How does a Thermocouple Work?

Thermocouples are a type of sensor used for the measurement of temperature. Thermocouples use the Seebeck effect, discovered by Thomas Johann Seebeck in the early 1800s. He was able to prove that if two dissimilar electrical conductors have a temperature differential across them, then a voltage difference is produced.

How Thermocouples work

So how does a thermocouple work? The voltages generated are extremely small, generally only a few microvolts per degree of temperature difference. Through the development of particular alloy and conductor combinations, these voltages have been tuned to the needs of industrial instrumentation to both maximise the voltage change per degree change of temperature and also to provide the greatest linearity possible.

In an industrial application the thermocouple probe or sensor will have a thermocouple junction. This is the temperature-sensitive part of the product and this should be placed at the position where you wish to measure temperature. This junction can be built into a variety of different designs to enable to the measurement of semi-solid, liquid, gas or surface temperatures. The thermocouple probe will generally have a cable which is either wired or plugged into an electronic instrument which will then convert the voltage output from the thermocouple into a temperature value.

Thermocouples are rugged sensors well known for being versatile and suitable for a huge range of applications. Due to the simplicity of the design, simply two wires joined together, thermocouples are suitable in processes where shock and vibration are present. They are also capable of withstanding very wide ranges of temperature. Depending on the thermocouple type and construction the range can extend down to -200°C and up to over 2000°C.

The versatility of thermocouples is further extended by the ability of thermocouple manufacturers such as us to build a very wide range of model types to suit almost any application. They can be mounted in very small housings to monitor bearings and other fast response processes; mounted in handheld probe designs for the food industry; built into high-pressure housings for the plastics extrusion market and made with ceramic components for the heat treatment and furnace industry. Thermocouples are the most numerous of all temperature sensors used today due to their flexibility, resilience and relatively low cost.

How to Test a Thermocouple

a 2D diagram showing from Process Parameters showing a small and a medium thermocouple sensor with two pins and how they work

2D diagram of thermocouple sensors

Thermocouples are very simple devices but knowing that they are working correctly can sometimes be a little confusing particularly if a thermocouple probe is giving wrong readings. Here are some tips on how to make sure your thermocouple is working correctly.

How to test a thermocouple – The best instrument to use is a thermocouple thermometer as this will give you a temperature measurement rather than a voltage which will mean much less.

It is important that any testing is done on a thermocouple which is not measuring ambient. If there is no temperature differential from one end of the sensor to the other then the signal is zero making any testing ineffective. It is important to elevate the temperature of the measuring tip.

If the sensor is working correctly, the temperature measurement will be an accurate representation of the object being measured. You should also notice a swift response from the probe to any temperature change.

There are two primary modes of failure. The first is the thermocouple becoming open circuit. In other words one or both of the conductors of the thermocouple develops a break. It is highly likely that your instrument will display O.L or similar to denote this open circuit.

The second mode of failure is more subtle and can require some careful investigation to find the fault. This is where the insulation between the two conductors breaks down, perhaps from being overheated or flexing too much. This causes a short circuit at this point. The issue is that your thermocouple thermometer will continue to give a measurement but now it will not be accurate. The response time is also likely to be very slow or non-existent. The reason for this is that the short circuit is now acting as the thermocouple junction and the temperature is being measured at this point and not at the tip. Heating of the short, if you can locate it, will show a normal response.

In both cases above it is highly probable that the thermocouples are irreparable and need replacement. Contact us for a quote to replace your faulty thermocouple probes.

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How does a thermocouple measure Temperature?

Two dissimilar metals are joined together at both ends in an electrical circuit. One “junction” is the measuring junction or “hot end”. The other is the reference junction or “cold end”. A sensitive voltmeter is connected into one of the conductors.

Under laboratory conditions the reference junction would be held at a known temperature, usually 0°C but in normal industrial practise the junction is left at ambient temperature and an external sensor is used to compensate for this variation (known as cold junction compensation, usually a thermistor bead is used to measure the ambient temperature).

Quite simply as the temperature rises or falls at the measuring junction a voltage is generated within the circuit which correlates directly to temperature and can easily be converted by reference to the appropriate tables.

Thermocouples have been developed as a result of the discovery of the Seebeck effect, by Thomas J. Seebeck in 1821.

How much does a Thermocouple Cost?

So, how much does a thermocouple cost? The cost of a thermocouple can vary from thermocouple type, construction and design.

To give you an idea of the cost variation. A simple type K sensor could cost £10.20 where a type R thermocouple would cost £200! Nearly twenty times more. Why is this? Well, it simply comes down to materials, Type R is made from Platinum and Platinum/Rhodium conductors. Rare earth materials are a lot more expensive than Nickel, Chromium and Copper conductors.

Pt100-principle

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A Process Parameters type J thermocouple made from Iron / Copper-Nickel (aka Constantin) with black casing.

Type J Thermocouple

Type J thermocouples are most commonly used in the plastics industry. This is a historic relationship where they were developed for early electronic temperature controls. As time has progressed the control equipment has changed but the requirement for type J has not.

Type J thermocouples are based on a pure Iron positive conductor and a Copper-Nickel alloy called Constantan for the negative conductor. Both conductors are mechanically tough and can withstand substantial flexing and stress making them perfect for applications such as injection moulding machines.

The temperature range for type J is more limited than type K at 0 to 750°C continuous and -180 to +800°C for short periods. The conductors are suitable for welding and brazing/silver soldering, so it is very easy to manufacture thermocouples with grounded hot junctions to give the fastest response time possible.

Class 1 accuracy for type J is ±1.5°C in the range -40 to +375°C. Beyond that the tolerance 0.4% of the measured value. Class 2 is a wider tolerance than this and is generally not used.

As mentioned above type J thermocouples are by far the most common type used in the plastics industry and there are some standard designs which fit the various processes. These include bayonet flexicouples for extrusion and injection moulding; melt bolt thermocouples for us on extrusion machines; bolt, nozzle and washer thermocouples for smaller heads and packaging machines.

Special care is required when using type J where water is present as the positive conductor is pure Iron and therefore is extremely susceptible to rusting. The cables generally used are glass fibre based and therefore not waterproof. Corrosion of the positive conductor will give the potential for measurement errors and ultimately failure of the thermocouple.

A Process Parameters Type K thermocouple made from Nickel-Chromium or Nickel-Alumel with green casing.

Type K Thermocouple

By far the most common type of thermocouple in use today is type K. It possesses a really good set of features which make it suitable for the widest possible range of applications.

The conductors for the thermocouple are Nickel-based alloys (Nickel-Chromium and Nickel-Aluminium) which makes the conductors of any cable extremely tough and robust. It also contributes to a usefully wide operating temperature range of between 0 and 1100°C continuously and in the range -180°C to +1300°C for short periods. Note that these temperature ranges are also highly dependent on the construction of the sensor.

The conductors of the thermocouple are ideal for welding and brazing/silver soldering. This enables manufacturers such as us to bond thermocouple junctions to other components with a high-temperature join (rather than adhesive). This could be a simple washer but could also be a weld pad or bolt type fixing.

Class 1 accuracy for type K is ±1.5°C in the range -40 to +375°C. Beyond that the tolerance 0.4% of the measured value. Class 2 is a wider tolerance than this and is generally not used.

Type K thermocouples used in certain application may require “special tolerance” wire. This means that conductor pairs are carefully selected and tested to provide improved levels of accuracy generally over a limited temperature range. If you have a special tolerance requirement, please contact us.

Type K thermocouples are used almost anywhere where the measured temperature falls within the possible range. Process Parameters manufactures type K thermocouples in a variety of designs including washer types for surface measurement, handheld designs for food measurement, small sheathed designs for use in packaging machines and ceramic sheathed designs for use in high-temperature furnace type processes.

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