Converts a thermocouple's raw millivolt reading to a cold-junction-compensated temperature using the official NIST ITS-90 reference polynomials, or converts an RTD's measured resistance to temperature using the IEC 60751 Callendar-Van Dusen equation with lead-wire compensation for 2/3/4-wire installations.
Standards followed
NIST ITS-90 thermocouple reference functions (Monograph 175)IEC 60751 (industrial platinum resistance thermometers)
What it can do
- Thermocouple types K, J, T, E — official NIST inverse polynomial (mV → °C), each fit within its standard sub-ranges.
- Cold-junction compensation using the superposition law: measured mV + cold-junction-equivalent mV, then converted to the true hot-junction temperature.
- RTD (Pt100 / Pt500 / Pt1000, alpha=0.00385): resistance-to-temperature via Callendar-Van Dusen, both above- and below-zero branches.
- RTD lead-wire compensation for 2-wire, 3-wire, and 4-wire (Kelvin) installations.
Example problem
Type K thermocouple reads 8.13mV raw, cold junction (terminal block) at 25°C. Separately, a Pt100 RTD (3-wire) reads 138.51Ω with negligible lead resistance.
- Type K equivalent mV at 25°C ≈ 1.000mV (from the NIST inverse-then-forward relationship).
- Total compensated voltage = 8.13 + 1.00 = 9.13mV.
- NIST inverse polynomial: 9.13mV → hot-junction temperature ≈ 222°C.
- Pt100: R(100°C) = 100×(1 + 3.9083e-3×100 − 5.775e-7×100²) = 138.51Ω exactly — matches the reading, so RTD temperature = 100°C.
Type K compensated temperature ≈ 222°C; Pt100 reads exactly 100°C — both hand-checked against the official NIST/IEC reference values, agreeing to well within their stated tolerances (±0.05°C class for the polynomials).
Want the full step-by-step version?
Rare-metal/high-temperature thermocouple types (N, R, S, B) are not yet implemented in this calculator — types K, J, T, E cover the large majority of general industrial instrumentation. Cold-junction-equivalent mV is obtained by numerically inverting the same trusted NIST inverse polynomial (bisection), rather than a separately-fitted forward polynomial. 3-wire RTD lead compensation assumes matched lead resistances on all three conductors.