Bicron Surveyor 50 Geiger Counter
© Brooke Clarke 2006 - 2022
This is a modern (solid state) type
GM counter that has a removable probe. The Bicron PGM probe
is called a "pancake" probe and has a much larger capture area for
particles than the conventional "hot dog" type probes.
When the two clasps are opened the
bottom box can be removed from the chassis giving access to the 9
volt battery attachment clips. There is provision for
storing a spare battery.
Controls & Indicators
The rotary switch has positions:
off - power off
bat. - the top of the meter scale is marked bat. ok
HV - about 1/3 up from meter zero is a narrow band marked HV ok
(Fluke 87 DMM shows 695 volts)
x100 - 50 mR/h or 60,000 cpm
x10 - 5 mR/h or 6,000 cpm
x1 - 0.5 mR/h or 600 cpm (maybe 66 counts in 1 minute background)
Of all the pancake probes shown on
the LND, Inc. web page only the 7311
correct voltage rating and physical configuration that will fit
the PGM housing and is rated for 900 volt operation. The PGM
pancake probe housing has a 55 mm window opening which has a
window screen covering and the GM tube is rated as having an
effective opening of 44.5 mm.
When the probe is disconnected and a Fluke model 87 DMM (specified
to have 1 volt accuracy on the 0 - 1000 VDC range) is used to
measure the meter HV the result is 695 Volts, not the 900 volts
mentioned in the sales literature for this model. This is a
measurement problem since the Fluke 87 has an input impedance of
10 M and the schematic for the Bicron 50 shows 3 M of resistance
after the HV sampling resistors. When a 10 M meter is used
to measure 900 volts with a source resistance of 3 M the voltage
should be about 692 volts which is very close the the observed 695
volts. Using the voltage divider in reverse an indicated
voltage of 695 corresponds to a source voltage of 695 * 13/10 =)
If the meter is in one of the counting range positions (x100, x10
or x1) and the Digital Multi Meter is used to measure the voltage
the meter indicates full scale. I'm sure that this is caused
by digital signals coming out of the DMM and triggering the
counting circuits of the GC.
type BNC connectors
on the cable are marked UG-932/U on one
end and KV-59-37
on the other end.
The PGM probe connector is marked Kings KV-79-15
and the Bicron 50 HV connector is marked Kings KV-79-15.
Coaxial Cable Connector, July 13, 1965, H.E. Miller et al, 439/454
174/75C assigned to Douglas Aircraft. - traps the center pin.
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Page Created 24 Jan 2006.