242 Advanced Operation Manual
8 Auxiliary Devices
Auxiliary Devices
About Auxiliary Pressure Control
Pressure units
There are two common ways of expressing gas pressures:
psia Absolute pressure, measured relative to vacuum.
psig Gauge pressure, measured relative to atmospheric
pressure. This name is used because most pressure gauges
have one side of the sensing element exposed to the
atmosphere.
The two measurements are related by:
psia = psig + atmospheric pressure
Two-channel pressure control module (PCM)
The PCM has two different channels. Channel 1 may be
either flow or pressure controlled and may be flow/pressure
programmed. It is essentially identical to the packed column
inlet flow module (see “About the Multimode Inlet).
Channel 2 is pressure only, but may be used in either a
forward- or back- pressure mode by changing connections.
Both channels are controlled by the same parameter list. The
first two lines are for channel 1; the remaining lines are for
channel 2.
The forward- pressure mode requires the user to supply a
downstream flow resistance, possibly a frit.
The back- pressure mode is most useful with gas sampling
valves. By connecting the sample exit line from the valve to
the vent fitting of the PCM, pressure in the sample loop can
be controlled.
Three-channel auxiliary pressure controller (Aux PCM)
All channels are 3- ramp programmable. Up to three modules
may be installed, for a total of 9 pressure- regulated
channels.
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