Vacuum gauges are devices for measuring gas pressures below
atmospheric pressure. In many cases the pressure indication depends
on the nature of the gas. For instruments with indirect
pressure measurement, the pressure is determined as a function of
a pressure-dependent property (thermal conductivity, ionization
probability, electrical conductivity) of the gas that are proportional
to the number density of particles and thus to the pressure. The
vacuum gauges with gas-dependent pressure reading include the
ionization vacuum gauge, the viscosity gauge and the thermal
conductivity vacuum gauge [1e11]. Thermocouple gauges belong to
the class of vacuum gauges which rely on the thermal transport
qualities of gases. The thermocouple gauge uses the thermal conductivity
property of gases, by incorporating a wire filament which
is heated by a constant source of power. Attached to this filament is
a thermocouple, which measures the temperature of the wire. At
high pressures, the large number of gas molecules striking the
heated wire carries energy away and cools the wire. At low pressures,
the smaller number of gas molecules striking the wire cause
less cooling, and thus a higher temperature. The thermocouple
output voltage responds to these temperature changes to give an
indication of pressure: low gas pressure gives high filament temperature
which gives high thermocouple output voltage; high gas
pressure gives low filament temperature which gives low thermocouple
output voltage. The meter measuring the thermocouple
voltage is calibrated in pressure units to give a direct indication of
pressure. At pressures below about 101 Pa, the heat loss from the
filament is primarily through radiation since the density of gas
molecules is so low. Since the heat loss due to radiation is constant,
the resulting temperature corresponds to the “zero” reading on the
meter. The thermocouple gauge is a simple, rugged device which is
very useful at rough vacuum pressures.