Thermocouple type K and J calculator

Hand-rolled thermocouple code reads about 25 C low because it skips cold-junction compensation. NIST ITS-90 polynomials in both directions for types K and J, with cold-junction compensation built in.

Open the calculator in EM·LINKBelow is a worked example. The working calculator runs free in EM·LINK.

Worked example

Example inputs

K (chromel / alumel)▾
Millivolts to temperature▾
10.000mV
25.0°C

Example result

Hot junction270.7 °C

The cold-junction EMF of 1.000 mV is added before inverting.

StepValue
Measured10.000 mV
Cold-junction EMF at 25.0 °C1.000 mV
Compensated EMF11.000 mV
Hot junction270.7 °C
Without compensation246.2 °C
Error if skipped−24.5 °C

Reference points: type K 4.096 mV and type J 5.269 mV at 100 °C.

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Run it with your own values

The working calculator is Thermocouple K / J in EM·LINK, our free browser tool. The button opens it directly.

Open in EM·LINK

How to read the result

Checked against NIST reference points: type K gives 4.096 mV at 100 C and 41.276 mV at 1000 C; type J gives 5.269 mV at 100 C.

A thermocouple only measures the difference between its two junctions. Add the cold-junction EMF before inverting, or you lose roughly the ambient temperature from every reading.

How this is calculated

The NIST reference function gives the EMF for a junction at temperature T against 0 °C. The tool adds the cold-junction EMF to the measured voltage, then inverts the function.

emf(T) = sum(c_i × T^i) Type K above 0 C adds a0 × exp(a1 × (T - a2)^2) emf_abs = emf_measured + emf(T_cj) T_hot = inverse(emf_abs)
10.000 mVChromel (+)Alumel (−)Copper Hot junction, 270.7 °CCold junction at the terminals, 25.0 °C E measured = E(T hot) − E(T cold), so E(T hot) = 10.000 + 1.000 = 11.000 mV

The full derivation, step by step, is in the EM·LINK documentation.

Calculated per NIST ITS-90. These tools are an engineering aid, not a certification.

Next step

Logging these sensors across the plant? Embedos Edge takes analog and digital I/O.

Common questions

What is the type K millivolt output at 100 C?

4.096 mV per NIST ITS-90.

Why is my thermocouple reading 25 C low?

Cold-junction compensation is missing. Add the EMF of the terminal temperature before converting.

Are types E, N, T, R, S and B supported?

K and J today. Ask us at team@embedos.io if you need another type.