Thermocouple calibrations
Thermocouples are used in various industries to measure temperatures from as low as -200 °C to as high as 2500 °C, across a wide range of challenging environments. These environments could include:
- High vacuum conditions
- Oxidizing or reducing atmospheres
- High pressures (up to 2000 atmospheres)
- Acidic or salty environments
To accommodate these diverse environments, different types of metals and alloys are used, and these combinations are referred to as calibrations.
These calibrations are standardized to ensure consistent EMF generation at given temperatures.
There are three primary groups of calibrations:
- Base metal calibrations (type K, N, J, E, T)
- Noble metal calibrations (type R, S, B)
- Refractory metal calibrations (type C)
Base metal calibrations (type K, N, J, E, T)
Type K Calibration
The most commonly used calibration is type K (Chromel-Alumel) also called NiCr-Ni. Both legs of this thermocouple are high nickel content conductors with varying alloying components. This calibration is for temperatures up to 1250 °C.
Type N Calibration
This calibration uses Nicrosil- Nisil conductors and was developed more recently to cover applications where type K generates some problems. Can also be used up to 1250 °C but is more expensive than type K.
Type J Calibration
Also a very popular calibration is type J or Iron-Constantan. Type J generates a higher EMF than type K but has a maximum temperature rating of 750 °C.
Type E Calibration
This calibration was developed as a compromise between K and J. The two conductors are Chromel-Constantan and that allows a maximum temperature up to 900 °C.
Type T Calibration
This calibration consisting of Copper-Constantan is specifically suited for low temperature and cryogenic applications from -200 °C up to 350 °C. The fairly low max temperature is caused by the copper wire.
Noble metal calibrations (type R, S, B)
For applications at temperatures where the base metal thermocouple cannot be used anymore, so certainly over 1250 °C but depending on the application sometimes already over 1000 °C the industry offers three noble metal calibrations. These calibrations are all thermocouples based on pure platinum and/ or platinum/rhodium alloys.
Type R/S calibrations
Type R is a thermocouple with a conductor of platinum with 13% rhodium versus pure platinum. Type S is a thermocouple with a conductor of platinum with 10% rhodium versus pure platinum. The application determines which of the two calibrations is best suited.
Both calibrations can be used up to 1500 °C.
Type B Calibration
The main problem with type R and S is the fact that the pure platinum can easily be contaminated during use which will lead to a deviation and inaccuracy of the reading. Typ B thermocouples consists of one conductor of platinum with 6% rhodium versus a conductor of platinum with 30% rhodium. This will allow a maximum operating temperature of 1700 °C.
Refractory metal calibrations (type C)
For the extreme high temperature ratings thermocouples of the five refractory metals are used. These can be very specific for a given application but the only commonly used combination is type C.
Type C Calibration
This thermocouple consist of a conductor tungsten with 5% rhenium versus a conductor of tungsten with 26% rhenium. This thermocouple can be used up to 2300 °C. It is obvious the both noble metal thermocouples and refractory metal thermocouple are very expensive.
Hamitherm thermocouple connectors
Hamitherm supplies connectors for all the above mentioned calibrations. For the base metal calibrations Hamitherm uses only original thermocouple materials. For both the noble metal and the refractory calibrations Hamitherm uses compensation materials that follow the EMF output as close as possible.
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