Temperature sensors come in different forms so selecting a temperature sensor for a particular application requires a bit of thought. However, having said that for most industrial applications the choice tends to be betweeen using an RTD, or a thermocouple.
What is a Thermocouple?
A Thermocouple is a sensor used to measure temperature. A thermocouple consists of two dissimilar metals, joined together at one end, which produce a small voltage when heated (or cooled). This voltage is measured and used to determine the temperature of the heated metals. The voltage for any one temperature is unique to the combination of metals used.
Thermocouples are typically selected because of their low cost, high temperature limits, wide temperature ranges, and durable nature.
Are There Standards Governing Types of Thermocouple?
British Standards Specification, BS 1041, Temperature Measurement provides guidance for the selection and use of devices for measuring temperature.
BS EN 60584-1: Thermocouples BS EN 60584-1 is an International Standard that relates to electromotive force (EMF) generated by the designated thermocouple types to temperature, based upon the International Temperature Scale of 1990 (ITS-90).
ASTM Standard E230 provides specifications for the common industrial grades, including the letter designations used for each type of thermocouple.
Why Are There Different Types of Thermocouple?
Thermocouples are available in different combinations of metals, usually referred to by a letter, e.g. J, K etc giving rise to the terms type J thermocouple, type K thermocouple etc. Each combination has a different temperature range and is therefore more suited to certain applications than others. Although it is worth noting that the maximum temperature varies with the diameter of the wire used in the thermocouple. The various types are detailed in the chart below.
Chart of Thermocouple Types
Type B Thermocouples
Type B thermocouples can be used up to 1600°C with short term excursions up to 1800°C. They have a low electrical output, therefore are rarely used below 600°C. In fact the output is virtually negligible up to 50°C, therefore cold junction compensation is not usually required with this type.
Type E Thermocouples
Type E thermocouples are often referred to as Chromel-Constantan thermocouples. They are regarded as more stable than Type K, therefore often used where a higher degree of accuracy is required.
Note - Constantan is Copper-Nickel.
Type J Thermocouples
Type J thermocouples degrade rapidly in oxidising atmospheres above 550°C. Their maximum continuous operating temperature is around 750°C though they can with stand short duration excursions to 1000°C. They are generally not used below ambient temperature due to condensation forming on the wires leading to rusting of the iron.
Note - Constantan is Copper-Nickel.
Type K Thermocouples
Type K thermocouples are the most widely used thermocouples in the Oil & Gas, and refining industries due to their wide range and low cost. They are occasionally referred to as Chromel-Alumel thermocouples. Note that above about 750°C oxidation leads to drift and the need for recalibration.
Type N Thermocouples
Type N thermocouples can handle higher temperatures than type K, and offer better repeatability in the 300 to 500°C range. They offer many advantages over Type R & S at a tenth of the cost, therefore prove to be popular alternatives.
Type R Thermocouples
Type R thermocouples cover similar applications as Type S but offer improved stability and a marginal increase in range. Consequently, Type R tend to be used in preference to Type S.
Type S Thermocouples
Type S thermocouples can be continually used at temperatures up to 1450°C. They can with stand short duration excursions up to 1650°C. They need protection from high temperature atmospheres to prevent metallic vapour ingress to the tip resulting in reduction of emf generated. Protection commonly offered is a high purity recrystallised alumina sheath. For most industrial applications thermocouples are housed in a thermowell.
Type T Thermocouples
Type T thermocouples are rarely used in industrial applications, and lend themselves more to use in laboratory situations.
The following pages on Control and Instrumentation.com give more detail on the techniques used in temperature measurement:
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For those who want to delve further into the world of thermocouples and temperature measurement, then the following books from Amazon will be of interest: