Recently, I have acquired an Eberline 120 survey meter however since it is pretty old, I was a little bit worried that the voltage coming from the unit might be too high for the probe due to the age of the internal components so I decided to measure it just to be sure.
Unfortunately, if you are going to measure the voltage using a cheap multimeter, you won’t get an accurate reading. That is because the impedance on the multimeter is different from the one on the Geiger counter. An easy way to fix this issue is simply to add a 1G Ohm (1000M Ohm) resistor between the positive probe and the Geiger counter.
Now simply set the multimeter to the 2000mV and connect probes to the BNC connector on the Geiger counter. The number on the display of the multimeter shows the voltage coming from the Geiger counter.
Since I have a lot of Geiger counters, I have decided to make my own probe with a 1G Ohm resistor built into it. Whenever I want to measure the voltage in my meters I can quickly change the standard probe for my moded one!
Here is another great post on this topic: