Thermocouple Calculator
Convert a measured thermocouple voltage into temperature, or find the millivolts you should read at a given temperature. Cold junction compensation is applied automatically using the NIST ITS-90 reference functions.
How Cold Junction Compensation Works
A thermocouple does not measure temperature directly. It produces a small voltage that depends on the difference between the hot (measuring) junction and the cold (reference) junction, which is usually the terminal block of your transmitter, PLC card or meter.
Standard thermocouple tables assume the cold junction is at exactly 0 °C. In a real panel it sits at room temperature, so the instrument must add back the voltage that the cold junction "steals". This is cold junction compensation (CJC).
Thot = table lookup of Etotal
Where Emeasured is the voltage at the terminals, E(Tcj) is the table voltage at the cold junction temperature and Thot is the process temperature.
Worked example
A Type K thermocouple reads 15.000 mV at the transmitter terminals, which are at 25 °C.
- From the Type K table, 25 °C corresponds to 1.000 mV.
- Total EMF = 15.000 + 1.000 = 16.000 mV.
- From the table, 16.000 mV corresponds to 390.60 °C.
Thermocouple Types at a Glance
Reference ranges are those of the NIST ITS-90 functions. Practical operating limits are lower and depend on wire size and sheath. Click a type to open its full table.
| Type | Positive leg (+) | Negative leg (−) | Reference range | Sensitivity at 0 °C | IEC 60584-3 colour | ANSI colour |
|---|---|---|---|---|---|---|
| Type K | Nickel chromium (Chromel) | Nickel aluminium (Alumel) | −270 to 1372.0 °C | 39.5 µV/°C | Green | Yellow |
| Type J | Iron | Copper nickel (Constantan) | −210 to 1200.0 °C | 50.4 µV/°C | Black | Black |
| Type T | Copper | Copper nickel (Constantan) | −270 to 400.0 °C | 38.7 µV/°C | Brown | Blue |
| Type E | Nickel chromium (Chromel) | Copper nickel (Constantan) | −270 to 1000.0 °C | 58.7 µV/°C | Violet | Purple |
| Type N | Nicrosil | Nisil | −270 to 1300.0 °C | 26.0 µV/°C | Pink | Orange |
| Type R | Platinum 13 % rhodium | Platinum | −50 to 1768.1 °C | 5.3 µV/°C | Orange | Green |
| Type S | Platinum 10 % rhodium | Platinum | −50 to 1768.1 °C | 5.4 µV/°C | Orange | Green |
| Type B | Platinum 30 % rhodium | Platinum 6 % rhodium | 0 to 1820.0 °C | 9.1 µV/°C (at 1000 °C) | Grey | Grey |
IEC colour is the cable sheath and positive leg; the IEC negative leg is always white. ANSI negative leg is always red.
Where the Numbers Come From
This calculator uses the NIST ITS-90 thermocouple reference functions (NIST Monograph 175 and SRD 60), the same polynomials used to print the standard tables in IEC 60584-1. For each type the EMF is a polynomial in temperature:
Type K above 0 °C adds: a0 × exp(a1(T − 126.9686)²)
Converting voltage back to temperature is done by solving this equation numerically, so the forward and reverse results always agree. Values match the published NIST tables to within 0.001 mV.
Frequently Asked Questions
What is cold junction compensation?
It corrects for the fact that the thermocouple reference junction is not at 0 °C. The instrument measures the terminal temperature, converts it to the equivalent table voltage, and adds that voltage to the measured signal before converting to temperature.
Why can I not just add the room temperature to the result?
Because thermocouple voltage is not proportional to temperature. The correction must be made in millivolts and only then converted to temperature. Adding temperatures can create errors of several degrees, especially for Types K and J at higher temperatures.
How do I check a thermocouple with a multimeter?
Measure the mV at the terminals and the terminal temperature, enter both here and compare the result with the process indication. A difference of more than the sensor tolerance points to a wiring, extension cable or transmitter problem.
What does a negative mV reading mean?
The hot junction is colder than the cold junction, or the wires are connected with reversed polarity. If the process is hotter than ambient, check the + and − legs and the extension cable colours.
How accurate is this thermocouple calculator?
The calculation reproduces the NIST ITS-90 reference tables to within 0.001 mV. The real measurement accuracy is limited by the sensor tolerance class (for example ±1.5 °C or ±0.4 % for Class 1 Type K) and by the instrument.
Learn more on the blog
Thermocouple and RTD installation precautions
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article