can be measured to a minimum -269 ℃ (such as gold and iron nickel-chromium), up to +2800 ℃ (such as tungsten - rhenium).

③ simple structure, easy to use. Thermocouple is usually composed of two different metal wire, and not subject to the size and the beginning of the restrictions, the outer protective sleeve, with very convenient.

1. Fundamentals of thermocouple temperature measurement

Two different materials of the conductor or semiconductor A and B welded together to form a closed loop, as shown in Figure 2-1-1

. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated therebetween,

Loop to form a size of the current, this phenomenon known as the thermoelectric effect. Thermocouple is the use of this effect to work

.

2. Thermocouple type and structure formation

(1) the type of thermocouple

Common thermocouple can be divided into standard thermocouple and non-standard thermocouple two categories. The standard thermocouple referred to is the state

Standard provides the relationship between its thermoelectric power and temperature, allowing the error, and a uniform standard table thermocouple, it

There are supporting the display of instruments available. Non-standardized thermocouple in the use of scope or magnitude are less than the standard

The thermocouple, generally there is no uniform indexing table, mainly for some special occasions.

Standardized thermocouple China from January 1, 1988 onwards, thermocouple and thermal resistance all according to IEC international standard production, and specify S, B, E, K, R, J, T seven standardized thermocouple for our unified design type Thermocouple.

(2) thermocouple structure In order to ensure reliable and stable operation of the thermocouple, its structural requirements are as follows:

① thermocouple composition of the two hot electrodes welding must be strong;

② two hot electrodes should be well insulated from each other to prevent short circuit;

③ compensation wire and the free end of the thermocouple connection to be convenient and reliable;

④ protective sleeve should be able to ensure adequate separation of the hot electrode and harmful media.

3. Temperature compensation of cold junction of thermocouple

Because thermocouple materials are generally more valuable (especially when using precious metals), and the temperature measurement point to the instrument are far away, in order to save the thermocouple material, reduce costs, usually using the compensation wire to the cold end of the thermocouple (free End) to a relatively stable temperature control room, connected to the instrument terminal. It must be noted that the role of thermocouple wire compensation only play the role of extension of the hot electrode, so that the cold end of the thermocouple to the control room of the instrument terminal, which itself can not eliminate the temperature change of the cold end of the impact of temperature compensation. Therefore, the need to use other correction methods to compensate for the cold side temperature t0 ≠ 0 ℃ on the temperature measurement.

In the use of thermocouple compensation wire must pay attention to the model match, polarity can not take the wrong, compensation wire and thermocouple junction temperature can not exceed 100 ℃.


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