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.

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.