Significance

• The Seebeck effect is named after Thomas Johann Seebeck. He discovered that he was able to generate an emf, which is a voltage, when he formed a junction with two metals held at distinct temperatures. Seebeck found that the larger the temperature differences between the metals, the higher the generated voltage, regardless of their shapes. His discovery is the basis of all thermocouples.
  
  Thermocouples are valuable in science and engineering, and they are inexpensive, have a fast reaction time, are small in size, and are dependable. They can accurately measure extreme temperatures, with ranges from --270 to 2,500 degrees Celsius, and errors within 0.5 to 2 degrees C.
  
  Thermocouples have some disadvantages. They need to be calibrated very carefully before usage. Their output signals are very small, and so they may have a problem with noise. They may be prone to metal corrosion and strain.
  
  

Types

• Thermocouples are classified according to the maximum amount of temperatures they can measure, their function, and their ruggedness. There are hundreds of types, but the most common ones are J, K, T, and E. They are constructed from rare metals, nickel based alloys, or constantan.
  
  

Rare Metals

• Rare earth thermocouples are types B, R, and S and are based on platinum-rhodium alloys. They are the most stable thermocouples, and are used for extreme temperatures. They are also very expensive, and are prone to contamination. B is not useful below 50 degrees C, while R has low sensitivity. S is so stable that it is used to calibrate the melting point of gold.
  
  

Nickel Alloys

• These are types K and N. K is the most popular, and is general and all-purpose. It comprises chromel paired with alumel. Chromel alloys are made from nickel-chromium, while alumel ones are based on nickel-aluminum. N is very stable, and is an inexpensive alternative to platinum for high-temperature measurements.
  
  

Constantan

• Constantan is a copper-nickel alloy. Thermocouples based on it are types E, J and T. E is paired with chromel, is especially useful for cryogenics, and is nonmagnetic. J is made using iron, and is used with older instruments. T is constructed with one lead being copper. A common usage is in moist and extremely cold environments. T is noted for resisting corrosion.
  
  The drawback is that copper has a large thermal conductivity, so care must be taken in the calibration and usage of constantan based thermocouples.
  
  

Uses

• Thermocouples are useful for performing measurements that need to be done quickly, especially for sites that need extreme temperature measurements. They can even be constructed quickly with soldering or welding at the place of interest. When it is necessary to collect many data points, they can be combined to form thermopiles.