Ludlum Measurements 2241-2, Manual

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Model 2241-2RK Technical Manual
Section 8
Ludlum Measurements, Inc. Page 8-19 July 2012
D
ETECTOR
O
VERLOAD
(OVL) C
ALIBRATION
Note:
The detector operating voltage (HV) must be determined and
adjusted before the
OVL
adjustment is performed. If the HV is
varied or another detector is substituted,
OVL
must be
readjusted. If the overload feature is not used, adjust the
control to the maximum counterclockwise position.
The detector overload circuit senses current flow through the detector. As
the radiation intensity is increased, the detector may start to saturate
(decrease pulse production), and the readout may decrease or read 0. But as
the pulse output continues to decrease in the saturated field, the detector
current drain continues to increase. This increase in current is detected by a
comparator circuit, which triggers the
OVERLOAD
enunciator on the LCD by
way of the microprocessor.
For GM and gamma scintillation detectors, the
OVL
trip point is adjusted to
the point to where the readout no longer increases with increasing radiation
intensity. In the event that the overload point cannot be determined due to
radiation field limitations, adjust the overload point from 5 to 10 times the
upper operating range of the detector.
Adjust the
OVL
control to the maximum counterclockwise position.
Place the detector in an increasing radiation field in which the readout
no longer increases. Adjust the
OVL
control until the
OVERLOAD
alarm
appears. Position the detector between the upper operating limit and the
OVL
set point and ensure the
OVERLOAD
alarm is defeated. Adjust the
OVL
control accordingly.
Example:
Ludlum Model 44-9 is calibrated with Model 2241-2 in the
R/hr units display, utilizing DT. The upper linear operating
point is 400 mR/hr for the Model 44-9. Place the detector at
the 1000 mR/hr point and adjust the
OVL
control to initiate
the
OVERLOAD
alarm. Place the detector in the 600-700
mR/hr field and ensure that the
OVERLOAD
is off.