Thermocouples
Thermocouples are the most commonly used temperature sensors.
A thermocouple is created when two dissimilar metals touch and the contact point produces a small open-circuit voltage that corresponds to temperature. This thermoelectric voltage is known as Seebeck voltage and is nonlinear with respect to temperature. Thermocouples require signal conditioning.
Thermocouple types differ in composition and accurate range:
| Thermocouple Type | Positive Conductor | Negative Conductor | Temperature Range (°C) for Polynomial Coefficients or for Table Conversion | Temperature Range (°C) for Inverse Polynomial Coefficients |
|---|---|---|---|---|
| J | Iron | Constantan | –210 to 1200 | –210 to 1200 |
| K | Chromel | Alumel | –270 to 1372 | –200 to 1372 |
| N | Nicrosil | Nisil | –270 to 1300 | –200 to 1300 |
| R | Platinum-13% Rhodium | Platinum | –50 to 1768 | –50 to 1768 |
| S | Platinum-10% Rhodium | Platinum | –50 to 1768 | –50 to 1768 |
| T | Copper | Constantan | –270 to 400 | –200 to 400 |
| B | Platinum | Rhodium | 0 to 1820 | 250 to 1820 |
| E | Chromel | Constantan | –270 to 1000 | –200 to 1000 |
Use the temperature ranges for polynomial coefficients when converting temperature to voltage. For most thermocouples, the equation used for converting temperature to voltage is the following:
where E is the voltage in millivolts, t90 is the temperature in degrees Celsius, and ci is the coefficient.
Use the temperature ranges for inverse polynomial coefficients when converting voltage to temperature. For most thermocouples, the equation for converting voltage to temperature is the following:
where t90 is the temperature in degrees Celsius, E is the voltage in millivolts, and Di is the coefficient.
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Note For coefficients to use with each thermocouple type, visit the NIST ITS-90 thermocouple database available at nist.gov. |