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Thermocouple operating principle

Technical Characteristics

To understand how a thermocouple works, it is not necessary to know the Seebeck effect in detail. It is sufficient to understand that the device consists of two elements with a zero potential difference, which changes depending on the thermal effect on the element. A variety of thermocouples represent both nickel-based alloys and compounds such as:



-tungsten-rhenium; -tungsten-molybdenum.

Thermocouple operating principle

The principle of operation is based on the thermoelectric effect, when heated there is a potential difference between conductors of different materials. For more accurate readings when heating is insignificant, thermocouples with high thermoelectric force are used. They have positive electrodes: copper or chromel or iron, and negative electrodes: alumel or copel or constantan. Among copel thermocouples, the thermocouple with a chromel electrode (TCK) occupies the first place for thermal emf value, which is used at temperatures up to 800 °C, the second place - with a copper electrode, after descending: - with an iron electrode (up to 760 °C). Compensation wires carry out potential transfer to the recording sensor. For such wires, cheaper wires are used: from constantan, chromel of K or KM grade, copel, etc. Copel and constantan are used in electric measuring instruments for production of resistance standards.

Types of thermocouples and measuring ranges

t° 200 - 1000 °C - alumel - chromel (TXA);

t° 200 to 800 °C - copel - chromel (TXA);

t° -40 to 900 °C - constantan - chromel (TXA);

t° up to 300 °C - copper - constantan;

t° from 0 to 760 °C - iron-copper;

t° in an inert medium up to 3000 °C - tungsten-rhenium.

measures ultra-low cryogenic temperatures - iron-constantan thermocouple;

Percent alumel composition

Fe Co Mn Impurities C Mn Al Ni+Co Si
≤ 0.3 0.6 - 1.2 1.8 - 2.7 ≤ 0.7 ≤ 0.1 1.8 - 2.7 1.6 - 2.4 91.5 - 95.15 0.85 - 1.5

Percentage chromel composition

Ni+Co Fe C Co Si Mn Si Impurities Al
87.4 - 90.4 ≤ 0.3 ≤ 0.2 9 - 10 ≤ 0.4 ≤ 0.3 ≤ 0.4 ≤ 1.4 0.6 - 1.2

Percentage composition of the copel

C Fe Ni+Co Si Impurities Cu Mn
≤ 0.1 ≤ 0.15 42.5 - 44 ≤ 0.1 ≤ 0.6 54.4 - 57.4 0.1 - 1


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