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.
Without 1G Ohm resistor 
With 1G Ohm resistor
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:
3 Comments
julian entsminger · 6 November 2025 at 19:11
just wondering what the other specs are on your resistor, as i need to do exactly the same thing?
allRadioactive · 6 November 2025 at 19:27
I think I just got 1G Ohm resistor and that’s it.
Solomon Rtutny · 19 July 2026 at 10:56
There’s a simpler method: use a low-leakage capacitor (polymer film) with a capacitance of 4 μF or higher. Connect it to the output of the high-voltage inverter and wait a few of minutes. (If the inverter doesn’t start (because this sort of “short-circuit” for it), connect the capacitor through a 10-20 megaohm resistor and wait longer (for example, 20 minutes). Then measure the voltage across the capacitor using a standard multimeter with an R internal 10 megaohm. The voltage across the capacitor will drop slowly, and the measurement will be quite accurate.