The ability to measure temperature has created a mountain of opportunities, which have consequently shaped the world we live in today.
From facilitating the cooking of food, to complex engineering and medical science, monitoring temperature lies at the heart of these operations.
So… how is temperature measured?
Thermocouple Wire Types Explained
What Is A Thermocouple Used For?
A thermocouple presents one method of measuring temperature. Generally speaking, a thermocouple will consist of two wire conductors, or legs, which can generate a voltage when one end, or junction, is heated. A meter is used to read this voltage created within the closed circuit, and when considered alongside the temperature of the cold junction, it can be used to calculate the temperature of the heated end.
Whilst a thermocouple can register a wide range of temperatures, its overall capacity will depend on the types of wire used.
Thermocouples are used in many industries, including aerospace and automotive for use within aircraft and vehicle engines, alongside domestic and public applications - being used as thermostats and for the careful regulation of gas appliances.
What Is A Thermocouple Wire?
As established, the wires within a thermocouple are essential to its operation, and it pays to understand the wires within your thermocouple.
A thermocouple utilises two different metallic materials to create – these materials will depend on the application of the thermocouple, and the conditions it must withstand. The grade of the wire is likewise affected by the combination of metals, or purity of alloys.
These two materials are joined at one end, which is heated and therefore known as the ‘hot’ end, and left separate at the other - the ‘cold’ end. Whilst temperature is measured at the hot end, it is the difference in temperature between the two ends which creates a voltage.
Thermocouple Wire Types
The type of thermocouple wire refers to the combination of materials, or alloys, used. There are several established and commonly used types of wire.
Type K is one of the most popular types of thermocouple wires. It consists of a positive nickel/chromium alloy leg and a negative nickel/aluminium leg. It can withstand a vast temperature range (from -200°C to 1250°C), making it suitable for many applications in a variety of industries. This is furthered by type K being an affordable option for many when compared with the Platinum based Type R and Type S thermocouples.
Type J is another common wire type, made from a positive iron leg, and a negative constantan (copper/ nickel alloy) leg. It does have a smaller temperature range than type K – ranging from 0°C to 750°C – meaning it has fewer applications. It may typically be seen in manufacturing operations or aircraft.
With a positive copper leg, and negative constantan leg, type T can be used in temperatures from -250°C to 350°C. Due to its capabilities in cold conditions, type T may measure temperature in freezers.
Type E consists of a positive nickel/ chromium leg and a negative constantan leg. Type E has a temperature range of -200°C to 900°C, and is particularly sensitive to measuring temperatures.
Type N is a relatively new thermocouple wire which arguably rivals the popular type K due to its superior repeatability characteristics around 400°C. It features two nickel alloy legs, and can withstand temperatures from -270°C to 1300°C. Type N may be used for measuring engine exhausts, among other applications.
Thermocouple Wire Colours
The thermocouple wire type can be identified by the colour of the individual wires, as well as their jackets. However, this colour coding will differ depending on nationality.
For example, the old British Standard type K was identified by a red jacket encompassing a brown positive leg and a blue negative leg whereas the newer IEC/ISO standard Type K has a green jacket over a positive green leg and a negative white.