BERTHOLD TECHNOLOGIES Uni-Probe LB 490, User Manual

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38477BA2B
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25.5.09
6 Explanations Volume 7
Further information on scenario B :
A differential limit value is monitored, i.e. each fast rise of the dose
rate triggers an alarm.
Even minor outside radiation is detected, when it occurs erratically.
Operative factors such as fast emptying of the vessel can result in
false alarms.
To rule out false alarms with sufficient statistical safety, you should
enter n > 5. The mathematical correlation shows that the distance
of the alarm threshold is dependent upon the respective mean
count rate In.
For calculation it holds:
Example
Count rate In = 300cps, n = 6
Thus, an alarm is signaled as soon as Is exceeds 404cps.
IMPORTANT
Due to the dynamic behavior of the interfering radiation detection,
a quick increase of the pulse rate due to operational factors (e.g.
very fast emptying of the vessel or large short-term level changes
caused by agitators) can be interpreted as interfering radiation.
Due to a high Sigma value, false triggering by these operating con-
ditions can be suppressed. However, the radiation interference
detection also becomes less sensitive.
For example, opening the useful beam channel on the shielding can
also cause a quick increase of the count rate. You have to reset the
alarm that is then triggered, or better, do not enable the interfering
radiation detection at first. However, it would be better not to
enable the interfering radiation detection at first. Enable the inter-
fering radiation detection only after calibration.
Sigma
CPS =
Is Im n Im
×
+ =
Is 300 6 300 × 404 CPS
= + =