Pt100, Pt1000 and thermocouples
Check a sensor with a meter: enter the measured resistance or voltage and see the temperature. It also works the other way, for example when simulating a sensor with a calibrator.
Result
- Class B tolerance
- ± 0.425 °C
- Sensitivity
- 0.3879 Ω/°C
- R / R₀
- 1.0973
R = R₀ · (1 + A·T + B·T²)
A = 3.9083·10⁻³ B = −5.775·10⁻⁷ C = −4.183·10⁻¹²
| T [°C] | R [Ω] | class A ±°C | class B ±°C |
|---|---|---|---|
| −200 | 18.5201 | 0.55 | 1.3 |
| −100 | 60.2558 | 0.35 | 0.8 |
| −50 | 80.3063 | 0.25 | 0.55 |
| 0 | 100 | 0.15 | 0.3 |
| 25 | 109.735 | 0.2 | 0.425 |
| 50 | 119.397 | 0.25 | 0.55 |
| 100 | 138.506 | 0.35 | 0.8 |
| 150 | 157.325 | 0.45 | 1.05 |
| 200 | 175.856 | 0.55 | 1.3 |
| 300 | 212.052 | 0.75 | 1.8 |
| 400 | 247.092 | 0.95 | 2.3 |
| 500 | 280.978 | 1.15 | 2.8 |
| 600 | 313.708 | 1.35 | 3.3 |
| 850 | 390.481 | 1.85 | 4.55 |
Pt100 and Pt1000 resistance thermometers
A Pt100 is 100 Ω at 0 °C and a Pt1000 is 1000 Ω. Resistance versus temperature follows the Callendar–Van Dusen equation from IEC 60751. A Pt100 changes by about 0.385 Ω per degree, so 1 Ω of lead resistance in a 2-wire circuit gives an error of roughly 2.6 °C. That is why industrial installations use 3- and 4-wire connections.
Thermocouples
A thermocouple produces a voltage that depends on the temperature difference between the measuring junction and the cold junction. ITS-90 tables and formulas assume the cold junction is at 0 °C. When the terminals are at another temperature, add E(T₀), the thermocouple’s voltage at the terminal temperature, to the measured voltage. That is what cold-junction compensation in a transmitter does.
Thermocouple voltages are calculated from the NIST ITS-90 reference functions (NIST SRD 60).
Common questions
What is the resistance of a Pt100 at 20 °C?
107.79 Ω. At 100 °C it is 138.51 Ω.
How do I check a thermocouple with a multimeter?
Measure the mV at the terminals, measure the terminal temperature and enter both values in “mV → °C” mode.
Why does type B start at 50 °C?
Below about 50 °C a type B thermocouple’s voltage is almost zero and ambiguous, so temperature can’t be read from it.