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Bi-Metallic Temperature Sensors

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Solids tend to expand when heated. The amount that a solid sample will expand with increased temperature depends on the size of the sample, the material it is made of, and the amount of temperature rise. The following formula relates linear expansion to temperature change:
L = Lo (1 + αΔT)

L = Length of material after heating
Lo = Original length of material
α = Coefficient of linear expansion
ΔT = Change in temperature

For any given metal, the coefficient of expansion is usually very small. To increase the expansion when the metals are heated, two metals of differing coefficient of expansion are bonded together and used in practical bi-metallic temperature sensor applications. The two bonded strips of metal produce a bending motion that greatly exceeds the linear expansion. This device is called a bi-metal strip. The bending motion of a bi-metal strip is significant enough to drive a pointer mechanism, activate an electromechanical switch, or perform any number of other mechanical tasks, making this a very simple and useful primary sensing element for temperature.

In the common bi-metallic strip industrial temperature gauge shown below:

The bi-metal strip is shaped into a helix. The helix is fixed at one end. The otherend of the helix is free to rotate a shaft which is fixed to it. The heat applied to the bi-metal strip at the fixed end causes the helix to unwind and turn the pointer on the dial.

The operating ranges vary to cater for different applications, the lower range being from -70 oC to 50 oC, with a higher range available of up to 100 oC to 550 oC. Bimetallic thermometers can be used in a Thermowell. A Thermowell has the added advantages of allowing the removal or replacement of the device without opening up the process tank or piping. They are typically in the helical form when used in thermowells.

Vibration and heat transfer can be a problem with some applications. However, selecting a unit that is completely sealed can overcome these limitations. A dry gas is generally used in the dial face portion of the assembly while silicone fluid fills the stem and surrounds the coil. Having fluid around the coil can assist in mechanical damping and heat transfer.

Advantages of Bimetallic temperature sensors
  • Inexpensive
  • Simple construction
Disadvantages of Bimetallic temperature sensors
  • Limited accuracy
  • Indication or simple switching only
  • Localised measurement only
  • Easily decalibrated due to mechanical shock
  • Hysteresis

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