Thermocouples can be found in several different types, each differing due to the combinations of alloys being used in making the thermocouple. The type of thermocouple is chosen considering their application, stability, cost, availability, chemical properties, temperature ranges, and output.
This blog is written to give a better understanding of the various types of thermocouples available in the market today along with their range and characteristics.
The K-type thermocouple has the widest temperature measuring range and is the most commonly used type of thermocouple. In Type K thermocouple, around 10% chromium is used along with 90% nickel to construct the positive lead. Then a mixture of 95% nickel, 2% manganese, 2% aluminium, along with 1% silicon is created in order to form the negative lead.
Positive lead in the Type K thermocouple is a non-magnetic material that is colored in yellow. The negative lead is a magnetic material and colored in red, with the overall jacket being colored in yellow. The type K thermocouple operates in the temperature ranges that fall between -328˚ F to +2300˚ F (-200˚C to +1260˚C). These types of thermocouples are mostly used inside appliances that are in need of temperature sensitivity in the range of 41μV/˚C . They are also inexpensive.
Deviation in alloys is responsible for determining the accuracy of thermocouples. The type K thermocouple provides or offers an accuracy of ±2.2˚ C (0.75%).
For a Type K thermocouple to give its peak performance, a clean oxidising atmosphere is required. It is recommended not to use Type K for partially oxidising conditions in a vacuum.
The T-Type thermocouple mostly finds its use case in measuring low temperatures. The negative lead is composed of constantan (55% copper and 45% nickel) and the positive lead is composed of copper.
With a sensitivity of 43 µV/°C, the type T thermocouple is suitable for work in an oxidising atmosphere. The type-T thermocouple operates most efficiently between the temperature range of -200°C to 350°C.
The Type J thermocouple has gained the reputation of being the most widely used thermocouple, and that could be because of its low-cost. In J type thermocouples, constantan which is a mixture of 55% copper and 45% nickel is used in making the negative lead while iron is used in the making of the positive lead. The negative terminal is colored red and the positive lead is colored white, with the overall jacket colored in black.
For type J thermocouples the temperature range is between -346˚F to 1400˚F (-210˚C to 750˚C). This type of thermocouple has a short life span compared to type K thermocouple and a smaller temperature range. But this type of thermocouple is best when it comes to oxidising atmospheres.
The accuracy of type J thermocouple is ±2.2˚C (0.75%). These types of thermocouples are not suggested for lower-temperature applications. This type of thermocouple has a sensitivity of approximately 50μV/˚C.
This type of thermocouple has stronger signal and higher accuracy than type K and J thermocouple while at moderate temperature ranges. The type E thermocouple is responsible for producing the highest EMF per degree when compared with other types of thermocouples.
A mixture of 1% silicon, 2% manganese, 2% aluminium and 95% nickel are used in making constantan which forms the negative terminal, while a mixture of 10% chromium and 90% nickel called nickel-chromium forms the positive lead. The negative lead-colored red and the positive lead is colored purple. The type E thermocouples have a standard accuracy of ±1.7˚C and have temperature ranges between 454˚F to 1600˚F (-270˚C to 870˚C)
This type of thermocouple has its use cases mainly where fast response and high accuracy are required. Sulphuric environments and low oxygen or vacuum applications are something to be avoided when using this type of thermocouples. The price of type E thermocouples is much higher when compared to the likes of type J and K thermocouples.
Designed by Noel A. Burley, Type N thermocouple have temperature and accuracy limits which are pretty similar to that of the type K thermocouple. The temperature for type N thermocouples ranges between -270 °C and 1300 °C. Sensitivity is 39μV/˚C which is slightly lower than type K thermocouples .
Type N thermocouple is made up of a mixture of Nisil and Nicrosil. Where Silicon, Nickel, and Chromium are mixed together to form Nicrosil, which is used to make the positive wire. Likewise, Nickel and Silicon are combined to form Nisil which is used in making the negative wire.
This thermocouple is the best alternative available for type K thermocouples to operate in low oxygen conditions. This type of thermocouple finds a use case in the field of oxidising atmosphere, inert atmosphere, dry atmosphere or vacuum.
There is a huge demand for Type S thermocouples for applications in higher temperatures. As this type of thermocouples provide stability and high accuracy, they are sometimes used for low temperatures. The temperature of type S thermocouples ranges between 630 °C to 1064 °C. Platinum is used in making the negative lead while a mixture of 10% Rhodium and 90% platinum is used to make the positive lead. Usually, this type of thermocouple is utilised in applications like Biotech industries and Pharmaceutical where the high temperature must be measured with high accuracy.
Type R thermocouple is made from a mixture of Rhodium and Platinum. But type R thermocouples have a slight advantage when it comes to stability and output range when compared to that of the type S thermocouple.
The negative lead is composed of Platinum and the positive lead of type R thermocouple is composed of 13% Rhodium and 87% Platinum. These types of thermocouples have a temperature range between 0 °C to 1600 °C.
Type B thermocouples are also composed of a combination of Rhodium and Platinum. The negative lead is composed of 6% Rhodium and 94% Platinum, and the positive lead of a thermocouple is composed of 30% Rhodium and 70% Platinum. Type B thermocouples are used in measuring temperature up to 1800°C. But the output of this thermocouple is much lower compared to type S and R thermocouples.
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