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Temperature transducers

 Temperature transducers

Aircraft use a variety of temperature transducers, or sensing devices, ranging from the simple bi-metallic strip or thermostat through to thermistors and thermocouples.

The type of sensor used depends on the application and takes into account:

temperature range

accuracy requirements

type of output required

cost.

 

Bi-metallic strip

A bi-metallic strip converts temperature changes into mechanical displacement as an on/off measurement.

The device consists of two strips of different metals with different coeffi cients of thermal expansion. The strips or elements are joined together by rivets, by brazing or by welding .

 

Differential expansion causes the element to bend one way when heated, and in the opposite direction when cooled below its nominal temperature. The metal with the higher coefficient of expansion is on the outer side of the curve whilst the element is heated and on the inner side when cooled.

 

 The element can be formed as a switch contact, called a thermostat, to open or close a circuit when temperature limits are exceeded.

Alternatively, the element is formed in a spiral shape so that temperature changes cause a shaft to rotate; the typical application for this is an outside air temperature indicator.

 

The sensing portion of the indicator projects through windscreen

into the ambient air.

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