MKS HPS 947 Series, Руководство по эксплуатации и обслуживанию, Страница: 37 / 40

HPS
®
Products Series 947 CEP System
A.1
Appendix A: How the Series
947 CEP System Works
The Series 947 CEP 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.
Convection enhanced 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 but not
eliminate these terms.
A convection enhanced Pirani sensor may be operated at constant current,
voltage, or resistance (equivalent to temperature) at the sensor wire. The
Series 947 System's CEP 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 the sensor.
At pressures above 200 Torr, convection currents contribute to energy
transport from the filament, increasing sensitivity at these pressures. In the
case of the Series 947 System's CEP Sensor, the upper measurement limit
is determined by the amount of energy the electronics is able to provide to
the filament.