PCM-2 Manual.pdf - Voss Associates

possible accuracy, or if the sources used are of relatively low activities,
background information acquired with the background plateau routine may be
subtracted from source plateaus. In order to do this, the background plateau must
include the voltage range of the source plateau and have been run at the same
voltage steps.
Source plateau graphs show four curves representing alpha and beta rates and
crossover rates from alpha to beta and beta to alpha. To select the optimum high
voltage setting for a detector while viewing its source plateau first move the
cursor line to that point where the Beta as Alpha counts begin to rise. Select a
high voltage setting just below that point, move the cursor there and press F2 to
save that high voltage. If you want to set all other detectors to the same high
voltage use the Override edit display.
As with background plateau data, source plateau data can also be displayed as
actual or normalized data. The alpha and beta source plateaus are normalized for
each source type; the two curves that result from the beta source are normalized
to the maximum value in that data. The curves generated by the alpha source are
normalized in an analogous fashion. Thus, all four curves are displayed in
comparable scale -- a feature that is especially useful if the alpha and beta sources
differ greatly in activity level.
If the hand probe option is installed and the hand probe board is communicating
properly, a source plateau may be obtained for the hand probe detector as well.
Pressing F5 in the "Select Detectors" screen toggles the selection of the hand
probe detector. If the hand probe option is installed but the board is not
communicating, a message will appear in the "Select Detectors" screen informing
the user that the hand probe is not communicating. Pressing F5 when choosing to
view plateau data also selects the hand probe detector. Plateau data may be
viewed even if the hand probe board has a communication failure.
Special Considerations
The Modular Detector Boards' operating voltages have been preset at 1550 vdc
during the initial unit testing and checkout at Eberline's Santa Fe, New Mexico
facility which is at @6480 ft. ASL. The MDB's have an upper operating voltage
of 1750 vdc which the PCM-2's operating software deliberately limits. Users are,
therefore, not allowed to set their operating voltages above this high limit. This
feature of the PCM-2 software prevents detector high voltage arcing failure
potential due to over-volting at sea level. The potential for over-volting does,
however, exist at altitudes above sea level. The general rule for adjusting the
MDB's operating voltage is to increase it by 30 vdc per each 1000 f1. of
elevation drop below the initial 6480 ft. setup elevation in Santa Fe, NM.
Users should try to minimize their MDB operating voltages, as much as
possible for their monitoring needs, to avoid this destrUictive over-voltage
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PCM2.MANI REV AI April 1995 7-3