Models 375 (Including Series One), 375/1, 375/2 & 375/4 Section 7
Ludlum Measurements, Inc. Page 7-2 May 2017
The dead time correction (DEAD TIME) is set when the detector is 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. GM tubes typically have long dead times from 50-150
microseconds. Neutron and scintillation detectors generally have short dead times
of 1-5 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.
Analog Output
The analog output is a five-decade logarithmic voltage-out. The maximum
voltage-out while under primary power is 6 volts. The maximum voltage-out
while under battery backup power is 4.5 volts. The five decades are:
0.1 <units> - 1.0 <units>*
1 <units> - 10 <units>*
10 <units> - 100 <units>*
100 <units> - 1000 <units>*
1000 <units> - 10000 <units>*
When the CAL MODE dip switch is switched to the right, the analog output
goes to full scale. The analog output goes to full scale during a DET FAIL
condition.
Discriminator
The DISC potentiometer located under the calibration cover is used to set the
threshold for pulses coming from the detector. The desired pulse threshold
depends on the type of detector used. It is adjustable from 2.0 mVdc to 100
mVdc.
Battery Charge
The potentiometer labeled BAT, located under the calibration cover, is used
to set the backup battery trickle-charge voltage. This is typically set to 6.9
Vdc with the battery disconnected.
* See note on page 2-3 regarding Model 375 Series One.
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