Pt100 & Thermocouple Converter
Convert Pt100/Pt1000 resistance and type K, J, T, E, N, R, S, B thermocouple mV to temperature and back. IEC 60751 and NIST ITS-90, with cold junction.
Resistance thermometer (RTD)
α = 0.00385, -200 to 850 °C
Thermocouple
Temperature → mV works over the full «Range»; mV → temperature only within the NIST inverse-function range.
Everything is computed in your browser; nothing you type is sent anywhere. Values out of range are reported, not computed.
Temperature table
| °C |
|---|
Thermocouple tables use a 0 °C cold junction, as in the NIST tables.
Equations and sources
RTD: the Callendar–Van Dusen equation of IEC 60751. R(t) = R0·[1 + A·t + B·t² + C·(t − 100)·t³], A = 3.9083×10⁻³, B = −5.775×10⁻⁷, C = −4.183×10⁻¹² (C = 0 at or above 0 °C). Resistance → temperature uses the quadratic root at or above 0 °C and iteration below.
Thermocouples: the reference functions E = Σ cᵢ·tⁱ (type K adds a₀·exp(a₁·(t − a₂)²) above 0 °C) and inverse coefficients published in the NIST ITS-90 Thermocouple Database (NIST SRD 60, NIST Monograph 175), copied from the per-type files at its90.nist.gov. mV → temperature starts from the inverse function and is refined against the reference function. Temperature → mV here matches every entry of the NIST tables (0.001 mV).
| Type | Temperature → mV | mV → temperature |
|---|---|---|
| K | -270 ~ 1372 °C | -200 ~ 1372 °C |
| J | -210 ~ 1200 °C | -210 ~ 1200 °C |
| T | -270 ~ 400 °C | -200 ~ 400 °C |
| E | -270 ~ 1000 °C | -200 ~ 1000 °C |
| N | -270 ~ 1300 °C | -200 ~ 1300 °C |
| R | -50 ~ 1768.1 °C | -50 ~ 1768.1 °C |
| S | -50 ~ 1768.1 °C | -50 ~ 1768.1 °C |
| B | 0 ~ 1820 °C | 250 ~ 1820 °C |
FAQ
How does cold junction compensation work?
A thermocouple's voltage depends on the difference between the measuring junction and the cold junction (terminals). Add the cold junction's EMF to the measured mV and convert: E(t) = E measured + E(cold junction). The other way, the expected reading is E(t) − E(cold junction).
What is the difference between Pt100 and Pt1000?
Only the resistance at 0 °C (R0): 100 Ω or 1000 Ω. Both use the same equation (α = 0.00385), so at any temperature a Pt1000 reads ten times a Pt100.
Why is the mV → temperature range narrower?
NIST publishes the inverse of the temperature → mV reference function as piecewise approximations. This tool converts mV → temperature only where those approximations apply, e.g. type K −200 to 1372 °C, type B 250 to 1820 °C.
To read temperature controllers or transmitters over Modbus, use PortDock Modbus Master.