Selection note

Type K vs Type E

Type E gives you nearly twice the signal. Type K gives you twice the range. Almost every other difference follows from those two facts.

	Type K	Type E
Positive leg	NiCr10	NiCr10
Negative leg	NiSi	Constantan
EMF at 100 °C	≈ 4.10 mV	≈ 6.32 mV
EMF at 500 °C	≈ 20.64 mV	≈ 37.01 mV
Approx. sensitivity	≈ 41 µV/K	≈ 68 µV/K
Useful range	−200 to 1200 °C	−200 to 900 °C
Drift in oxidising service	Green-rot in reducing atmospheres	Better stability at low temperature

Why type E has the bigger signal

Both types share the same positive leg. The difference is on the negative side: constantan has a much larger Seebeck coefficient against NiCr than NiSi does. That is the whole mechanism — and it is also why type E's negative leg is the same wire used as the negative leg of types J and T.

Where type K still wins

Above 900 °C, where type E is out of its range
Where the thermocouple must survive thermal cycling in air
Where the installed base and the controllers already assume type K

One practical warning

Type K's negative leg is prone to "green rot" in reducing atmospheres and to short-range ordering effects around 250–550 °C. If your process is a vacuum furnace with a carbon environment, that is worth designing around rather than discovering.

Questions we get asked

Can I use the same constantan for type E, J and T?

Yes — the negative leg is constantan in all three. The positive leg is what changes, which is convenient for a probe maker stocking one negative wire.

Which drifts less at low temperature?

Type E and type T are both better than type K below ambient. Type T is usually preferred for cryogenic work because copper behaves predictably there.

Send the specification — diameter, grade, temper, quantity.

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