Model 78 and 78-1
Technical Manual
Section 8
Ludlum Measurements, Inc.
Page 8-6
March 2021
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 zero. 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 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, or at the maximum calibrated radiation field.
Adjust the
OVL
control until the
OVERLOAD
alarm just appears.
