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 by a thermocouple?
Thermocouple Wire Types Explained
Thermocouple Wire Types
The different types of thermocouple wire refer to the combination of metals or alloys used.
Type K Thermocouple Wire
Type K is one of the most popular types of thermocouple. It consists of a positive Nickel/Chromium alloy leg and a negative Nickel/Aluminium leg. It can be used across a wide temperature range (from -200°C to 1250°C), making it suitable for many applications in a variety of industries. It is a much more affordable option than the Platinum based Type R and Type S thermocouples which can be used at higher temperatures.
Type J Thermocouple Wire
Type J is another common type, made from a positive Pure Iron leg, and a negative Constantan (Copper/ Nickel alloy) leg. It has a smaller temperature range than type K – ranging from 0°C to 750°C but the materials in the conductors make it very cost effective. The Iron positive leg makes it very susceptible to moisture, but it is used extensively in indoor, dry applications.
Type T Thermocouple Wire
With a positive Pure 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 be used to measure temperature in freezers. It is also one of the most accurate thermocouple combinations, making it popular in circumstances where a close control of temperature is required.
Type E Thermocouple Wire
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 as it has the highest voltage output per °C.
Type N Thermocouple Wire
Type N is a relatively new thermocouple combination having been created in the 1960s. It is an improvement on the popular Type K due to its superior repeatability characteristics around 400°C. Type N consists of a positive Nicrosil (Nickel/Chromium/Silicon) leg and a negative Nisil (Nickel/Silicon) leg. The addition of the specific quantities of Silicon reduces the ‘ageing’ effect at 400°C which adversely affects Type K . It can be used at temperatures from -270°C to 1300°C. Type N may be used for measuring engine exhausts, among other applications.
As each of these thermocouple combinations produces a different voltage output per °C (Seebeck Coefficient) it is not possible to exchange one for the other without ensuring the meter is recalibrated and that no part of the circuit contains a thermocouple conductor of another type.