Fluke Biomedical 05-450 - Manual (Page 23)

Digital Doorway Monitor
Calibration
15
Calibration
High Voltage
The high voltage is adjustable from 600 V dc to 1200 V dc using the HV
potentiometer located under the calibration cover. Ensure that the high voltage is
checked only with a high impedance (≥1000 megohm) voltmeter only. A high-
voltage checkpoint is located next to the HV potentiometer. The high voltage
required will depend on the type of detectors used. Normally, each detector in a
two detector system is separately plateaued and documented. The two plateau
sheets are then compared and an operating voltage is selected that is compatible
to both detectors.
Warning
To prevent possible electrical shock, fire, or personal injury 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. Once the calibration constant is set and checked at a low
radiation field, the dead time correction can be set.
The dead time correction (DEAD TIME) is set when the detectors are exposed to
a "high" radiation field. A "high" radiation field in this case is defined as a field
where dead time losses exceed 30 %. The dead time correction will elevate the
ratemeter reading to account for counts arriving at the detector during the
detector's dead time. Scintillation detectors generally have short dead times from
1 microsecond to 10 microseconds.
Note
Once parameters are set, it is important to remember to switch the
CAL MODE switch back to the left. This action protects the
parameters from inadvertent changes.