PCM-2 Manual.pdf - Voss Associates

Gas Management Option (PCM2 OPT12)
Theory of Operation
In concept, gas proportional detectors are very simple. In the real world,
however, seals leak and materials come with random flaws. In practice, detectors
require a constant flow of counting gas primarily to overcome the following:
1. Loss of counting gas through pinholes in mylar detector faces and through
imperfect seals elsewhere.
2. Contamination with air (oxygen, water vapor, etc.) via the same routes.
3. Organic vapors released from plastics, gaskets, foam seals, adhesives and
other construction materials, particularly in newly (re)built detectors.
4. Gradual degradation of the counting gas itself, especially when exposed to
high radiation fields.
The ideal solution is to provide each detector with a continuous flow of fresh
counting gas to sweep out air and other contaminants as fast as they appear. In
large multiple-detector instruments, this may not be economically or logistically
feasible. Compromises may be made, such as plumbing several detector
chambers in series, however the performance of each detector in such a chain
becomes lower as gas purity decreases.
A corollary of this is that instruments should ideally be plumbed in a parallel
manner, so that no detector receives the used gas from another. Unfortunately if
each detector is operated at a normal flow rate of 200 cclmin, gas consumption in
a large system is relatively high. In addition, each detector must be provided with
its own flow meter and adjustment valve to insure that counting gas is evenly
distributed among all of the detectors.
Finally, if the outlet of a detector is left open, chamber pressure will be very low
and air will diffuse in relatively quickly. Flow rates must be adequate to maintain
adequate working pressure and to prevent air from diffusing upstream from the
gas omleI.
The PCM-2 Gas Manager System incorporates the following features to reduce the
gas consumption rate of gas flow proportional detectors:
1. Detectors are plumbed in parallel so that each receives fresh counting gas
directly from the source.
2. A flow restrictor is inserted in front of each detector to insure even
distribution of gas to every chamber.
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