Chapter 3 Parameters
Mini-Switch LB 471
Second alarm relay (EVU soft version 1.01 and higher)
The function is foreseen for „requirement-oriented maintenance“.
It helps, for example, to discover wall caking at an early stage,
because wall caking may result in switching errors.
A switch point between 10 to 100% can be set.
Since this function also triggers an alarm when the level changes,
a time window has to be set in the PCS which detects the differ-
ence between caking and actual level changes.
As soon as the
warning relay for the wall caking has been enabled, the alarm re-
lay also has to be triggered within e.g. 10 minutes. If the alarm
relay does not respond within these 10 minutes, then the signal of
the warning relay has to be interpreted as a maintenance alarm in
the PCS
1
. For the function to work properly, the time constant has
to be set manually to a value to ensure that in the regular course
of operation this alarm will not be triggered by statistical fluctua-
tions, but only if a wall caking is actually building up. When setting
the time constant to manual, the source decay has to be taken
into account as well. Because the source decay requires a larger
time constant to ensure that the device will work without stepping
errors even after several years.
Example with CS -137 source:
Empty count rate: 300 cps
Full count rate: 40 cps
Warning relay: 30%
Alarm relay: 60%
Maximum fluctuation margin permitted: ±10% (means the reading
may not vary by more than ±10%)
1) Calculation of fluctuation margin in cps:
ΔI = 10% of (300-40) = 0.1 x 260 = ±26 cps
2) Calculation of the required time constant:
T = 18 x I
Empty
/ (ΔI
Empty
)² = 18 x 300 / 26² = 8s
The time constant has to be set to 8s in order to limit the maxi-
mum fluctuation margin to ±10%.
For a Co-60 source, the source decay has to take into account for
the expected service life of the measurement. For a Cs-137
source, the source decay for a service life of approx. 10 years is
virtually negligible.
Automatic monitoring for source replacement is not possible any
more when working with a manual time constant. To signal a nec-
essary source replacement, you have to enter the year in which
the source replacement is to be indicated in code 55.
PCS = process control system
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