What are the different types of thermocouple detector?
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 Temperature 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.
