PCM-2 Manual.pdf - Voss Associates

• Detector channels whose corrected average background count rate falls within
the tolerance band are considered to be normal. If a detector's alpha or beta
average background count rate exceeds the upper limit of the tolerance band,
that detector is flagged with a HI SENSITIVITY FAILURE. A LOW
SENSITIVITY FAILURE occurs whenever a detector channel falls below the
lower limit.
The environment that a PCM-2 operates in can cause sensitivity failures even
though all detectors can be operating properly. For that reason, the first
recommended step is to analyze the Geometry Factors and Sensitivity Factors and
make changes to those parameters as necessary.
• Default Geometry Factors are set in inverse proportion to the detectors area,
using detector #O's area as the benchmark. Factors other than a detector's
size can contribute to its cout rate, e.g., a streaming background or a shield
that affects only part of the instrument. If changing Geometry Factors is
warranted, observe the background averages in the VIEW menu -- insure that
the Correction is toggled off -- and assess relative values of count rates to
establish appropriate Geometry Factor values. Geometry Factors are edited in
the Detector Parameter screen under the EDIT menu.
• Default sensitivity factors are 0.50 for the beta channels and 0.10 for the
alpha channels. While these values will work for many installations, an
environment that involves wide fluctuations of background accompanied by
directional characteristics may warrant lowering the sensitivity factors.
Sensitivity factors are established in the System Parameters screen under the
EDIT menu.
If count rates are high or low enough to indicate a true sensitivity failure, the
following is suggested.
• A low sensitivity failure may be caused by counting gas starvation. Verify
positive gas supply, correct pressure and flow rate settings and search out any
leaks or kinked hoses that may be present. Once corrected, allow sufficient
purge time before placing the channel back into service.
• Other possible problems will be narrowed down to a failed detector assembly
or its complimentary MDB (Modular Detector Board). By exhanging the
MDB with that of a known good channel it is possible to isolate the problem
to either the board or the detector. The faulty assembly should be replaced.
NOTE: Whenever and MDB is replaced, the DETECTOR DOWLOAD routine
must be invoked to restore the correct parameters to the affected detector.
High Voltage Fail
Verify that the high voltage failure message was not induced by some transient
condition by first executing the DETECTOR DOWNLOAD routine from the
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PCM2.MAN/ nP.v A/April 1995