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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 voltage which
can be read as a pressure on meter. The pirani gauge
head indicates the total pressure of combined gases and
other vapors in the system on the heated wire, which can
loss heat to both gas and vapor molecules. The change
in resistance due to temperature build-up, of a coil
filament, which is supplied with constant voltage placed
in Wheatstone bridge configuration, consequent to the
pressure and thermal conductivity which is directly read
by the gauge in terms of m.bar.
The gauge consists of
the stabilized power supply and the indicating meter
with the arrangement to select any of two gauge heads.
Each gauge head consists of bridge and calibration
elements. These heads are factory calibrated with full
interchangeability. Each gauge head is provided with `O'
ring coupling, to braze to customer's vacuum system.
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