You can find there types of voltmeter around:
Chinese digital voltmeter: The cheaper ones, from 3 to 10 euros. They have only 1 MΩ of input impedance. At the left you can see how to measure the impedance of one of these voltmeters with the aid of another one.
The one at the left is in the 2V scale, and the one at the right is acting as a ohmmeter with 2 MΩ scale. You can see the impedance of the voltmeter (at left) is only 1004 kΩ.
Decent digital voltmeter: Most of them have an input impedance around 10 MΩ.
Analog voltmeter: The good ones usually have a sensitivity of 20.000 Ω/V, so about 20 MΩ at 1000 volts scale. Even 20 MΩ is a huge load for most Geiger tube's power supplies so for a precise measurement, you need input impedances of 100 MΩ or even greater.
1 GΩ resistor
The one at the picture is a 1GΩ 1/2 watt resistor, and it is perfect to transform a cheap 1MΩ voltmeter in a really nice high voltage, high impedance voltmeter. How? Making a voltage divisor with the resistor :
A piece of plastic from a ball pen, a metallic tip, a socket and the resistor. That's all you need. Just solder the parts and glue the plastics. That's all!
You don't need a high voltage to test it. Just use your 12 volts power supply. It must read 12.0 +/- 0.1 at the 200 mV scale and 12 +/- 1 at the 2000 mV scale.
Does it worth it?Examine the following table and extract your own conclusions. It was obtained measuring an unregulated power supply for a SBM-10 geiger tube:
|Cen-Tech 92020 + 1GΩ resistor||1 GΩ||400 V|
|ICE 680E||20 MΩ||375 V|
|Fluke 79 series II||11 MΩ||336 V|
|Uni-T UT81B||5 MΩ||307 V|
|Cen-Tech 92020||1 MΩ||214 V|
Just a final tip: Do not try to measure over 2000 volts with this probe!!!
2kV is high enough to measure all geiger tubes and photomultipliers you can find.