What is a thermocouple and where is it used?
What is a thermocouple?
A thermocouple is a sensor that is used for measuring temperature. The sensor has two dissimilar metal wires joined at one end and is connected to a thermometer or another thermocouple-capable device at the other end. When they are correctly configured, they can provide temperature measurements across a broad range of temperatures.
Stable thermocouples are highly versatile as temperature sensors, and so are often used in various applications, from industrial use to a regular thermocouple you can find on utilities and standard appliances. There are many different models and technical specifications for thermocouples, so it’s very important to understand the basics of how it works, its structure, and its ranges to get a better insight into what type of thermocouple and material are right for your application.
How does a thermocouple work?
When two wires made of different metals are joined together at both ends and one of the ends is then heated, there is a consistent current which flows in the thermoelectric circuit. If the circuit breaks at the centre, the net open circuit voltage is a part of the junction temperature and composition of the two metals. This means that when heat or cold is applied to the junction of the metals a voltage is produced that can be linked back to the temperature.
Thermocouples can come in multiple different calibrations or combinations of metals. The most commonly used are the base metals referred to as N, T, E, J, and K types. As well as this there are high temperature calibrations called noble metals. These are types R, S, C, and GB.
Where are thermocouples used?
Thermocouples are the most commonly used temperature sensors in the world because they can measure a wide range of temperatures, are durable, and are relatively inexpensive. When it comes to high temperatures, fast response, small temperatures, and a high vibration, you will likely find a thermocouple wire collecting the temperature measurements. Below we’ll be looking at just a few examples of where these sensors are used, so you will know how they can be applied in your operations.
Thermocouples are used in many different types of applications within the food and drink industry, such as:
- Clean-in-place sensors
- Penetration probes
- Oven control
- Food chain monitoring
- Hotplate control and monitoring
- Steam kettle temperature control
Extruders need high temperatures and pressures. Also, they have a unique thread adapter that works to position the tip of the sensor in the molten plastic under the high-pressure conditions located there.
Measuring low temperature
Type E, K, T, and N thermocouples can all be used to record low temperatures, as low as -200C. However, the alloys used need to be specially chosen for use at these temperatures to meet the published accuracies.
Many manufacturers ensure their alloys are calibrated for use from 0C and above. The same alloys can be used to measure down to -200C, but the accuracy might change slightly from the established values. If you purchase individual calibrations, you can determine offset values.
The right thermocouple for this application will depend on the furnace conditions it will be exposed to. When deciding on the right thermocouple, some factors that will need to be considered are:
- The temperature capabilities of the thermocouple cables
- Temperature capabilities of the sheath or protective coating (metal or ceramic)
- The environment it will be used in (air, reducing, oxidising)
- The configuration for mounting
It is difficult to measure the temperature of molten metal due to the high temperatures and harsh conditions involved. The only option for getting contact measurements in this area is to use Type K or N Base Metal thermocouples, or Types R, S, and B Platinum thermocouples.
When base metal thermocouples are used, the wires are generally large in diameter, with ceramic insulators and ceramic and/or metal protection tubes. The increased diameter of the Type K or N wires degrade slower to enable enough time for measurements to be taken before the high temperature conditions break down the wires.
Platinum thermocouple wires, unlike the base metal Type K and N, will become soft instead of becoming corroded, due to long term annealing and ultimately fail because of grain growth.
Contact TRM today for help with temperature measurement solutions for your business’ operations.