Appendix A Algorithms
288
Extend Count Time set in the F2 – Wait Timeouts screen. Following the extended
count, the UTP reverts to the higher of LLD and ATP calculated for the total (normal
plus extended) counting time, which is higher than the corresponding values at the
end of the initial count time, because the error term is smaller. Note that if any
zone(s) registers definitely contaminated (above the CTP) then the unit will alarm at
the end of the initial (i.e. not extended) counting time and the UTP for the remaining
zones reverts to the higher of LLD and ATP.
When counting Gamma Body in two steps and the occupant is in the first position
(Monitor Position 1) then a “Contaminated” decision for a Gamma Body detection
zone, if in doubt, could be postponed until the end of the second counting period;
hence the UTP for the detectors is the higher of LLD and CTP. In the second
position (Monitor Position 2) the UTP reverts to the higher of LLD and ATP
calculated for the total (Position 1 plus Position 2) counting time, which is higher
than the corresponding values at the end of the first count time, because the error
term is smaller. This permits the use of a shorter counting time for Gamma Body
zones than would otherwise be possible. If it happens that a non-Gamma Body zone
is controlling the count time such that the count time required for each Gamma Body
zone is less than the count time being used, then the “Count Gamma Body in Two
Steps” is not used in Position 1. However in Position 2 the counts in both positions
are still combined when computing the measured count rate. Details of the actual
count time used are available in the F5 – Contamination Check Results Printout.
The diagram shown below illustrates the relationship between CTP, ATP, ALS, LLD
and average background count rate, R
b
. Notice how the spread between ATP, ALS
and CTP decreases when the Count Time is extended.
Figure 134 Extended Counts Under Conditions of Potential Contamination
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