MKS HPS 945 Series, Руководство по эксплуатации и обслуживанию

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A.1
Digital Pairani Vacuum Sensor System
Appendix A: How the Series 945
Pirani System Works
The Series 945 Pirani System is a heat-loss manometer which infers the
pressure of a gas by measuring thermal loss from a heated wire.
Theory of a Thermal Conductivity Sensor
A hot wire suspended from supports in a partial vacuum loses thermal energy
in three ways:
gas transport, which is pressure dependent,
end loss to the supports, and
radiation to surrounding surfaces.
Pirani and thermocouple sensors use pressure-dependent gas transport from a
hot wire to measure pressure. Because end loss and radiation are constant for
a wire at constant temperature, they provide a masking signal which largely
determines the low pressure limit of the sensor. Optimizing parameters of the
wire length and diameter, thermal emissivity, thermal conductivity, and wire
temperature can control end loss and radiation but not eliminate these terms.
A Pirani sensor may be operated at constant current, voltage, or resistance
(equivalent to temperature) at the sensor wire. The Series 945 Pirani System's
Sensor is operated at constant temperature to increase high pressure
sensitivity. At constant current or voltage, the wire temperature at high
pressure is much less than the temperature value at vacuum, reducing the
high pressure sensitivity.
Measurement Limits
At 10
-3
Torr, the sum of end and radiation losses is about 10 times the energy
lost to the gas through conduction. This determines the practical lower limit for
thermal conductivity sensors.
At pressures between 20 to 200 Torr, the gas acts as an insulation layer
around the filament, which reduces the sensitivity of a Pirani sensor.
At pressures above 200 Torr, only convection currents contribute to energy
transport from the filament. Convection currents are extremely sensitive to
orientation. However, since the Pirani Sensor for the Series 945 System
operates to 100 Torr, it can be installed in any direction.