PCM-2 Manual.pdf - Voss Associates

Counting Gas Installation
'~_==~~='-="_.===" ='m===~==· =._==~~~" _~~=~_~=
_
The PCM-2 uses gas proportional detectors which require an approximate total of
200 cc/rnin of P-lO counting gas for their operation. Two gas inputs are
provided, for which supply hose feed through grommets at the rear side columns
of the instrument, are provided. The instrument will select one gas source upon
power-up and continue to use that source until its pressure fails, at which time the
alternate gas source will automatically be selected via internal gas switching
electronics. If both inputs drop below approximately 2 psi, the instrument will
cease to operate.
If portable cylinders are used to supply counting gas, each must be regulated
down to approximately 5 psi and connected to one of the gas inputs. The
standard PCM-2 unit includes one (1) gas bottle pressure regulator located in the
extraneous parts box provided, as previously noted. If the optional Gas Bottle
Enclosure (Option 9) is ordered, one (1) additional gas pressure regulator is
included. If two counting gas sources are installed it is important that both
regulators be set to the same pressure so that gas flow will remain constant when
the second cylinder is selected. If the pressure at either input fails, a message will
be displayed on the CRT indicating that the gas cylinder needs to be replaced.
When the instrument is operated from a permanently installed counting gas
manifold, both inputs may be connected to the same 5 psi source. This will
eliminate false "gas bottle empty" messages.
Gas Flow
Adjustment
Two flow meters are provided, along with a single needle valve to adjust the gas
flow rate. For normal operation, use the low-range meter to set a flow rate of
approximately 200 cc/minute. When setting up a new instrument or an instrument
which has been disconnected from its gas supply, adjust the flow to 0.8 liter per
minute (800 cc/min) for four hours to purge the detectors completely. Do not
exceed 1 liter per minute (1000 cc/miu) at any time to avoid bursting the
unit's detectors. If replacing a detector, the replacement detector should first be
purged on the bench prior to installaton or purged in place after installation using
the "Quick Detector Purge" gas supply hose coil located behind the flowmeter
mounting bracket. If the latter method is used for detector purging, simply
disconnect the red gas supply hose from the subject detector and plug the quick
purge line in its place. This effectively re-routes the gas flow from the system's
gas supply harness to the purging detector only. Utilization of this quick purging
hose feature will provide complete purging of a large PCM-2 detector within
approximately 20 minutes at a nominal gas flow rate of 200cc/min. Faster puging
times can be achieved by increasing the gas flow rate, however, caution must be
exercised to avoid bursting the detector's mylar face. A more thourough
explainatiol1 of the quick purging line feature appears in the maintenance section
(Section 9) of this manual.
2-2
rCM2.MI\N/ Hcv I\/I\pril 1995