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Introduction
Theory of Operation
Gas Usage
Theory of Operation
Operational Mode
One size 1A P-10 gas cylinder = approximately 24 days @ 200cc / min flow.
Users have reported up to a factor of 10 reduction in gas usage, using the
PCM-2 Gas Manager (see “OPT12, Gas Management” on page 19-57).
Radioactive emissions cause ionization of the counting gas in the detector
chambers. The ions are collected at the detector anode wire causing small
voltage pulses on top of the static high-voltage that is applied to the anode
wire. Through capacitive coupling, the pulses are stripped from the
high-voltage, amplified, and discriminated by pulse height into alpha and
beta gamma channels. The detector microprocessors count the pulses, and
convert the counts to count rates. Count rate information is communicated
to the system controller over an RS-485 bus. The system controller applies
the appropriate algorithms to update background count rates and measure
for contamination.
In its main task loop, the computer program continually updates
background count rates for all detector channels, performs diagnostic
checks, and monitors input devices to determine if a person will be
measured. Numerous I /O devices are used to prompt the user and verify
correct positioning for a contamination measurement. The results of the
measurement are annunciated audibly and visually. The measurement is
principally a qualitative determination (an alarm indicates a high probability
that the person is contaminated; no alarm indicates a high probability of no
contamination present). Notwithstanding, alarm annunciation includes
presentation of quantitative information, i.e., activity levels are stated.
Three counting modes are supported: Preset All, Maximum Sensitivity
(Fixed Count Time), and Minimum Count Time. In each mode, statistical
control of the counting exercise ensures that the performance of the monitor
is optimized for that mode’s key parameters. The alarm set points (all three
modes), RDA (mode 2), and minimum count time (mode 3) are all
computed for each new background measurement. The parameters used
include average background count rate, count time, sigma factor (which
controls false alarm probability), confidence level, RDA and detector
efficiency.
Preset All
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PCM-2 Technical Manual
This mode maintains a fixed confidence level (probability of detection) for
the user-selected RDA and count time. False alarm probability is
maintained at or below a user-prescribed maximum. This mode is best used
when a fixed release limit is established and lower levels of activity are of
negligible concern. This mode is also useful when a fixed count time and
fixed alarm set points are preferred.
Thermo Electron Corporation
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