What the calculator is for
A thermocouple produces a voltage that depends on the temperature difference between its measuring (hot) junction and its reference (cold) junction, where the thermocouple wires meet copper. Tables give that voltage with the reference junction at 0 °C. In the field the reference junction is at the terminal temperature, so a reading has to be corrected before it means anything. Use the calculator to check a transmitter with a millivolt source, to read a thermocouple with a multimeter, or to look up a value.
- Temperature → mV: measured EMF = E(t) − E(tref)
- mV → temperature: E(t) = measured EMF + E(tref), then solve for t
- E(t) is the NIST ITS-90 reference function for the type, the definition behind IEC 60584-1.
Worked example
A type K thermocouple at 500 °C produces 20.644 mV against a 0 °C reference. Now read a type K in the field with a multimeter: it shows 8.000 mV and the terminals are at 25 °C.
Looking 8.000 mV up in the table directly gives 196.5 °C, which is wrong. The terminals at 25 °C hide E(25 °C) = 1.000 mV, so the thermocouple’s full EMF is 8.000 + 1.000 = 9.000 mV, and the hot junction is at 221.5 °C. Simply adding 25 °C to the table answer only works where the curve is nearly straight.
Thermocouple reference table
| °C | K | J | T | E | N | R | S | B |
|---|---|---|---|---|---|---|---|---|
| −250 | −6.404 | — | −6.180 | −9.718 | −4.313 | — | — | — |
| −200 | −5.891 | −7.890 | −5.603 | −8.825 | −3.990 | — | — | — |
| −150 | −4.913 | −6.500 | −4.648 | −7.279 | −3.336 | — | — | — |
| −100 | −3.554 | −4.633 | −3.379 | −5.237 | −2.407 | — | — | — |
| −50 | −1.889 | −2.431 | −1.819 | −2.787 | −1.269 | −0.226 | −0.236 | — |
| 0 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 |
| 50 | 2.023 | 2.585 | 2.036 | 3.048 | 1.340 | 0.296 | 0.299 | 0.002 |
| 100 | 4.096 | 5.269 | 4.279 | 6.319 | 2.774 | 0.647 | 0.646 | 0.033 |
| 150 | 6.138 | 8.010 | 6.704 | 9.789 | 4.302 | 1.041 | 1.029 | 0.092 |
| 200 | 8.138 | 10.779 | 9.288 | 13.421 | 5.913 | 1.469 | 1.441 | 0.178 |
| 250 | 10.153 | 13.555 | 12.013 | 17.181 | 7.597 | 1.923 | 1.874 | 0.291 |
| 300 | 12.209 | 16.327 | 14.862 | 21.036 | 9.341 | 2.401 | 2.323 | 0.431 |
| 350 | 14.293 | 19.090 | 17.819 | 24.964 | 11.136 | 2.896 | 2.786 | 0.596 |
| 400 | 16.397 | 21.848 | 20.872 | 28.946 | 12.974 | 3.408 | 3.259 | 0.787 |
| 450 | 18.516 | 24.610 | — | 32.965 | 14.846 | 3.933 | 3.742 | 1.002 |
| 500 | 20.644 | 27.393 | — | 37.005 | 16.748 | 4.471 | 4.233 | 1.242 |
| 550 | 22.776 | 30.216 | — | 41.053 | 18.672 | 5.021 | 4.732 | 1.505 |
| 600 | 24.905 | 33.102 | — | 45.093 | 20.613 | 5.583 | 5.239 | 1.792 |
| 650 | 27.025 | 36.071 | — | 49.116 | 22.566 | 6.157 | 5.753 | 2.101 |
| 700 | 29.129 | 39.132 | — | 53.112 | 24.527 | 6.743 | 6.275 | 2.431 |
| 750 | 31.213 | 42.281 | — | 57.080 | 26.491 | 7.340 | 6.806 | 2.782 |
| 800 | 33.275 | 45.494 | — | 61.017 | 28.455 | 7.950 | 7.345 | 3.154 |
| 850 | 35.313 | 48.715 | — | 64.922 | 30.416 | 8.571 | 7.893 | 3.546 |
| 900 | 37.326 | 51.877 | — | 68.787 | 32.371 | 9.205 | 8.449 | 3.957 |
| 950 | 39.314 | 54.956 | — | 72.603 | 34.319 | 9.850 | 9.014 | 4.387 |
| 1000 | 41.276 | 57.953 | — | 76.373 | 36.256 | 10.506 | 9.587 | 4.834 |
| 1050 | 43.211 | 60.890 | — | — | 38.179 | 11.173 | 10.168 | 5.299 |
| 1100 | 45.119 | 63.792 | — | — | 40.087 | 11.850 | 10.757 | 5.780 |
| 1150 | 46.995 | 66.679 | — | — | 41.976 | 12.535 | 11.351 | 6.276 |
| 1200 | 48.838 | 69.553 | — | — | 43.846 | 13.228 | 11.951 | 6.786 |
| 1250 | 50.644 | — | — | — | 45.694 | 13.926 | 12.554 | 7.311 |
| 1300 | 52.410 | — | — | — | 47.513 | 14.629 | 13.159 | 7.848 |
| 1350 | 54.138 | — | — | — | — | 15.334 | 13.766 | 8.397 |
| 1400 | — | — | — | — | — | 16.040 | 14.373 | 8.956 |
| 1450 | — | — | — | — | — | 16.746 | 14.978 | 9.524 |
| 1500 | — | — | — | — | — | 17.451 | 15.582 | 10.099 |
| 1550 | — | — | — | — | — | 18.152 | 16.182 | 10.679 |
| 1600 | — | — | — | — | — | 18.849 | 16.777 | 11.263 |
| 1650 | — | — | — | — | — | 19.540 | 17.366 | 11.848 |
| 1700 | — | — | — | — | — | 20.222 | 17.947 | 12.433 |
| 1750 | — | — | — | — | — | 20.877 | 18.503 | 13.014 |
| 1800 | — | — | — | — | — | — | — | 13.591 |
Common mistakes
- Ignoring the cold junction. The commonest error: a meter reading is the difference between two junctions, not the hot junction alone.
- Copper wire in the run. Thermocouple or matching extension cable must run all the way to the reference junction; copper in between moves the reference point.
- Reversed polarity. In IEC 60584-3 colours the negative leg is white. A reversed pair reads low and runs the wrong way as temperature rises.
- Type B at low temperature. Its EMF is almost zero up to about 50 °C, so type B needs no cold-junction correction in practice but cannot measure low temperatures.
Basis
Reference functions and coefficients are those of NIST Monograph 175 (ITS-90), published in the NIST Temperature Scale Database SRD 60, and adopted in IEC 60584-1. Temperature from EMF is found by solving the reference function itself, not the approximate inverse polynomials, so both directions agree exactly. Every 10 °C value of the NIST tables is checked in SILProof’s tests.