Models 375 (Including Series One), 375/1, 375/2 & 375/4 Section 5
Ludlum Measurements, Inc. Page 5-7 May 2017
Sigma Alarm Modification Option
With this option, special firmware allows the Model 375 to have a sigma-
based alarm point in addition to a regular fixed alarm point. This sigma-
based alarm point allows the user to have a floating alarm point that will stay
at “x” sigma above the radiation background. As the background changes,
the sigma alarm also changes. The sigma alarm, when activated, activates a
rapid beeping and activates the HIGH ALARM indicator on the front panel of
the Model 375.
To set the sigma alarm, one first needs to consult a probability table showing
one-sided sigma values. If the sigma alarm (read or set by the
LOW ALARM
button) is set to 3.0, that setting statistically means that 99.87% of normal
background readings would be less than the alarm point. To look at the false
alarm rate, it means that 0.13% or 1 out of 769 comparisons would result in
a false alarm. Since comparisons are made every second, a setting of 3.0 will
result in a false alarm about every 13 minutes. Similarly, a setting of 5.0
would result in a false alarm every 38 days. To actually calculate the sigma
alarm point, it is necessary first to determine the background radiation level
in cps (counts per second). The sigma alarm point is then BKGND + (x
sigma × square root of BKGND).
The HIGH ALARM has not been changed; it is still a fixed alarm point and will
be activated when the radiation level exceeds that setpoint. This feature
allows the sigma alarm to trigger quickly if a small amount of radiation is
present and allows the fixed alarm to warn that the background radiation is
too high. Since the sigma alarm is allowed to rise if the background rises, the
HIGH ALARM is necessary to have an absolute value or ceiling for the
radiation level. The time constant for the background radiation level and the
displayed radiation reading is 20 seconds. The sigma alarm is not activated
until 60 seconds after the Model 375 is turned ON, in order to allow the
Model 375 to accumulate a stable background radiation reading.
Two other changes were made to the Model 375. The first change was to
deactivate the
LOW ALARM indicator. Both the sigma-based alarm (set by the
LOW ALARM button) and the fixed alarm (HIGH ALARM button) trigger the
HIGH ALARM indicator. The second change was to lower the detector loss-
of-count time frame to 15 seconds. This change means that the DET FAIL
indicator is activated if no pulses are received from the radiation detectors in
15 seconds. Since the sigma alarm is most useful for scintillation detectors
that have several hundred pulses per minute, this change allows a faster
determination of detector failure.
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