Rare Earth

• Rare earth thermocouples are types B, R and S, and are based on platinum-rhodium alloys. They are the most stable of the thermocouples, are expensive, and are used for extreme temperature measurements.
  
  

Nickel Alloys

• These are types K and N, and they both use a chromel alloy as one of the probes. Chromel alloys are made from nickel-chromium. K is the most popular, being general and all-purpose, and N was constructed as a replacement for K.
  
  

Constantan

• Constantan is a copper-nickel alloy, and thermocouples based on it are types E, J and T. E is especially useful for cryogenics, J for older instruments and T for moist and extremely cold environments.
  
  

Function

• Thermocouples are useful for performing measurements that need to be done quickly. Each type has different roles, but among them are applications in the food and medical industry as thermometers, and in household appliances as safety features.
  
  

Considerations

• Thermocouples need to be calibrated very carefully before usage, and their output signals may have problems with noise. Each type has specialized problems as well; for example, K cannot be used in high temperature environments where this is not enough oxygen, lest it turns green and corrodes.