Models 375 (Including Series One), 375/1, 375/2 & 375/4 Section 7
Ludlum Measurements, Inc. Page 7-1 May 2017
Calibration
High Voltage
The high voltage is adjustable from 450-2500 Vdc using the HV
potentiometer located under the calibration cover. The high voltage required
will depend on the type of detector used. Internal GM detectors usually
require 550 Vdc. Ensure that the high voltage is checked only with a high-
impedance (≥1000 megohm) voltmeter. A high-voltage checkpoint is
located next to the HV potentiometer.
Warning!
Do not touch the circuit board in the calibration window
due to potential for electric shock.
Calibration Parameters
The calibration parameters, LOW ALARM, HIGH ALARM, CAL CONST, and
DEAD TIME can only be changed while in calibration mode. Switch the top
dipswitch CAL MODE to the right to switch into calibration mode. Changing
any parameter is done by holding down the parameter key and pressing the
up or down arrow buttons. Any parameter can be set in the range of 0.1 to
9999*. If a parameter is changed, the instrument will beep to confirm the
saving of the parameter, and then return to displaying the current radiation
level.
The calibration constant (
CAL CONST) is set when the detector is exposed to
a "low" radiation field. A "low" radiation field in this case is defined as a field
where dead time losses do not exceed 5%. The calibration constant is usually
given for a certain detector. A Ludlum Model 133-4 detector, for example,
has a calibration constant of approximately 100 cpm/mR/hr. Once the
calibration constant is set and checked at a low radiation field, the dead time
correction can be set.
Sectio
7
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