The shunt error of armored thermocouples and corresponding countermeasures

Armored thermocouple is one of the commonly used temperature detection components in industry.

Its advantages are:

① High measurement accuracy. Because the thermocouple is in direct contact with the measured object and is not affected by the intermediate medium.
② Wide measurement range. Commonly used thermocouples can measure continuously from -50~+1100 ℃, while some special thermocouples can measure temperatures as low as -269 ℃ (such as gold iron nickel chromium).
③ Simple construction and easy to use. Thermocouples are usually composed of two different types of metal wires, and are not limited by size or opening. They have protective sleeves on the outside, making them very convenient to use.

Basic principle of armored thermocouple temperature measurement:

Weld two different materials of conductors or semiconductors A and B together to form a closed circuit. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between them, resulting in a current of a certain magnitude in the circuit. This phenomenon is called thermoelectric effect. Thermocouples work by utilizing this effect.

Types and structural formation of armored thermocouples:

(1) Types of thermocouples
Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The so-called standard thermocouple refers to a thermocouple that has a national standard that specifies the relationship between its thermoelectric potential and temperature, allows for errors, and has a unified standard scale. It has a matching display instrument available for selection. Non standardized thermocouples are not as widely used or of the same order of magnitude as standardized thermocouples, and generally do not have a unified calibration table. They are mainly used for measurements in certain special occasions. Standard thermocouples are produced according to IEC standards, and the eight standard thermocouples S, R, B, K, N, E, J, and T are unified design thermocouples.
(2) The structural form of a thermocouple, in order to ensure its reliable and stable operation, requires the following structural requirements:
① The welding of the two thermoelectric electrodes that make up the thermocouple must be firm;
② The two thermoelectric electrodes should be well insulated from each other to prevent short circuits;
③ The connection between the compensating wire and the free end of the thermocouple should be convenient and reliable;
④ The protective sleeve should ensure sufficient isolation between the thermoelectric electrode and harmful media.
3. Temperature compensation of thermocouple cold end
Due to the fact that the materials of thermocouples are generally expensive (especially when using precious metals), and the distance between the temperature measuring point and the instrument is far, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the thermocouple to a relatively stable temperature location and connect it to the instrument terminal. It must be pointed out that the function of the thermocouple compensation wire is only to extend the thermoelectric electrode and move the cold end of the thermocouple to the instrument terminal. It cannot eliminate the influence of temperature changes at the cold end on temperature measurement and does not have a compensation effect. Therefore, other correction methods need to be used to compensate for the impact of cold end temperature t0 ≠ 0 ℃ on temperature measurement.
When using thermocouple compensation wires, it is necessary to pay attention to matching the model and ensure that the polarity is not connected incorrectly.