What is a Thermocouple?
A thermocouple is a device for measuring temperature. It comprises two dissimilar metallic wires joined together to form a junction. When the junction is heated or cooled, a small voltage is generated in the electrical circuit which can be measured and this corresponds to temperature.
In theory any two metals can be used to make a thermocouple but in practise there are a fixed number of types that are commonly used. They have been developed to give improved linearity and accuracy and comprise specially developed alloys.
Thermocouples can be made to suit almost any application. They can be made to be robust, fast responding and to measure a very wide temperature range.
What are the different types?
Each of the thermocouple types is given a coding letter to denote each of the alloy combinations. The most common code letters are K, J, T and N. Other types include R, S, E, B, C, G and W.
Each thermocouple type has its own alloy pair which are carefully selected and calibrated to meet the international standards to which we must comply. The standard that governs thermocouples in the UK and Europe is BS EN 60584 and covers the characteristics, tolerances and colour coding of the different thermocouple types.
Each thermocouple type is suited to different applications and temperature ranges. See the table below for appropriate operating ranges. More specific advice is freely available from our sales engineers.
How does a Thermocouple work?
Thermocouples have been developed as a result of the discovery of the Seebeck effect, by Thomas J. Seebeck in 1821.
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.
Are Thermocouple’s interchangeable?
Yes. All thermocouples manufactured by Process Parameters comply with BS EN 60584 and are therefore interchangeable with those of other manufacturers.
In general all thermocouples manufactured by us will be colour coded to what is known as IEC colour coding, a coding system that has almost universal acceptance across Europe. It is quite possible however the thermocouples you are using have different colour coding, perhaps American or even old British or German colours. These are still interchangeable with those colour coded to European standards as the characteristics have not been altered by the adoption of new colour coding. Note that care must be taken when making connections to ensure the polarity is maintained.
How accurate are Thermocouples?
The accuracy of thermocouples is relatively complicated and is best shown in tabular format.
Each thermocouple type has different tolerance bands known as Class 1 (the highest standard tolerance), Class 2 and Class 3 (vary rarely available). Within each of these Classes, tolerances are given according to the measured temperature.
Note that it is quite possible for a thermocouple to be more accurate that an RTD and careful consideration should be given to the various factors in the application. It should be noted that whilst the probe itself may be more accurate, this benefit may be lost with your instrumentation.
In some cases, special tolerance thermocouple wire is available for certain applications. For example we can supply special tolerance type K thermocouples for use in Aerospace applications that require conformance to AMS2750. Note however that minimum order quantities normally apply.
What is meant by Exposed, Ground and Insulated Hot Junction?
Exposed Hot Junction – An exposed junction is where the two wires that make the junction are left without an external protection sheath. In the simplest form this could mean just a bare wire design or the junction can be constructed so that it extends out the end of a protection sheath. Exposed junctions give the fastest response time of any thermocouple construction but are only suitable for unpressurised, dry and non-corrosive applications.
Grounded Hot Junction – A grounded thermocouple has the thermocouple electrically connected, or grounded, to the protection sheath, normally by welding or silver soldering so that it becomes an integral part of the sensor tip. This type of construction can also give fast response times but with the added advantage of being suitable for pressurised applications. This junction type is popular in the Plastics Industry.
Insulated Hot Junction – As the name suggests the thermocouple junction in this design is electrically insulated from the protection sheath. Response times are slower with this construction but this is now the most common type due to its ability to avoid interference from earth loops in machinery.
What temperature range are Thermocouples suitable for?
This is very much dependent on type but the overall range for all thermocouple types is -200°C to +1800°C. There are many factors to consider when selecting a thermocouple type for an application and we would be happy to offer assistance where possible. The life of a thermocouple is also something to consider, particularly at extreme temperatures and it may be worth considering Infrared Pyrometers instead which can measure temperature from outside the process.
What does Mineral Insulated mean?
A Mineral Insulated Thermocouple is a reference to the type of construction used. The thermocouple conductors are extruded along with the protection sheath and a densely packed oxide powder that provides the electrical insulation. Various protection sheaths are available as well as many of the standard thermocouple types giving massive versatility.
A mineral insulated thermocouple has advantages over a traditional fabricated thermocouple as follows:
Semi-flexible – The probe part of the thermocouple can be bent to almost any shape on site without special equipment. The oxide powder inside the probe helps to support the tubular sheath during bending, avoiding kinking. Minimum bend radii apply.
Fast Response – Mineral insulated thermocouples can be made in remarkably small diameters, down to 0.25mm diameter. More common sizes range from 1mm to 6mm. The smaller the diameter, the faster the response time and they can be used in pressurised applications.
Robust – The mineral insulated cable used to make this type of thermocouple is extremely resilient and can be used in high pressure applications. It can even be used in applications where there is a risk of crushing.
Long lengths – Due to the way the mineral insulated cable is made, it is supplied in long lengths. It is therefore quite possible to manufacture a thermocouple probe that is many metres long. This makes it ideal for temperature measurement in arduous or extreme applications where traditional cables are not appropriate.
Cables and Connectors
Due to the way a thermocouple works, it is very important to maintain the conductor combination through the complete measuring circuit. This includes all cables and connectors from the probe all the way to your instrument. You must not use copper based cables or cable with a different thermocouple type. The polarity must be maintained throughout as well, you need to keep an eye on positive and negative.
If you have very long cable runs, this can add up to being expensive as the quantity of cable builds up. One way around it is to use a temperature transmitter mounted locally to the probe. This will enable you to use a single pair copper cable which potentially has a lower cost.
Process Parameters can advise on which cables and connectors you need and of course supply them with your probes.
What is the difference between extension and compensating cable?
Nearly all thermocouple types, with the exception of the rare metal types such as R, S and B, have Extension cables available for them. This means that the cable is made from the same alloy combination as the thermocouple itself and means you can maintain the thermocouple combination throughout the measurement circuit.
For some thermocouple types however, the cost of the conductors makes this prohibitive for even the shortest of cable runs. Clearly this is most apparent with type R, S and B which include Platinum in their make-up. It also applies to a lesser degree to type K.
In these cases it is appropriate to use a Compensating cable. These cables do not use the same alloy pair as the thermocouple itself but a lower cost combination that gives characteristics as close as possible to the thermocouple pair. This gives potentially major cost savings.
Note that there are some stringent operating conditions when choosing to use Compensating cable over Extension cable. For type K the junction between the thermocouple and cable must be at a temperature below 100°C otherwise it is possible to introduce errors.
If you would like additional assistance, we can help you. Please contact us and we would be glad to discuss your application and help with temperature probe selection.