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Thermocouple Sensors - Custom Built Thermocouples to your Specifications

What is a Thermocouple?

A thermocouple is a simple device that is used to measure temperature. Thermocouple sensors are made from two wires of different metals joined (welded) together to form a measuring junction (also known as a hot junction) which generates an EMF output (see below). This junction is usually enclosed in a metal sheath and is inserted into the medium where temperatures are to be measured. The opposite end of the two wires are also (effectively) joined at a point known as the cold junction.

How does a thermocouple work?

As the temperature changes at the hot junction, it creates a thermal gradient between the hot and cold junctions which generates an electron flow and a resultant electromotive force (EMF), measured in Millivolts. Thus a thermocouple provides an EMF output and the voltage measured at the ends of the conductors is a function of the temperature difference along the conductor length. This principle or effect was discovered by TJ Seebeck in 1822 and is known as The Seebeck Effect.

When used with a temperature instrument, the cold junction is electronically referenced to 0ºC and a formula is applied to the millivolt signal, which is then be converted to °C or °F, using instrumentation. A thermocouple can also be connected to a transmitter to provide a 4 to 20mA current for standard process instrumentation. For a more detailed explanation of how a thermocouple works, please see our Thermocouple Theory page.

What are the various thermocouple types?

There are several different thermocouple calibration types (K Type Chromel / Alumel thermocouples, type J, type T, type N, R Type etc) each with different characteristics and covering various temperature ranges. Type K is one the most popular thermocouples used in industrial applications, due to its versatility and wide temperature range, T type thermocouples are generally preferred in applications below 0ºC and Type R thermocouples are preferred in furnace thermocouple applications. A detailed list of the different thermocouple types, thermocouple colour codes and other characteristics can be found here. Please click here for information and details on thermocouple codes.

What is the maximum temperature for a thermocouple?

The maximum operating temperature is dependant on diameter, sheath material, thermocouple type and environmental factors. The most popular style of thermocouple is the metal sheathed, mineral insulated sensor, as such these sensors have the widest choice of sheath materials and diameters, further guidance is available here. For very high temperature applications up to 1600°C, ceramic sheathed thermocouples are generally used.

Popular Thermocouple Probes

Mineral Insulated
Thermocouple Sensors

Our most popular thermocouple style. Rugged sensors, ideal for most applications. Vast choice of terminations e.g. pot seals, Wire, connectors, heads etc.

Swaged Tip
Thermocouple Sensors

Fast temperature response time thermocouple probes ideal for industrial and other applications.

Miniature
Thermocouples

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

Heavy Duty
Thermocouples

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

High Temperature Thermocouple Sensors

Various thermocouples specifically for high temperature applications. Wide choice of sheath materials and terminations.

General Purpose
Thermocouples

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

There are many types of thermocouple sensors available in a wide range of designs and constructions making them a practical choice for almost all temperature measurement applications and temperature ranges in industry, science and beyond.

Practical Thermocouple Sensors / Probes

Popularity of the Thermocouple Sensor

The reasons for the popularity of thermocouples over other temperature sensors, such as a Pt100 or thermistor, are not just the existence of a range of types designed to cover almost all temperature, environmental and accuracy requirements, or the fact that they are small. Others include the ease with which they can be made and applied, and the availability of a vast assortment of housings and special packages to match almost every imaginable application.

For simple applications, the thermocouple probe can easily be made from lengths of bare thermocouple wire or insulated cable, the insulation material being selected for compatibility with the application, and likewise the cable itself. As for wire diameter, 0.004” to 0.118” OD or larger for industrial use is common.

The measuring junction is best constructed by welding the two wires together. Soldering or twisting are less satisfactory, although with the aid of a clamping screw in a connecting block, greater security can be obtained. The key to success is a good electrical connection which does not disrupt the composition of the thermocouple wires themselves. Bear in mind the expected operating conditions for the measuring junction.

Base Metal Type Thermocouple Sensors

Base metal thermocouples (like Type K (Nickel Chromium / Nickel Aluminium) or Type T (copper / constantan) are usually welded electrically in an argon atmosphere, while platinum thermocouples can be welded using a small oxy-hydrogen flame or a spot welder with a tungsten electrode. Beyond this, base metal thermocouple wires are normally supplied ready annealed and are thus prepared for use directly after welding. The same is not usually the case with the platinum equivalents which therefore have to be annealed after inserting the wires into the insulators and making up the junction.

At the other end of the circuit cables, each thermocouple wire can be joined to a copper wire to form the reference junction. Again, welding is the best bet, but silver soldering using a very small quantity of solder in paste form together with a miniature flame is a reasonable alternative - as long as all traces of corrosive flux are removed. The junctions can then be fitted into closed end tubes or potted for immersion in an ice-water mixture.

This method of thermocouple construction is simple, versatile, and fine for experimentation in the laboratory. Reasonable accuracies can be expected.

Recommended Thermocouples for General Measurement Use

Mineral insulated Thermocouples

The most popular thermocouple style for industry today is the mineral insulated (MI) style. These are comprised of a seamless metal sheath enclosing highly compacted mineral insulant powder (typically, magnesium oxide) which supports and electrically insulates the thermocouple wires held inside. Click here for details of available sensors.

Temperature ranges covered are from -200°C to +1250°C. These assemblies provide a high integrity, compact, hermetically sealed, self armored construction. They can be bent or formed and are suitable for the most difficult operating conditions. MI cable is generally available with two to six conductors, and with diameters from 0.010” to 0.5”.

Advantages of the Mineral Insulated Thermocouple

There are many advantages with this construction. They include small size, ease of installation (they can be bent, twisted and flattened without impairing performance), good mechanical strength, excellent isolation of the junction from hostile environments, high long term accuracy and stability, fast response and good insulation resistance. They are also readily available off the shelf and are reasonably priced. They are thus ideal for accurate measurement in a very wide range of applications, including extreme environments, like high vibration and high pressure/vacuum.

Additionally, they allow the use of a wide range of outer sheaths and seal termination styles to suit tremendously diverse operating conditions. Sheath materials include stainless steels, Inconel, and the Nicrobell alloys. The selection of the sheath material is based on the temperature and environment (click here for schematic and details). Finally, platinum-rhodium alloy sheaths are often used with platinum thermocouples. The finished assembly length can be from a few inches to hundreds of feet. Beyond this, all of the usual thermocouple alloy combinations are available as MI thermocouples - both rare and base metal types. Also, the measuring junction can be exposed or insulated from the sheath, or grounded to the sheath. The insulated version has an insulation resistance reading of over 100M Ohms. By being insulated, ground loops on associated instrumentation are prevented. Although, with the grounded junction, the response time is faster.

Limitations of the Mineral Insulated Thermocouple

On the down side, limitations can include problems due to the different thermal coefficients of expansion of the stainless steel sheath variants, for example, as compared particularly to the Type K and N thermocouple materials - sometimes leading to premature mechanical fatigue failure. Also ironically, with both the stainless steels and Inconel 600 sheaths, there are possibilities of material contamination problems due to vapor diffusion of the elements, leading to actual contamination of the thermocouple wires by the sheath material itself.

There can also be problems relating to the ingress of water vapor resulting in reduced insulation resistance, causing calibration instability and possibly premature failure. This latter phenomenon, however, is really a matter of care in manufacture and repair.

Higher Temperature Applications

Special sheathing alloys have been developed to deal with these limitations, particularly for higher temperature applications with Type K and N thermocouples. These include the Nicrotherm D^TM alloy which take on the inherent advantages of Type N thermocouple materials.

This means that MI thermocouples using Nicrotherm D^TM can last four to six times longer than their stainless steel based alternatives. And, remembering the transmutation reductions achieved using Nicrotherm D^TM, sheaths constructed from this material also out-perform Inconel 600 in terms of long term drift due to thermocouple wire contamination.

In general, it is recommended that the smallest diameter mineral insulated metal sheathed thermocouple's should be avoided if possible for very high temperature or corrosive environment measurements. There does seem to be a correlation between MI cable diameter and its survival and long term performance. For details of sheath materials and available configurations for non mineral insulated thermocouples click here.

Practical Information for Thermocouples

Technical Information for Thermocouples