What is a Type J Thermocouple? - Technical Information

Type J - Copper vs Copper-Nickel, IEC 60584-1

Thermocouple Type J, commonly referred to as Iron/Constantan, is one of the few thermocouples that can be used safely in reducing atmospheres. However, in oxidising atmospheres above 550°C, degradation is rapid. Maximum continuous operating temperature is around 800°C, although for short term use, temperatures up to 1,000°C can be handled. Minimum temperature is -210°C, but beware of condensation at temperatures below ambient - rusting of the iron arm can result, as well as low temperature embrittlement. Type J thermocouples have been widely adopted in the plastics machinery industry with typical styles including bayonet thermocouples, nozzle thermocouples and melt bolt thermocouples.

Type J Thermocouples - Typical Constructions and Styles

Type J Mineral Insulated

Type J Thermocouples with Mineral Insulation

Rugged sensors, ideal for most applications. Vast choice of terminations e.g. pot seals, cables, connectors, heads etc.

Type J Swaged Tip
Type J Thermocouples with Swaged Tip

Fast response thermocouples ideal for industrial and other applications.

Type J Miniature
Miniature Type J Thermocouples

Ideal for precision temperature measurements where minimal displacement and a fast response is required.

Type J Heavy Duty
Heavy Duty Type J Thermocouples

For industrial applications such as furnaces, kilns, ovens, boliers, flues etc. Many types of sheath material available.

General Purpose
Type J Thermocouples
General use Type J Thermocouples

A wide range of thermocouples to suit many applications. Hand held, surface, bayonet, bolt, patch styles etc.

ATEX/IECEx Approved
Type J Thermocouples
ATEX Approved Type J Thermocouples

A large range of thermocouples with a variety of terminations such as pot seal, terminal heads etc.

Type J Thermocouple - 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
Also known as: Fe
Also known as:
Constantan, Advance™,
54 56 59 +20 to +700°C –180 to +750°C Temperature Range
Tolerance Value
Temperature Range
Tolerance Value
–40°C to +375°C
375°C to 750°C
±0.004⋅ |t|
–40°C to +333°C
333°C to 750°C
±0.0075 ⋅ |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.

J Type Cable Colour Codes for Thermocouples

ANSI MC96.1 IEC 60584-3 Redundant national colour coding for thermocouple cables
BS 1843DIN 43714NFC 42324
ANSI MC96.1 Type J ThermocoupleIEC60584-3 Type J ThermocoupleBS1843 Type J ThermocoupleDIN 43714 Type J ThermocoupleNFC 42324 Type J Thermocouple
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Common Type J Thermocouple Sensors for Temperature Measurement Applications

Thermocouples with Basic End Seal 0.010" to 0.313" dia. Type J Thermocouple Basic Seal

Internal seal with bare conductors

Type J Thermocouples with Pot Seal 0.010" to 0.313" dia. Type J Thermocouple with Pot Seal

a large selection of plain and threaded pot seals supplied with tails or extension Wire (PVC, FEP, Fiberglass etc.)

Thermocouple Probes with Miniature Plug 0.010" to 0.125" dia. Type J Thermocouple with Miniature Plug

fitted with a miniature thermocouple plug rated to either 428ºF, 572ºF, 797ºF or 1112ºF

Thermocouples with Miniature Jack
0.010" to 0.125" dia.
Type J Thermocouple with Miniature Socket

fitted with a miniature thermocouple jack rated to either 428ºF, 572ºF, 797ºF or 1112ºF

with Standard Plug
0.062" to 0.250" dia.
Type J Thermocouple with Standard Plug

fitted with a standard thermocouple plug rated to either 428ºF, 572ºF, 797ºF or 1112ºF

with Standard Jack
0.062" to 0.250" dia.
Type J Thermocouple with Standard Socket

fitted with a standard thermocouple jack rated to either 428ºF, 572ºF, 797ºF or 1112ºF

with Lemo Connector
0.125" to 0.250" dia.
Type J Thermocouple with Lemo Plug fitted with a size 1 Lemo plug
or jack
with Terminal Entry Gland
0.040" to 0.313" dia.
Type J Thermocouple with Terminal Entry Gland terminated with a 16mm ISO x 1.5mm compression gland seal supplied with tails or extension Wire (PVC, FEP Fiberglass etc.)
with Micro die cast alloy head 0.125" to 0.250" dia. Type J Thermocouple with Micro HeadMicro die cast alloy screw down terminal head with ceramic terminal block. Suitable for simplex and duplex assemblies with Miniature IP67 die cast head 0.125" to 0.313" dia.Type J thermocouple with Miniature HeadWeatherproof die cast alloy screw top terminal head with ceramic terminal block. Suitable for simplex and duplex assemblies with Standard IP67 die cast head 0.188" to 0.500" dia. Type J Thermocouple with Standard HeadWeatherproof die cast alloy screw top terminal head with ceramic terminal block. Suitable for simplex, duplex and triplex assemblies

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.


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