PCM-2 Manual.pdf - Voss Associates

3. Chamber outlets are connected together and exhausted through a flow
restrictor to maintain uniform gas pressure within all detectors. A solenoid
valve is used to bypass this restrictor during initial high rate purging of the
detectors.
4. Additional solenoid valves are used to meter gas into the intake manifold
(upstream of the flow restrictors) as needed to maintain a constant working
pressure. This pressure is measured at the outlet, and is therefore
representative of all detectors in the system.
The overall gas consumption of such a system is determined by the pressure
setting and the diameter of the outlet flow restrictor. In addition, enough gas
must be supplied to compensate for leakage through detector faces and chamber
seals.
Practical working pressures for typical detectors faced with thin mylar range from
approximately 0.05 to 0.50 inch of water. Pressures above 0.5 inch may damage
detector faces and/or seals; below 0.05 inch the mylar will not be adequately
inflated and air will diffuse into the chambers more rapidly. A starting system
pressure of 0.25 inches is recommended for the PCM-2.
In a parallel-plumbed system, maximum detector efficiency is typically obtained at
a flow rate of approximately 10 cc/min per detector chamber. At 6 cc/min per
detector chamber, a very slight decrease in efficiency may be observed, however
operation will still be very satisfactory. Depending upon the size and number of
leaks present, many detectors will operate well down to even 4 cc/min/chamber.
Note that PCM-2 has three different sizes of detectors and that these are average
values. Note also that new detectors may require slightly higher flow rates to
remove organic vapors released by internal materials.
The absolute minimum useful flow rate for any given system must be determined
by experiment and may therefore not be achieved in practice. It is quite easy,
however, to operate instruments on only 15-20% of the gas which would be
consumed without the gas manager. This equates to a gas usage rate of
approximately one size #IA P-I0 gas cylinder every 160 days for the PCM-2 unit
under continuous normal operation at a system inlet pressure of 5 psi and flow
rate of approximately 150 cc/min.
Operating Instructions
When operating normally, the gas manager displays three screens of data in
rotation:
1. System status (gas off, gas flowing, over pressure, etc.).
2. Current working pressure in inches of water.
3. Short and long-term gas consumption.
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