What is a Thermocouple: A thermocouple is a device used for the temperature measurement of physical bodies, components, or substances. Thermocouple measure the temperature of the process media flowing with the help of a sensing element called a sensor. The thermocouple is an essential type of temperature-measuring instrument which is very often used in the oil and gas industry, pharmaceutical, petrochemical, and other industries.
Construction of Thermocouple: Thermocouples mainly consist of two wires having different metals, when these dissimilar wires are joined together, complete the circuit. Each wire has its junction, so typically there are two junctions. The first junction of unknown temperature is known as the hot temperature junction and the second junction whose temperature is stable and identical known as the cold or reference junction.
Working of Thermocouple: The working of a thermocouple is based on the thermoelectric principle. A thermocouple consists of two dissimilar wires which are joined together to form a thermoelectric circuit. One junction is connected to the body source whose temperature is to be measured (Unknown Temperature) referred to as a hot junction and the other is the reference junction whose temperature is known.
The difference in temperature of these two-state junction voltages is produced across the circuit. This generated voltage correlated back to calculate the temperature of the unknown or hot junction.
Types of thermocouples: Types of Thermocouple are found generally four which are listed below:
- Type J
- Type T
- Type K
- Type E
Now we will understand each type of thermocouple in the description:
Type J: Type J Thermocouple is a common and mostly used Thermocouple. The measurement range for the J Type thermocouple is smaller and the same is mentioned below. If the Type J Thermocouple is used for high-temperature measurement purposes regularly than the life cycle of the J-type thermocouple becomes shorter.
Type J Thermocouple is made up of Iron wire for the positive leg and copper-nickel alloy for the negative leg. This type of thermocouple is inexpensive and suitable for reducing (non-oxidizing) atmosphere and vacuums (not be used in a condition where moisture is available).
Temperature range suitability for J Type Thermocouple:
- Thermocouple grade wire, -346 to 1,400 °F (-210 to 760 °C)
- Extension wire, 32 to 392 °F (0 to 200 °C)
Type K: Type K Thermocouple is made up of wires material which combination of nickel (chrome-alumel). The positive leg is composed of 90% nickel, and 10% chromium, whereas the negative leg contains 95% nickel, 2% aluminum, 2% manganese, and 1% silicon on an approximation basis.
The chrome-alumel material section makes Type K Thermocouple cost-effective with accuracy with standard deviation and reliable technique which can be used for high-temperature applications. The temperature range for Type K Thermocouple is more than the Type J thermocouple indicated below.
Temperature range suitability for K Type Thermocouple:
- Thermocouple suitable for (-200 to 1260 °C)
- High accuracy with standard deviation +/- 0.75.
Type N: Type N thermocouples consist of wires made up of nicrosil-nisil alloy (nickel, chromium, and silicon). This type of thermocouple contains properties of K type thermocouple. The type N Thermocouple temperature range is higher than the Type K thermocouple. They are expensive, and stable in comparison with K-type thermocouples.
Temperature range suitability for N Type Thermocouple:
- Thermocouple suitable for (-270 to 1300 °C)
- Accuracy with standard deviation +/- 0.75.
Type T: Type T type thermocouple is generally used for low-temperature and cryogenic applications. Type T thermocouple wire is made up of copper constantan alloy. It can perform well in oxidizing conditions too.
Temperature range suitability for T Type Thermocouple:
- Thermocouple suitable for (-200 to 350 °C)
- Accuracy with standard deviation +/- 0.75.
Selection of Thermocouple: Selection of the thermocouple is dependent upon the several factors mentioned below:
- Application and location of thermocouple sensor to be used.
- Temperature range for which thermocouple probe to be exposed.
- Installation requirement (compatibility to be checked with equipment to be used).
- Thermocouple resistance to the process media of the equipment (corrosive or high-pressure gas).
Images Reference:
https://www.yourelectricalguide.com/2019/06/thermocouple-working-principle-types-advantages
https://grabcad.com/library/thermocouple