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Change
of pressure in vacuum systems brings about a rise or
fall in number of gas molecules present and hence a rise
(or) fall in the thermal conductivity of the gas. Thus
the heat loss of constant voltage electrically heated
filament in the system varies with the pressure.
The pirani gauge head filament has high temperature
co-efficient of resistance. So a slight change in system
pressure brings about useful change in filament
resistance resulting in an out of balance of the wheat
stone bridge where filament forms one of the arms.
This electrically heated filament is an arm of a self
balancing wheat stone bridge circuit. An automatic
control amplifier corrects bridge voltage automatically,
so that the filament temperature is kept constant around
120°C throughout the whole measuring range. Thus the
required bridge voltage (which varies depending upon the
pressure at the sensor head filament) is a measure of
the pressure which after proper conditioning and
calibration is displayed on the meter.
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