4. THEORY OF OPERATION page 17
Model PRM-470B
If the RAM test passes, the PRM-470B will display the current settings. If the RAM test fails, it
will display "---1".
After the instrument has completed the RAM test it will display the current settings for
approximately 3 seconds each. This delay also gives the high voltage time to stabilize.
The instrument will then count down from 10 seconds and will display a flashing background
count followed by an automatic switch to "SEARCH" mode.
4.3. BACKGROUND MODE
The PRM may be returned to background mode by pressing and holding the mode switch until it
is acknowledged with a beep. The display will flash in background mode, while the instrument
constantly updates the background count to reflect changes in the environment. The background
is accumulated in 10 second increments, with the current background displayed as the one
second average.
4.4. SEARCH MODE
In search mode, the alarm threshold is determined by the Sigma switch setting. The instrument
updates the display each second.
The instrument counts incoming pulses in 50 millisecond periods, and sums eight of these
periods into a 400 millisecond moving average.
That is, the most recent 50 millisecond count is added to the moving sum and the oldest count
discarded. This moving average sum is then compared to the alarm threshold.
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