MKS 907 Series, Инструкция по эксплуатации

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5 Operation
22
Series 907 Analog Convection Transducer
Instruction Manual - #109070001
5.2 Typical Applications
• Measuring fore line and roughing pressures generated by mechanical vacuum pumps
• Controlling valves and pumps to automate system pump down using the relay set point(s)
• Sensing abnormal pressure and taking appropriate security measures using the relay set point(s)
• Controlling system pressure using the analog output as input to an automatic pressure controller
• Starting or stopping system processes using the relay set point(s)
• Measuring pressures of backfilled gases in the range of 10
+3
to 10
-3
Torr
• Activating high-vacuum sensors within their operating range
5.3 Theory of Operation
The Series 907 Transducer is a heat-loss manometer which infers the pressure of a gas by measuring
thermal loss from a heated wire. 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
• 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 907 Transducer 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.
5.3.1 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 907 Transducer, the upper measurement
limit is determined by the amount of energy the electronics is able to provide to the filament.