A Type K thermocouple is a base-metal sensor with a chromel (~90% Ni, 10% Cr) positive leg and an alumel (~95% Ni, with Al, Mn, Si) negative leg. It spans roughly −270 °C to +1372 °C on the ITS-90 scale with a sensitivity near 41 µV/°C, which makes it the default general-purpose thermocouple in industrial service.
What standards govern a Type K thermocouple?
Two standards apply. The EMF–temperature relationship is fixed by the ITS-90 reference function in NIST Monograph 175, and the permissible error is set either by IEC 60584-1 (Class 1, 2, 3) or by ASTM E230 / ANSI MC96.1 (Standard and Special limits of error).
This site evaluates that reference function directly: a piecewise polynomial for the −270 °C to 0 °C and 0 °C to 1372 °C ranges, with the additional exponential term that Type K requires above 0 °C. Conversion back from voltage uses the NIST inverse polynomials, whose residual fit error is roughly −0.05 to +0.06 °C across the working range — small relative to the sensor's own limits of error. The two tolerance regimes are not interchangeable; the tolerance class page sets them side by side. For the full chromel/alumel construction range, see the Type K thermocouple catalog.
Polarity and color code are a frequent source of field error: under ANSI MC96.1 the Type K overall jacket and positive (KP) leg are yellow and the negative (KN) leg is red; under IEC 60584-3 the jacket and positive leg are green and the negative leg is white. A reversed leg produces a plausible-but-wrong reading rather than an open circuit.
How do you convert a Type K millivolt reading to temperature?
Measure the EMF across the leads and the reference-junction temperature, add the reference junction's table EMF to the reading, then invert the ITS-90 curve. The converter above does exactly this. Compensating in temperature instead of voltage introduces error because the response is non-linear.
T_hot = InverseITS90( E_total )
A thermocouple generates an EMF proportional to the temperature difference between its hot and reference junctions — the Seebeck effect — not absolute temperature. For manual lookups the printable millivolt chart carries the same ITS-90 values at 1 °C resolution. For a primer on signal conditioning and CJC hardware, see cold-junction compensation notes.
Why do Type K readings go wrong?
The characteristic Type K degradation modes are preferential oxidation (“green rot”) in low-oxygen atmospheres near 800–1050 °C, the ~354 °C alumel magnetic anomaly, EMF aging and calibration drift, insulation-resistance shunting from moisture, and mechanical or vibration fatigue at the junction.
Each mode has a distinct symptom signature, which makes them diagnosable in the field. The troubleshooting guide works through them symptom-first; installation practice prevents most of them. Recalibration and certificate services are covered under thermocouple calibration.