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For a stable operation of the PSPC, the detector gas must be
replenished at a flow rate of several cm 
min 
.
There are several reasons for this constant flow.
The first is to maintain the relative composition of the detector
gas mixture (65% Ar, 20% Xe, 15% CH4 ),
which would otherwise be changed by the differential gas diffusion
through the thin polypropylene/lexan window.
The second reason is to prevent gas poisoning by polymerization
products of the quench gas as well as outgasing from glue and plastic.
In order to maintain a temperature-independent gas gain,
the detector gas is density
rather than pressure controlled.
Otherwise, a one degree temperature change would result in
a 5% gain shift.
The basic components of the gas supply system are the gas tanks
(three tanks of 12 l each with an initial pressure of 200 bar)
with pressure regulators, two redundant solenoid valves for the
detector pressure regulation, and two artificial leaks
(2.5 cm 
min 
and 3.8 cm 
min 
)
at the outlet of each detector.
Sealed reference volumes are integrated in the detector housings
to guarantee a good thermal coupling between detector and reference gas.
They are filled with detector gas at the nominal operating pressure
(1465 mbar at 22 
C).
The gas control unit maintains the detector pressure at the pressure
of the reference volume, yielding a density control
(i.e., when the temperature goes up, the regulating pressure goes up
accordingly, and vice-versa).
The control unit is a bistable regulator with a selectable
duty cycle (1s - 60s).