Thermocouple Sensors

A thermocouple is a type of temperature sensor made from two dissimilar wires joined together to form a junction. When this junction is heated or cooled a voltage is generated which corresponds to the temperature being measured. This voltage can then be interpreted by thermocouple instrumentation to display the temperature in terms of ºC or ºF.

The principle of temperature measurement that thermocouples rely upon is known as the Seebeck effect. In practice there are two junctions, one is known as the 'hot junction' and is described above, the second junction is known as the 'cold junction'. The Seebeck effect essentially states that the voltage generated is proportional to the difference in temperature between these two junctions.

To make thermocouples practicable, the cold junction is referenced to 0ºC. Instrumentation will do this electronically in a process known as cold junction compensation where the difference between the ambient temperature and 0ºC is effectively added to the incoming signal from the hot junction. Find out more about how thermocouples work.

Thermocouples are usually manufactured in a sealed stainless steel casing to form a thermocouple probe and are suited to most industrial temperature measurement applications. They can be made in rugged flexible constructions and are available in many varied designs to suit applications including measuring the temperature of fluids, vapour, air and surfaces over a wide temperature range.

For a more detailed technical explanation on thermocouple theory, why not request our free guide to thermocouple thermometry which includes an illustrated 37,000 word explanation on the theory of temperature measurement?

Thermocouple probes for industrial temperature measurement applications come in a wide range of constructions each designed for a particular set of parameters. The simplest form of thermocouple is a welded tip thermocouple and these are best suited for air / gas temperature measurements due to their fast response. They are generally not suitable for use in liquids.Thermocouple probes are usually mineral insulated thermocouples which are suitable for most industrial applications, they are rugged yet flexible, cover a wide operating temperature range and are available in a wide range of terminations such as thermocouple plug / socket, terminal head, with fitted extension leads etc. mineral Insulated thermocouples are our most popular style of thermocouple.

Beyond these styles we offer miniature thermocouples which are ideal for applications requiring a fast response to changes in temperaure where the sensor is required to be more robust than a simple welded tip or where the minimal size is important. For on site temperature measurements using portable indicators, we recommend our range of hand held thermocouples where we have hand held sensors suitable for air, water, penetration and surface measurements.

Further, we offer a range of thermocouples for more specific applications such as autoclave thermocouples, bayonet thermocouples for use in plastics machinery, surface temperature sensors, vacuum thermocouples, heavy duty thermocouples etc and these are all shown in our general purpose thermocouple section.

Sheathed thermocouple probes are available with a choice of three 'hot' junction types: insulated, grounded or exposed. The hot junction is located at the tip of the sensor and in the case of a grounded junction, the two wires which form the thermocouple junction are attached to the tip of the probe. This gives a fast response to temperature changes as the there is a good heat transfer between the medium being measured and the hot junction. The grounded thermocouple is susceptible to electrical intereference.

An insulated junction, which tends to be the most popular, is where the thermocouple junction is electrically insulated from the sheath and so the thermoocouple signal is isolated from any electrical interference.

Finally, an exposed junction is where the thermocouple hot junction sits in front of the tip of metal sheath. This gives the fastest response of all to temperature changes, but the thermocouple is prone to corrosion. Recommended for fast changing non-corrosive gas / air measurements.

The majority of our thermocouples have insulated junctions, please contact us if you require thermocouples with a grounded or exposed junction.

Thermocouple types are designated by a letter (e.g. K, J, T etc.) which corresponds to the conductors used in the thermocouple. The most common types are the base metal thermocouples (K, T, J, N and E) which are ideal for most applications. For high temperature use, there the noble metal thermocouples (R, S, B and C).

Each thermocouple type has its own characteristics, performance, temperature range and environment uses, although the maximum temperature will depend on the sheath material and diameter of the thermoocuple probe, as detailed here.

The list below shows the more popular types of thermocouple in use in the UK and those listed under IEC 60584-1. There are some other thermocouples that you might come across, such as Type B and Type C thermocouples, please contact us for more information on these types.