Bicron Surveyor 50 Geiger Counter

© Brooke Clarke 2006

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.

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 has the 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 =) 903.5 volts.

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.

The High Voltage type BNC connectors on the cable are marked UG-932/U on one end and KV-59-37 pat 3195098 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.
patent 3195098 Coaxial Cable Connector, July 13, 1965, H.E. Miller et al, 439/454; 174/75C assigned to Douglas Aircraft. - traps the center pin.

LND, Inc. - Pancake Probes -