A thermocouple is a type of device used for measuring temperature. Consisting of two pieces of wire made of different metals, thermocouples rely on the thermoelectric effect whereby the two strips are attached at each end to form a loop with two junctions. If the temperatures at each junction is different, such as if one junction were to be inserted into something very hot or cold, then an electrical voltage is created in the loop.
There is a relationship between the voltage and the difference between the temperatures at each junction, so by measuring the voltage, the temperature of the ‘measuring’ junction can be ascertained. But why bother with all this? What’s wrong with a regular glass-and-mercury thermometer? The problem arises when you want to measure something very hot or very cold, such as a furnace or forge. Traditional thermometers cannot stand such temperatures themselves.
Thermocouples, on the other hand, have operating temperatures from below -250 °C up to 1700 °C, depending on the metals used. For this reason, they are predominantly used in industrial and scientific applications.
Thermocouple Types and Applications
There are many different types of thermocouples offering a variety of temperature tolerances and constructed from different metals. The main types are denoted by alphabetical letters.
The Type K is used as a general-purpose thermocouple, offering a wide range of temperatures from -200 °C to 1200 °C. Made from two alloys, Nickel-Chrome and Nickel-Aluminium, in a variety of probe types, the Type K boasts sensitivity of around 41 microvolts per degree Centigrade. The Type K is commonly used for carrying out safety tests on heating appliances as well as measuring temperatures in industrial process plants, such as petroleum refineries and chemical production facilities, but can found across many other industries too.
Another commonly used thermocouple, the Type J is made from iron and a copper-nickel alloy, offering a more limited range than the Type K, from -40 °C to 750 °C. The Type J does boast a slightly higher sensitivity of 50 microvolts per degree centigrade. Due to the metals used, the Type J can oxidise in damp environments, so it is most commonly used to monitor resin and plastics manufacturing processes, alongside measuring temperatures in vacuums and inert metals.
Widely used in the food industry, the Type T offers a high level of accuracy and is resistant to moisture without oxidising. Its relatively low temperature range, from -200 °C to 400 °C, and reasonable sensitivity of 43 microvolts per degree centigrade makes it even more ideal for food process monitoring. As such, the Type T is commonly used in food processing facilities to detect possible food safety hazards and ensure that regulations are complied with.
The Type N was developed as an improvement to Type K by the addition of Silicon. This reduced the effect of a hysteresis error found in Type K at around 400°C and also improved the corrosion resistance. Type N operates in the temperature range -270 °C to 1300 °C.making it suitable for temperature monitoring in furnaces, ovens and kilns, such as those found in brick manufacturing facilities. The Type N is also used for measuring engine exhausts and gas turbines, as well as in smelters to record temperatures throughout the entire production process of aluminium, steel and iron.
Using two alloys, nicrosil and nisil, the Type N offers slightly lower sensitivity than the Type K, at 39 microvolts per degree centigrade.