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:
-platinum-rhodium-platinum;
-tungsten-rhenium;
-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 |
Supplier
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