The Workhorse Thermocouple
Type K is by far the most common thermocouple in industrial use. The positive (KP) leg is Chromel — a nickel-chromium alloy nominally 90% Ni / 10% Cr. The negative (KN) leg is Alumel — nickel with about 2% aluminum, 2% manganese, and 1% silicon. The combination gives a usable range from -270 °C to +1372 °C, a nearly-linear sensitivity of ~41 µV/°C across most of the working range, and acceptable performance in oxidizing or inert atmospheres.
Type K is the default choice for general-purpose furnace, kiln, exhaust, oven, and industrial process applications up to about 1100 °C continuous service. Above that, drift becomes significant and Type N (Nicrosil/Nisil) is often preferred for new installations.
Conductor Composition
- KP (+): Chromel — 90% Ni / 10% Cr — yellow per ANSI MC96.1
- KN (–): Alumel — 95% Ni / 2% Al / 2% Mn / 1% Si — red per ANSI MC96.1
- Combined: yellow-jacketed thermocouple-grade wire (positive yellow / negative red)
- Extension grade (KX): same color code, lower-temperature wire used cold-junction to instrument
Sensitivity Snapshot
@ -200 °C → ~16 µV/°C
@ 0 °C → ~39 µV/°C
@ +200 °C → ~40 µV/°C
@ +500 °C → ~43 µV/°C
@ +1000 °C → ~39 µV/°C
@ +1300 °C → ~36 µV/°C
Type K Sub-Range Anomaly
Between approximately 300 °C and 550 °C, Type K shows a small magnetic ordering disturbance that produces a calibration shift of up to ~0.5 °C. The effect is reversible if the sensor is annealed; for metrology-grade work, use Type N or a Type S/R reference instead.
Avoid Sulfur
The nickel conductors corrode rapidly in sulfur-containing atmospheres — fuel gas, refineries, geothermal — and in low-oxygen ("green rot") environments. Use a gas-tight sheath, or specify Type N for these services.
Type K Tolerance per IEC 60584-1
Tolerance is expressed as the greater of a fixed °C value or a percentage of the measured temperature. The Class 1 limit is approximately three times tighter than Class 2 at the upper end of the range.
| Class | Tolerance Formula | Range | At 100 °C | At 500 °C | At 1000 °C |
|---|---|---|---|---|---|
| Class 1 | ±1.5 °C or ±0.004·|t| | -40 to +1000 °C | ±1.5 °C | ±2.0 °C | ±4.0 °C |
| Class 2 | ±2.5 °C or ±0.0075·|t| | -40 to +1200 °C | ±2.5 °C | ±3.75 °C | ±7.5 °C |
| Class 3 | ±2.5 °C or ±0.015·|t| | -200 to +40 °C | n/a | n/a | n/a |
Notes on Class 1 vs Class 2
"Special" or "Premium" grade thermocouples sold in the US are typically Class 1 equivalents. "Standard" grade is Class 2. The tolerance applies to the bare wire — measurement uncertainty in the installed system is generally larger due to cold-junction error, lead-wire heat conduction, and instrument resolution.
Type K EMF from -270 °C to +1372 °C
Millivolt output of a Type K thermocouple as a function of measuring junction temperature with the reference junction held at 0 °C. Values are interpolated to the nearest 0.001 mV from the NIST ITS-90 polynomials. Tolerance columns show the maximum deviation in °C for Class 1 and Class 2.
| Temperature (°C) | EMF (mV) | Class 1 (°C) | Class 2 (°C) |
|---|---|---|---|
| -270 | -6.458 | — | — |
| -260 | -6.441 | — | — |
| -250 | -6.404 | — | — |
| -240 | -6.344 | — | — |
| -230 | -6.262 | — | — |
| -220 | -6.158 | — | — |
| -210 | -6.035 | — | — |
| -200 | -5.891 | — | — |
| -190 | -5.730 | — | — |
| -180 | -5.550 | — | — |
| -170 | -5.354 | — | — |
| -160 | -5.141 | — | — |
| -150 | -4.913 | — | — |
| -140 | -4.669 | — | — |
| -130 | -4.411 | — | — |
| -120 | -4.138 | — | — |
| -110 | -3.852 | — | — |
| -100 | -3.554 | — | — |
| -90 | -3.243 | — | — |
| -80 | -2.920 | — | — |
| -70 | -2.587 | — | — |
| -60 | -2.243 | — | — |
| -50 | -1.889 | — | — |
| -40 | -1.527 | ±1.50 | ±2.50 |
| -30 | -1.156 | ±1.50 | ±2.50 |
| -20 | -0.778 | ±1.50 | ±2.50 |
| -10 | -0.392 | ±1.50 | ±2.50 |
| +0 | 0.000 | ±1.50 | ±2.50 |
| +10 | 0.397 | ±1.50 | ±2.50 |
| +20 | 0.798 | ±1.50 | ±2.50 |
| +30 | 1.203 | ±1.50 | ±2.50 |
| +40 | 1.612 | ±1.50 | ±2.50 |
| +50 | 2.023 | ±1.50 | ±2.50 |
| +60 | 2.436 | ±1.50 | ±2.50 |
| +70 | 2.851 | ±1.50 | ±2.50 |
| +80 | 3.267 | ±1.50 | ±2.50 |
| +90 | 3.682 | ±1.50 | ±2.50 |
| +100 | 4.096 | ±1.50 | ±2.50 |
| +110 | 4.509 | ±1.50 | ±2.50 |
| +120 | 4.920 | ±1.50 | ±2.50 |
| +130 | 5.328 | ±1.50 | ±2.50 |
| +140 | 5.735 | ±1.50 | ±2.50 |
| +150 | 6.138 | ±1.50 | ±2.50 |
| +160 | 6.540 | ±1.50 | ±2.50 |
| +170 | 6.941 | ±1.50 | ±2.50 |
| +180 | 7.340 | ±1.50 | ±2.50 |
| +190 | 7.739 | ±1.50 | ±2.50 |
| +200 | 8.138 | ±1.50 | ±2.50 |
| +210 | 8.539 | ±1.50 | ±2.50 |
| +220 | 8.940 | ±1.50 | ±2.50 |
| +230 | 9.343 | ±1.50 | ±2.50 |
| +240 | 9.747 | ±1.50 | ±2.50 |
| +250 | 10.153 | ±1.50 | ±2.50 |
| +260 | 10.561 | ±1.50 | ±2.50 |
| +270 | 10.971 | ±1.50 | ±2.50 |
| +280 | 11.382 | ±1.50 | ±2.50 |
| +290 | 11.795 | ±1.50 | ±2.50 |
| +300 | 12.209 | ±1.50 | ±2.50 |
| +310 | 12.624 | ±1.50 | ±2.50 |
| +320 | 13.040 | ±1.50 | ±2.50 |
| +330 | 13.457 | ±1.50 | ±2.50 |
| +340 | 13.874 | ±1.50 | ±2.55 |
| +350 | 14.293 | ±1.50 | ±2.62 |
| +360 | 14.713 | ±1.50 | ±2.70 |
| +370 | 15.133 | ±1.50 | ±2.77 |
| +380 | 15.554 | ±1.52 | ±2.85 |
| +390 | 15.975 | ±1.56 | ±2.92 |
| +400 | 16.397 | ±1.60 | ±3.00 |
| +410 | 16.820 | ±1.64 | ±3.07 |
| +420 | 17.243 | ±1.68 | ±3.15 |
| +430 | 17.667 | ±1.72 | ±3.23 |
| +440 | 18.091 | ±1.76 | ±3.30 |
| +450 | 18.516 | ±1.80 | ±3.38 |
| +460 | 18.941 | ±1.84 | ±3.45 |
| +470 | 19.366 | ±1.88 | ±3.52 |
| +480 | 19.792 | ±1.92 | ±3.60 |
| +490 | 20.218 | ±1.96 | ±3.67 |
| +500 | 20.644 | ±2.00 | ±3.75 |
| +510 | 21.071 | ±2.04 | ±3.82 |
| +520 | 21.497 | ±2.08 | ±3.90 |
| +530 | 21.924 | ±2.12 | ±3.97 |
| +540 | 22.350 | ±2.16 | ±4.05 |
| +550 | 22.776 | ±2.20 | ±4.12 |
| +560 | 23.203 | ±2.24 | ±4.20 |
| +570 | 23.629 | ±2.28 | ±4.27 |
| +580 | 24.055 | ±2.32 | ±4.35 |
| +590 | 24.480 | ±2.36 | ±4.42 |
| +600 | 24.905 | ±2.40 | ±4.50 |
| +610 | 25.330 | ±2.44 | ±4.58 |
| +620 | 25.755 | ±2.48 | ±4.65 |
| +630 | 26.179 | ±2.52 | ±4.72 |
| +640 | 26.602 | ±2.56 | ±4.80 |
| +650 | 27.025 | ±2.60 | ±4.88 |
| +660 | 27.447 | ±2.64 | ±4.95 |
| +670 | 27.869 | ±2.68 | ±5.02 |
| +680 | 28.289 | ±2.72 | ±5.10 |
| +690 | 28.710 | ±2.76 | ±5.17 |
| +700 | 29.129 | ±2.80 | ±5.25 |
| +710 | 29.548 | ±2.84 | ±5.33 |
| +720 | 29.965 | ±2.88 | ±5.40 |
| +730 | 30.382 | ±2.92 | ±5.47 |
| +740 | 30.798 | ±2.96 | ±5.55 |
| +750 | 31.213 | ±3.00 | ±5.62 |
| +760 | 31.628 | ±3.04 | ±5.70 |
| +770 | 32.041 | ±3.08 | ±5.77 |
| +780 | 32.453 | ±3.12 | ±5.85 |
| +790 | 32.865 | ±3.16 | ±5.92 |
| +800 | 33.275 | ±3.20 | ±6.00 |
| +810 | 33.685 | ±3.24 | ±6.08 |
| +820 | 34.093 | ±3.28 | ±6.15 |
| +830 | 34.501 | ±3.32 | ±6.22 |
| +840 | 34.908 | ±3.36 | ±6.30 |
| +850 | 35.313 | ±3.40 | ±6.38 |
| +860 | 35.718 | ±3.44 | ±6.45 |
| +870 | 36.121 | ±3.48 | ±6.52 |
| +880 | 36.524 | ±3.52 | ±6.60 |
| +890 | 36.925 | ±3.56 | ±6.67 |
| +900 | 37.326 | ±3.60 | ±6.75 |
| +910 | 37.725 | ±3.64 | ±6.83 |
| +920 | 38.124 | ±3.68 | ±6.90 |
| +930 | 38.522 | ±3.72 | ±6.97 |
| +940 | 38.918 | ±3.76 | ±7.05 |
| +950 | 39.314 | ±3.80 | ±7.12 |
| +960 | 39.708 | ±3.84 | ±7.20 |
| +970 | 40.101 | ±3.88 | ±7.27 |
| +980 | 40.494 | ±3.92 | ±7.35 |
| +990 | 40.885 | ±3.96 | ±7.42 |
| +1000 | 41.276 | ±4.00 | ±7.50 |
| +1010 | 41.665 | — | ±7.57 |
| +1020 | 42.053 | — | ±7.65 |
| +1030 | 42.440 | — | ±7.72 |
| +1040 | 42.826 | — | ±7.80 |
| +1050 | 43.211 | — | ±7.88 |
| +1060 | 43.595 | — | ±7.95 |
| +1070 | 43.978 | — | ±8.03 |
| +1080 | 44.359 | — | ±8.10 |
| +1090 | 44.740 | — | ±8.17 |
| +1100 | 45.119 | — | ±8.25 |
| +1110 | 45.497 | — | ±8.32 |
| +1120 | 45.873 | — | ±8.40 |
| +1130 | 46.249 | — | ±8.47 |
| +1140 | 46.623 | — | ±8.55 |
| +1150 | 46.995 | — | ±8.62 |
| +1160 | 47.367 | — | ±8.70 |
| +1170 | 47.737 | — | ±8.78 |
| +1180 | 48.105 | — | ±8.85 |
| +1190 | 48.473 | — | ±8.92 |
| +1200 | 48.838 | — | ±9.00 |
| +1210 | 49.202 | — | — |
| +1220 | 49.565 | — | — |
| +1230 | 49.926 | — | — |
| +1240 | 50.286 | — | — |
| +1250 | 50.644 | — | — |
| +1260 | 51.000 | — | — |
| +1270 | 51.355 | — | — |
| +1280 | 51.708 | — | — |
| +1290 | 52.060 | — | — |
| +1300 | 52.410 | — | — |
| +1310 | 52.759 | — | — |
| +1320 | 53.106 | — | — |
| +1330 | 53.451 | — | — |
| +1340 | 53.795 | — | — |
| +1350 | 54.138 | — | — |
| +1360 | 54.479 | — | — |
| +1370 | 54.819 | — | — |
| +1372 | 54.886 | — | — |
Reading the Table Correctly
The table assumes the cold (reference) junction is at exactly 0 °C. In any real measurement, the cold junction is at room temperature or instrument-terminal temperature — typically somewhere between 15 and 50 °C. The instrument's electronics measure that local temperature with a separate sensor and apply a software correction.
The correction procedure is:
- Measure cold-junction temperature T_cj with an internal sensor
- Look up EMF for T_cj in the Type K table → call this V_cj
- Measure raw thermocouple voltage V_raw
- Compute corrected voltage: V_corr = V_raw + V_cj
- Look up V_corr in the inverse table → this gives the true measuring-junction temperature
This is why "cold-junction-compensating" multimeters and data loggers need a few seconds to settle when first powered on — the internal sensor must equilibrate with the terminal block.
Worked Example
Measuring junction in oven
V_raw = 16.001 mV
T_cj = +23 °C
V_cj ≈ 0.919 mV (table)
V_corr = 16.920 mV
T_meas ≈ +412 °C
Without compensation: ~390 °C (22 °C error)
Extension Wire
The wire from the measuring junction to the cold junction must be Type K (or KX extension grade). Substituting copper anywhere in that path introduces an unintended secondary thermocouple and an arbitrary error.
Where Type K Is Used
Furnaces & Kilns
Heat treating, brazing, annealing, ceramic firing, glass tempering. Type K is the default sensor up to about 1100 °C continuous service.
Engine & Exhaust
Turbocharger, exhaust manifold, EGT (exhaust gas temperature) probes for gas turbines, diesel engines, and motorsport applications.
Industrial Process
Petrochemical reactor monitoring, food processing, plastics extrusion, chemical batch processing. Reliable signal across a wide range with simple instrumentation.
Avoid Type K When…
Sulfur or hydrogen atmospheres present, ultra-precise measurement < 0.5 °C uncertainty needed, or above 1100 °C continuous (use Type N, R, or S instead).
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