Endress+Hauser gammapilot FTG 671, Руководство по эксплуатации

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4.2 Standard calibration with background calibration
(for detector DG57)
The standard calibration requires the determination of three parameters, in any order:
•
the countrate when the barrier is free at V0H1, s. Fig. 4.1
the countrate when the barrier is covered at V0H2, s. Fig. 4.1(often simulated
by switching off the radiation source)
•
the countrate when radiation is switched off and barrier is covered, s. Fig. 4.2
(the background radiation from the source and environment is then
measured.)
The vessel should be completely free of build-up prior to calibration. If this cannot be
guaranteed, a value of e.g. 20 can be entered in V0H1. This prevents the instrument from
signalling an alarm due to too high a dose rate, if the build-up falls off after calibration.
As shown in Fig. 4.2, the Gammapilot FTG 671 comperates the calibration parameters
for the decay of the source. The switching points are influenced by the compensation,
too. The background calibration ensures, that the minimum countrate will not drop below
the backraound rate.
Of course, when the minimum rate is hold while the maximum rate continues to fall, the
distance between the two values decreases. The FTG 671 automatically perceives, when
this distance becomes too small, and indicates an error (E635).
BA082D28
»Covered«, radiation "ON"
V0H2 »Free«, radiation "ON"
V0H1 »Covered«, radiation "OFF"
V0H9
Fig. 4.1:
Parameters of the
standard calibration
Nmax
Nmin
BA082E46
Max. countrate
on calibration
Background radiation
Switchpoints
entered as
normalised values
time (years)
Count
rate
With background calibration
min countrate = background
from this point
At this point
E635 appears -
switch points too
near
Min. countrate
on calibration
Fig. 4.2:
Decay of countrate and
compensation of switchpoints
with background calibratrion
Gammapilot FTG 671 Chapter 4: Calibration and Operation
33