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Who this is for: lab staff, core-facility users and equipment buyers who need to know what a type K thermocouple is, what its letter guarantees, and how it differs from other types. After reading you should be able to read a probe label or spec sheet and know which standard its numbers come from.
A type K thermocouple is a temperature sensor whose two conductors are a nickel-chromium alloy (positive leg, often sold as Chromel) and a nickel-aluminum alloy (negative leg, often sold as Alumel). The NIST reference tables for type K cover -270 to 1372 °C.
What a type K thermocouple is
The letter K is not a brand or a probe style. NIST Monograph 175 states that letter designations identify a reference table of voltage against temperature, and that a letter may be applied to any thermocouple that follows that voltage-temperature relation. In that monograph the type K positive leg is designated KP and the negative leg KN, and Chromel and Alumel appear as trade names for the two alloys. A probe, a bare-wire junction and a mineral-insulated sheathed sensor can all be “type K” if their wire follows the type K table.
What is a thermocouple, and how does it work?
A thermocouple is a temperature sensor made from two dissimilar conductors joined at one end. When the joined (measuring) junction is at a different temperature from the other (reference) end, a small voltage appears across the pair. That voltage depends on the temperature difference and on which two materials are used. An instrument measures the voltage and converts it to temperature using the reference table or reference function for that letter type.
Because the voltage reflects a difference, the instrument must also know the temperature of the reference end. The NIST and IEC reference functions express voltage with the reference junction at 0 °C; readers and controllers either compensate electronically for the actual reference-end temperature or use an ice-point reference.
Type K compared with other letter types
The table below is taken from Table 1.1.1 of NIST Monograph 175 (the ITS-90 thermocouple tables). The ranges are the ranges covered by the reference tables, not a guarantee of safe operating limits for any particular probe.
| Letter type | Reference-table range (°C) | Conductor pair (positive vs negative) |
|---|---|---|
| B | 0 to 1820 | Platinum-30% rhodium vs platinum-6% rhodium |
| E | -270 to 1000 | Nickel-chromium vs copper-nickel |
| J | -210 to 1200 | Iron vs copper-nickel |
| K | -270 to 1372 | Nickel-chromium alloy vs nickel-aluminum alloy |
| N | -270 to 1300 | Nickel-chromium-silicon vs nickel-silicon-magnesium |
| R | -50 to 1768 | Platinum-13% rhodium vs platinum |
| S | -50 to 1768 | Platinum-10% rhodium vs platinum |
| T | -270 to 400 | Copper vs copper-nickel |
J-type vs K-type thermocouple
Type J (iron vs copper-nickel) has reference tables from -210 to 1200 °C; type K (nickel-chromium vs nickel-aluminum) covers -270 to 1372 °C, a wider span at both ends. Their voltage-temperature relationships differ, so a J-type wire cannot be read with a K-type table or on an instrument configured for K.
Thermocouple types in one sentence
The eight standardized letter types in the NIST tables are B, E, J, K, N, R, S and T; B, R and S use platinum-rhodium alloys, the others use base-metal alloys or copper.
Standards that define type K
- NIST Monograph 175 and the NIST ITS-90 Thermocouple Database provide the tables and reference and inverse function coefficients for types B, E, J, K, N, R, S and T on the ITS-90 temperature scale.
- IEC 60584-1:2013 gives reference functions and tolerances for letter-designated thermocouples. Its polynomials give EMF in volts against ITS-90 temperature in °C with the reference junction at 0 °C, and the 2013 edition restricts the type K EMF polynomial range to 1300 °C. It replaces the 1995 second edition and merges the former IEC 60584-2. Tolerances are defined in this standard; consult it for the limits that apply to a given probe.
Where type K is used in labs
For incubators, thermal cyclers and refrigerated storage, the relevant question is usually the sensor’s calibration and placement rather than its letter; see CO2 incubator calibration and temperature uniformity mapping and thermal cycler calibration. Where a sensor sits inside a protective sleeve, see what a thermowell is.
Common mistakes
- Treating the letter as a brand or quality grade: it only names the voltage-temperature table.
- Reading a probe with an instrument set to a different letter type, which gives wrong temperatures because the tables differ.
- Treating the table range as the probe’s usable range: check the probe’s own specification.
- Assuming a thermocouple is the only option: for trade-offs against resistance sensors see RTD vs thermocouple.
FAQ
What is a thermocouple?
A temperature sensor made of two different conductors joined at one end, which produces a voltage that depends on temperature.
How does a thermocouple work?
The voltage across the two conductors depends on the temperature difference between the measuring junction and the reference end; an instrument converts it to temperature using the reference table for the letter type.
What temperature range does a type K thermocouple cover?
The NIST reference tables for type K run from -270 to 1372 °C; IEC 60584-1:2013 restricts the type K polynomial range to 1300 °C.
Is a k type thermocouple the same as a type K thermocouple?
Yes. “Type K” and “K-type” are two word orders for the same letter designation.
Sources
- Burns, G.W. et al., NIST Monograph 175, Temperature-Electromotive Force Reference Functions and Tables for the Letter-Designated Thermocouple Types Based on the ITS-90, nvlpubs.nist.gov (read 2026-11-07).
- NIST ITS-90 Thermocouple Database, its90.nist.gov (read 2026-11-07).
- IEC 60584-1:2013, Thermocouples, Part 1: EMF specifications and tolerances, IEC webstore abstract (read 2026-11-07).
