F5 – Information Menu
Gem-5 User’s Manual 245
DARK MAGENTA = Alarm Level Set point (ALS).
LIGHT MAGENTA= Alarm Trip Point (ATP) used for Clean/Contam decision.
LIGHT MAGENTA= Contaminated Trip Point (CTP) used for Clean/Contam
decision.
GREEN = Background ± variance.
WHITE = Lowest Level of Detection (LLD).
LIGHT BLUE = Background Reset (K Delta) values. Two lines are shown, one
for the upper reset values and the other for the lower reset values.
The ALS, ATP, and LLD values are calculated using the equations of Alarm Trip
Points starting on page 286.
and the actual monitoring time (T
m
) in effect at the time. The spread between ALS,
ATP, LLD and the Average Background Rate values is an indication of the effect of
background error on monitoring sensitivity and accuracy. Note that the ALS, ATP
and LLD lines are referenced to the Effective Average Background Rate line (the
yellow line) and not to zero, i.e. they are shown as grossed-up values. This provides
for easier comparison of the margin between average background rates, LLD, and
alarm levels.
All averaging periods are set in the F2 – Wait Timeouts screen (on page 106).
The Average Background Rates (brown and yellow lines) show the smoothed
background used for net count rate calculations. The Average Background Rate
(brown line) is multiplied by the Self Shield Factor (“Occupied Position 1 / 2”,
“Occupied”, “Alarm Test”, “Detector Contamination Check” or Calibration, as
applicable) and then subtracted from the gross count rate to obtain the net count rate.
The yellow line is the “Occupied / “Occupied Position 1 / 2” Self Shield Factor -
adjusted Average Background Rate subtracted from the gross count rate for user-
monitoring net count rate calculations. If the “Occupied / “Occupied Position 1 / 2”
Self Shield Factor is unity (1.0) then the brown line is not visible beneath the yellow
line.
The Average Background Rate (brown line) is compared to the Low Background
Rate limit (the dark red line). This comparison, i.e., the low count rate alarm, is
discussed in Low Background Rate Setting and Strategy on page 282. This test does
not apply for sum zones.
The data at the bottom right of the graph includes the background average time Tb (s)
that is currently being used to calculate the Average Background Rates (yellow and
brown lines), the current “Occupied” Self Shield Factor -adjusted Average
Background Rate R
b
(in yellow) and the current Average Background Rate R
B
(in
brown). The third number is not shown if the “Occupied / “Occupied Position 1 / 2”
Self Shield Factor is 1 since R
b
= R
B
in that case. R
b
is defined in Alarm Trip Points
on page 286 and reflects the current value of the yellow line on the graph. R
B
is
defined in Background Rate Calculations on page 275 and reflects the current value
of the brown line on the graph. T
b
is defined on page 286. It indicates the quality of
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