PCM-2 Manual.pdf - Voss Associates

decontamination. Liquids under pressure can migrate into the internal electronics
enclosures through seams and gaps at sheet metal part interfaces and cause
potential damage.
Detector
Maintenance
The PCM-2's detectors do not require any specific periodic maintenance but,
because of the delicate nature of their thin mylar covered faces, often do require
face puncture/tear repair. The recommended approach to instrument servicing in
the event of a detector becoming punctured or otherwise disfunctional, is to
remove the subject detector and replace it with a spare purged detector of the
same configuration. This approach renders the unit out of service for only the
short time interval required for detector swapping, enabling the maintenance
technician to repair the damaged probe without further impacting the unit's
operation. Refer to Section 16 and Appendix B for detailed instructions
reguarding PCM-2 detector assembly repair and servicing.
Troubleshooting
The following subheadings list some of the problems which are subject to occur
with the PCM-2 and outlines their likely causes.
High/Low
Sensitivity Fail
With each background update a detector sensitivity analysis is performed on each
channel. A sensitivity failure is posted when a detector's sensitivity falls outside
of defined limits of sensitivity. The analysis is performed as follows:
• Detector average background count rates are normalized to a reference
detector size by applying that detector's GEOMETRY FACTOR. For
example, a small detector's Geometry Factor will be greater than 1. Its
background count rate is scaled up by its Geometry Factor so that the
corrected count rate can be compared to all other detector channel on an equal
count rate/unit area basis.
• The corrected background count rates of all active detectors are averaged to
provide a reference count rate for comparison. Separate averages are
computed for the alpha and beta channels.
• A tolerance band is established about the mean count rates computed in the
previous steps. The tolerance band upper limit is determined by dividing the
average count rate by the ALPHA or BETA SENSITIVITY FACTOR; (the
sensitivity factors are less than 1). The lower limit of the tolerance band is
derived by multiplying the average count rates by the appropriate sensitivity
factors.
PCM2.MANI REV AI April 1995 9-5