How the thermocouple calculator works
A thermocouple produces a voltage that depends on the temperature difference between its measuring (hot) junction and the point where its wires connect to copper: the reference or cold junction. The voltage–temperature relationship for each standard type is defined by the NIST ITS-90 reference functions, published in NIST Monograph 175 and adopted by IEC 60584-1. They are polynomials in temperature, with an extra exponential term for type K above 0 °C.
The calculator evaluates these reference functions directly. For the reverse direction, voltage to temperature, it inverts the same functions numerically. NIST also publishes separate inverse polynomials, but they only approximate the forward functions, to within a few hundredths of a degree. Inverting the forward function avoids that small extra error: converting a temperature to millivolts and back returns the same temperature to better than 0.001 °C for every type across its whole range.
Cold-junction compensation, step by step
All thermocouple tables are referenced to 0 °C. Your meter, however, sees the voltage between the hot junction and its own terminals, which are usually near room temperature. The correct way to compensate works in voltages, not temperatures:
- Find the EMF of the cold junction temperature from the table: E(T_cj).
- Add it to the measured voltage: E(T_hot) = E_meter + E(T_cj).
- Convert that total back to temperature.
Example, type K. The hot junction is at 500 °C and the terminals are at 25 °C. The 0 °C-referenced EMF at 500 °C is 20.644 mV and at 25 °C it is 1.000 mV, so the meter reads 19.644 mV. Enter 19.644 mV in the calculator with a 25 °C cold junction and it returns 500 °C.
If you look up 19.644 mV directly in a type K table, you get 476.5 °C: an error of almost 24 °C. Adding 25 °C to that result gives 501.5 °C, which is close but still wrong, because the thermocouple’s sensitivity is not the same at 25 °C (40.5 µV/°C) as at 500 °C (42.6 µV/°C). For other types the difference is larger: for type J, computing 300 °C this way is off by about 2 °C.
Choosing a thermocouple type
| Type | Range covered by the tables | Typical use |
|---|---|---|
| K | −270 to 1372 °C | General purpose, most common in industry |
| J | −210 to 1200 °C | Older equipment, reducing atmospheres; iron leg oxidises in air at high temperature |
| T | −270 to 400 °C | Low temperatures, food and laboratory work, good accuracy near ambient |
| E | −270 to 1000 °C | Highest output per degree of the base-metal types |
| N | −270 to 1300 °C | More stable than K at high temperature |
| R, S | −50 to 1768 °C | Platinum types for high temperatures and reference work; low output |
The ranges in the table are the ranges of the reference functions. The usable range of a real thermocouple is narrower and depends on wire diameter, sheath and atmosphere; check the probe datasheet.
Tolerance classes
IEC 60584-1 defines tolerance classes for thermocouple wire. For type K, class 1 is ±1.5 °C or ±0.4 % of the temperature, whichever is larger, from −40 to 1000 °C; class 2 is ±2.5 °C or ±0.75 %. At 500 °C that is ±2.0 °C and ±3.75 °C. The calculator shows both classes for the selected type and temperature, or “outside range” where the class is not defined.
The EMF table
The table under the converter lists EMF and sensitivity (Seebeck coefficient) for any range and step, in °C or °F. Full-range reference tables with CSV downloads are available for type K, J, T, E, N, R and S.
Common mistakes
Adding temperatures instead of voltages for cold-junction compensation, as shown above.
Using copper wire to extend a thermocouple. Every junction of dissimilar metals is a thermocouple. Extensions must use matching extension or compensating cable, otherwise the connection point becomes the new cold junction at an unknown temperature.
Reversed polarity. A reversed pair, or a reversed extension cable, subtracts instead of adds; the error is roughly twice the temperature difference across the reversed section.
Mixing up types. Many instruments are set to type K by default. A type J probe read as type K gives a wrong result without any obvious warning.
Frequently asked questions
Why does my meter show a small voltage when the probe is at room temperature?
It shouldn’t if the hot and cold junctions are at the same temperature; the voltage depends on the temperature difference. A few microvolts can come from offsets and thermal gradients in the terminals.
Can I use this calculator for a thermocouple with a 0 °C ice-point reference?
Yes. Set the cold junction to 0 and the meter reading equals the table EMF.
How accurate are the reference functions?
The functions themselves are the standard; they define the table values. The uncertainty of a real measurement comes from the thermocouple’s tolerance class, drift, extension cable and the instrument.
Does the calculator support °F?
Yes. Switch °C/°F at the top of the page; temperature fields, tables and sensitivities convert automatically.