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Snap-action type thermostats are commonly used in our homes for controlling the temperature set point of ovens, irons, immersion hot water tanks and they can be found on walls to control the domestic heating system. But it has large hysteresis range on the opening and closing of electrical contracts. That’s why commercially available bi-metallic thermostats have temperature adjustment screws, by which it is possible to adjust the temperature at desired level and pre-set hysteresis level.

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1. Thermocouple

Thermocouples are the most common temperature measuring device. It consists of two dissimilar metals, joined together at one end.

Working principle

  Thermocouple works on the basis of Seebeck effect .If two conductors of different materials are joined at one point, an EMF is created between the open end which is dependent upon the temperature of the junction. As T1 increases, so does V. The EMF also depends on the temperature of the open ends T2. The junction is placed in the process, the other end is in iced water at 0C. This is called the reference junction. The voltage is directly proportional to temperature.

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Types of thermocouple                       

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 http://www.thomasnet.com/articles/automation-electronics/Thermocouples-Types

Advantage

• It is simple and rugged in construction
• It can measure wide range of temperatures up to 2600°C
• Fast Response
• Inexpensive
• Calibration can be checked easily
• It gives fastest response to temperature change.
  
  

Limitations

• It is least stable and least repeatable.
• It requires cold junction compensation for accurate temperature measurement.
• The emf induced verses temperature characteristics is non linear.
• Lowest accuracy.

Application

• Thermocouple is extensively used in steel processing, turbine and diesel engine for temperature measurement
• It is used in gas feed heat appliances such as ovens and water heaters.
• It is used in power production.
• A thermocouple can be used as a vacuum gauge over the range of approximately 0.001 to 1 torr absolute pressure
• Thermoelectric cooling
• Medical equipment and Packaging equipment

references:

http://www.electronics-tutorials.ws/io/io_3.html

https://controls.engin.umich.edu/wiki/index.php/TemperatureSensors

http://www.thomasnet.com/articles/automation-electronics/Thermocouples-Types