The traditional pirani vacuum gauge, originally invented in 1906 by Marcello Stefano Pirani, is based on a hot metal wire suspended in a tube that is exposed to gas pressure media. The Pirani gauge measures the vacuum pressure dependent thermal conductivity from the heated wire to the surrounding gas. The heated pirani sensor filament is typically made of a thin (<25 µm) Tungsten, Nickel or Platium wire.
As gas molecules collide with the filament wire, heat is transported from the hot wire. The heat loss is a function of the gas pressure and at low pressure the low gas density and long mean free path between gas molecules provides a low thermal conductivity. At high pressure the high gas density and short mean free path between molecules will result in high thermal conductivity.
The pirani wire filament is typically operated in a balanced Wheatstone bridge circuit where one leg of the bridge is the pirani filament and the other three elements of the bridge circuit balance and temperature compensate the circuit.
The filament wire is maintained at a constant temperature and when the gas density changes and thereby thermal conductivity changes the energy required to maintain the wire changes accordingly. Consequently, the voltage supply to the Wheatstone bridge becomes vacuum pressure dependent and the measured bridge voltage can be converted to a pressure value.
The Pirani gauge measures the thermal conductivity of the gas and therfore the measurement is dependent of the gas properties.
High performance MEMS Pirani
The Pirani working principle was modernized with MEMS (microelectromechanical system) sensor technology where the wire filament has been substituted with a resistor deposited on an ultra-thin silicon suspended diaphragm. The MEMS Pirani sensor technology offers superior performance compared to the traditional wire pirani and convection type vacuum gauges. The miniaturization of geometry, reproduceable semiconductor manufacturing processes, low operating temperature and options for improved temperature compensation all contribute to much better measurement performance.
The first commercially available MEMS sensor based heat-loss pirani transducer was introduced in 1993 by the Danish company Wenzel Instruments.
In 2019 a new next-generation MEMS based SmartPirani™ transducer was introduced by Sens4. The novel patent pending SmartPirani™ technology extends the measuring range down to 1.0E-6 mbar (7.5E-7 Torr) and offers best-in-class performance with unmatched measurement stability.
For more information about the Sens4 SmartPirani™ visit the SmartPirani™ transducer product page.
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