Type K Thermocouple - Technical Information
Type K - Nickel-Chromium vs Nickel-Aluminium, IEC 60584-1
Thermocouple Type K, generally referred to as Chromel-Alumel, is the most common thermocouple in use today. Type K thermocouples are designed primarily for general temperature measurements in normal atmospheres. Maximum continuous temperature is about 1,100°C, although above 800°C oxidation increasingly causes drift and decalibration. For short term exposure, however, there is a small extension to 1,200°C.
Although Type K is widely used because of its wide temperature range and cheapness, it is not as stable as other base metal sensors in common use. At temperatures between 250°C and 600°C, but especially 300°C and 550°C, temperature cycling hysteresis can result in errors of several degrees.
Type K thermocouples should not be used in reducing atmospheres unless protected by a suitable sheath and whilst Type K thermocouples are popular for nuclear applications because of its relative radiation hardness, Type N is a better choice for those applications.
Type K Thermocouples - Typical Constructions and Styles
Type K Thermocouples - Mineral Insulated
Rugged sensors, ideal for most applications. Vast choice of terminations e.g. pot seals, cables, connectors, heads etc.
Type K Thermocouples - Swaged Tip
Fast response thermocouples ideal for industrial and other applications.
Type K Miniature Thermocouples
Ideal for precision temperature measurements where minimal displacement and a fast response is required.
Type K Heavy Duty Thermocouples
For industrial applications such as furnaces, kilns, ovens, boliers, flues etc. Many types of sheath material available.
General Purpose Type K Thermocouples
A wide range of thermocouples to suit many applications. Hand held, surface, bayonet, bolt, patch styles etc.
ATEX/IECEx ApprovedType K Thermocouples
A large range of thermocouples with a variety of terminations such as pot seal, terminal heads etc.
Type K - Technical Data
| Conductor Combination | Approximate generated EMF change in µV per ºC change (referenced to 0ºC) at: |
Approximate Working Temperature Range |
Thermocouple Output Tolerances to IEC 60584-1 | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| + Leg | - Leg | 100ºC | 500ºC | 1000ºC | Continuous | Short Term | Type | Class 1 | Class 2 | Class 3 |
| NICKEL - CHROMIUM Also known as: Chromel™, Thermokanthal KP*, NiCr, T1*, Tophel™ | NICKEL - ALUMINIUM (magnetic) Also known as: Ni-Al, Alumel™, Thermokanthal KN™, T2™, NiAl™ |
42 | 43 | 39 | 0 to +1100ºC | -180 to +1350ºC | Temperature Range Tolerance Value Temperature Range Tolerance Value |
–40°C to +375°C ±1.5°C 375°C to 1000°C ±0.004 ⋅ |t| |
–40°C to +333°C ±2.5°C 333°C to 1200°C ±0.0075 ⋅ |t| |
–167°C to +40°C ±2.5°C –200°C to –167°C ±0.015 ⋅ |t| |
1. The tolerance is expressed either as a deviation in degrees Celsius or as a function of the actual temperature.
2. Thermocouple materials are normally supplied to meet the tolerances specified in the table for temperatures above –40 deg C. These materials however, may not fall within the tolerances for low temperatures given under Class 3 for Types T, E and K thermocouples. If thermocouples are required to meet limits of Class 3, as well as those of Class 1 and/or Class 2, the purchaser should state this, as selection of materials is usually required.
| ANSI MC96.1 | IEC 60584-3 | Redundant national colour coding for thermocouple cables | ||
|---|---|---|---|---|
| BS 1843 | DIN 43714 | NFC 42324 | ||
 |  |  |  |  |
Thermocouple with Basic End Seal 0.25 to 10.8mm dia.
Internal seal with bare conductors
Thermocouple with Pot Seal 0.25 to 10.8mm dia.
a large selection of plain and threaded pot seals supplied with tails or extension cable (PVC, FEP, Fibreglass etc.)
Thermocouple with Miniature Plug 0.25 to 3.2mm dia.
fitted with a miniature thermocouple plug rated to either 220ºC, 300ºC, 425ºC or 600ºC
Thermocouple with Miniature Socket 0.25 to 3.2mm dia.
fitted with a miniature thermocouple socket rated to either 220ºC, 300ºC, 425ºC or 600ºC
Thermocouple with Standard Plug 1.5 to 8.0mm dia.
fitted with a standard thermocouple plug rated to either 220ºC, 300ºC, 425ºC or 600ºC
Thermocouple with Standard Socket 1.5 to 8.0mm dia.
fitted with a standard thermocouple socket rated to either 220ºC, 300ºC, 425ºC or 600ºC
Thermocouple with Lemo Connector3.0 to 6.0mm dia.
fitted with a size 1 Lemo plug
or socket
1.0 to 8.0mm dia.
terminated with a 16mm ISO x 1.5mm compression gland seal supplied with tails or extension cable (PVC, FEP Fibreglass etc.)
Thermocouple with Micro die cast alloy head 3.0 to 6.0mm dia.
Micro die cast alloy screw down terminal head with ceramic terminal block. Suitable for simplex and duplex assemblies
Thermocouple with Miniature IP67 die cast head 3.0 to 8.0mm dia.
Weatherproof die cast alloy screw top terminal head with ceramic terminal block. Suitable for simplex and duplex assemblies
Thermocouple with Standard IP67 die cast head 4.5 to 10.8mm dia.
Weatherproof die cast alloy screw top terminal head with ceramic terminal block. Suitable for simplex, duplex and triplex assemblies
Thermocouple with IP67 heavy duty cast iron head 4.5 to 10.8mm dia.
Weatherproof cast iron screw top terminal head with ceramic terminal block. Suitable for simplex, duplex and triplex assemblies
Thermocouple with IP67 Bakelite head 4.5 to 10.8mm dia.
Weatherproof Bakelite screw top terminal head with Bakelite terminal block. Suitable for simplex, duplex and triplex assemblies
Thermocouple with alloy straight through head 4.5 to 10.8mm dia.
Die cast alloy straight through terminal head with Bakelite terminal block. Suitable for simplex and duplex assemblies
Thermocouple with 316 Stainless Steel head 4.5 to 10.8mm dia.
Weatherproof 316 stainless steel screw top terminal head with ceramic terminal block. Suitable for simplex and duplex assemblies
Thermocouple with BUZ-H style top hat head4.5 to 10.8mm dia.
Weatherproof die cast alloy screw down 'flip' terminal head. Mainly used for duplex assemblies where two transmitters can be fitted
Thermocouple with spring loaded terminal block 3, 4.5, 6 and 8mm dia.
Spring loaded insert assemblies. The end seal is incorporated into a terminal block for mounting into any standard terminal head
Style of Thermocouples
However, DIY thermocouples are not for everyone, nor are they necessarily ideal for industrial applications. For industrial use, quantities, installation methods, maintenance, and replacement must be considered. It is more common to select a custom manufactured thermocouple from the wide variety described in this catalog.
Thermocouples may be insulated with anything from PVC to ceramics depending on the application. Frequently, the thermocouple conductors will be fitted into a closed end probe of some kind, outer sheath (protection tube), or thermowell made from a suitable heat-resistant alloy or refractory material.
There are almost as many sizes and styles as there are applications. Standard sizes range from 0.25mm to 6.35mm with larger diameters of 25.4mm and larger available to cover all of the possible requirements. As for styles, there are general purpose welded sheath probes, bolt style probes, hand-held probes, surface probes, moving surface probes and these can all be encased in a metal sheath. There are also several junction types and connector systems available.
Additionally, there are sheathed probes for autoclave temperature measurement (incorporating flexible stainless steel conduit and pressure entry glands), bayonet and compression fitting style thermocouples for the plastics and other industries) and heavy duty and high temperature industrial sheathed thermocouples with options like head-mounting terminal assemblies and thermowell extension pieces.
In all cases, great care is taken by the suppliers to ensure that the conductors are correctly manufactured, and installed into the sensor housing under closely controlled conditions. Thus, the amount of change the heated region of the conductors may experience during service (which affects uniformity) is minimized. This is important, since it is this unit that will almost certainly be sitting in the area of greatest temperature gradient, and therefore contributing to most of the output voltage.
An alternative form of construction uses mineral insulated (MI) cable, where the thermocouple conductors are embedded in a closely compacted, inert mineral powder, and surrounded by a metal sheath (like stainless steel or nickel alloy) to form a hermetically sealed assembly. The sheath functions as a useful protective cover. This type of device is available with sheath diameters ranging from 0.25mm to 12.7mm. Lengths can be from a few millimetres to hundreds of metres.
For rather special applications, where high speed response is needed, it can be advantageous for an MI thermocouple to be manufactured with the junction itself exposed. However, this style of junction requires the expertise and skill of an experienced technician.
Thermocouple sensors are often provided with a connection or terminal box which allows convenient linking to the rest of the circuit. Alternatively, a special plug and female connector can be supplied with connecting pins made from the appropriate thermoelectric material.
