Thermocouple Sensors

Select the right thermocouple for your application

We have a wide range of thermocouples designed to meet the requirements of most industrial and scientific temperature measurement applications available from stock for Next Day Delivery!

What is a thermocouple? How does a thermocouple work?

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?

What types of thermocouples are available?

Thermocouples are made from different combinations of wire, each with particular characteristics and temperature ranges. For example, Type K, the most widely used thermocouple, is made from a positive leg of Nickel-Chromium and a negative leg of Nickel-Alumel. Type K thermocouples are inexpensive, reliable and with a wide temperature range of -200ºC to +1100ºC, it is easy to see why they are so popular. Other common types of thermocouple are Type J (Fe/Con), Type T (Cu/Con) and Type N (NiCrSi/NiSi), details of all thermocouple types are shown below.

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.

Which type of junction should I choose?

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 Junctions for Mineral Insulated Thermocouples

Insulated Thermocouple Junction

insulated thermocouple junction

Grounded Thermocouple Junction

grounded thermocouple junction

Exposed Thermocouple Junction

exposed thermocouple junction

Hot junction insulated from sheath. Gives floating output with typical insulation resistance in excess of 100 megohms.

Hot junction welded to sheath tip giving earthed output and faster response to temperature changes.

Fastest response, mainly for the measurement of air temperature in ducts. Restricted to a maximum operating temperature of 600°C.

Thermocouple Types in detail

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.

Thermocouple Types to IEC 60584-1

Type K Thermocouple - Nickel-Chromium vs Nickel-Aluminium

Type K thermocouple is also known as 'Chromel-Alumel' is the most widely used of all thermocouples in industry. It has a wide temperature range of 0ºC to +1100ºC with short term use being -180ºC to +1350ºC. Type K thermocouple, also referred to as Chromel-Alumel, is the most common thermocouple in use today mainly because it is reliable, inexpensive, has a wide temperature range and is reasonably accurate. 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.

Type T Thermocouple - Copper vs Copper-Nickel

Type T thermocouple was originally called Copper-Constantan is often used in laboratoies over the temperature range -250°C to 400°C. It's repeatability over the range -200°C to 200°C is excellent (±0.1°C). The copper/nickel alloy used in the negative arm is not the same as that in Type J - so they’re not interchangeable.

Type J Thermocouple - Iron vs Copper-Nickel

Type J thermocouples are often referred to as Iron/Constantan. Type J is one of the few thermoocouples that can be used safely in reducing atmospheres up to 550°C. The maximum continuous operating temperature is around 800°C whilst the minimum is -210°C.

Type N Thermocouple- Nickel-Chromium-Silicon vs Nickel-Silicon

Type N thermocouples are a good alternative to Type K in higher temperature applications where a greater resistance to oxidation-related drift is required and so it can be used at a higher temperature than Type K (up to 1,280°C, and higher for short periods).

Type S Thermocouple - Platinum-10% Rhodium vs Platinum

Type S thermocouples are generally used for higher temperature applications up to 1600ºC in both oxidising or inert atmospheres and for short term use up to 1700°C. Generally the thermocouple wires are insulated with high purity recrystallised alumina insulators and usually with an impervious sheath to prevent contamination.

Type R Thermocouple - Platinum-13% Rhodium vs Platinum

Type R is similar to Type S but has a slightly higher EMF output and improved stability.

Further Information on Thermocouples

What are the temperature ranges for different thermocouples?

Whilst some thermocouple types like ceramic sheathed type S are capable of very high temperatures (up to 1970º kelvin) and suitable for applications such as in a furnace or kiln, the temperature range of a thermocouple is as dependant on the sheath material used as it is on the thermocouple type and the environmental conditions that the thermocouple is exposed to. Find out more about the temperature ranges for different types of thermocouples?

How accurate are thermocouples?

Whilst thermocouples are not quite as accurate as RTD pt100 sensors, each type has its own standard of accuracy defined as a tolerance class. See our detailed tables showing the accuracy of thermocouples

What is cold junction compensation?

Unlike other sensors such as a thermistor or bimetallic thermometer, a thermocouple needs to be referenced to 0ºC. This is usually accomplished in the digital circuit of associated instrumentation and so is not usually something for the industrial user to be concerned with. We have provided a detailed explanation of how cold junction compensation works along with aassociated schematic and diagram.

What are thermocouple colour codes?

All thermocouples (J, T K type etc.) have a designated standard color code to identify the thermocouple by its cable or connector. We have provided a complete guide to thermocouple colour codes here.

What is the response time of a thermocouple?

Thermocouples have an ectremely fast response time to changes in temperature. It is one of the advantages of thermocouples over other types of temperature sensor. We have tabulated the response times of thermocouples over a wide temperature range.

Popular Thermocouples