How to calibrate your Geiger counter

Old, vintage Geiger counters are a lot of fun. Not only they are built like tanks but also have a certain character and feel to them. Unfortunately, the calibration on them might be a bit off as a result of their ageing components. Today I want to show you how you can easily calibrate your Geiger counter so that it shows accurate readings again!

In order to do this you will need:

  • a multimeter with 2 probes and a 1G Ohm (1000M Ohm) resistor for measuring the HV
  • a pulser
  • and your Geiger counter with a cable to connect a GM probe

Firstly, you will need to check if the voltage on your meter if it is correct using a multimeter and a 1G Ohm resistor. I already made a post about it which you can find HERE.

The next step is to connect the pulser in place of the probe. Pulser will generate “clicks” at a consistent rate which will allow us to calibrate the meter. I personally use one made by GEOelectronics and I highly recommend it but others should also work fine.

Now turn on your meter and set your pulser to generate a desired amount of CPM. Make sure that the needle on the meter is showing the correct amount of CPM generated by the pulser on every scale. If it is not, then adjust the calibration potentiometer until it sits right where you want it.

Ludlum Model 3 calibration potentiometers

If your meter uses dose units such as uSv/h, then you will need to find a conversion rate from CPM to uSv/h for the Geiger tube used in your meter. THIS post by DIYGeiger covers most of the GM tubes commonly used and gives a rough conversion ratio from CPM to uSv/h. Please note that this isn’t very accurate and if possible, you should use CPM when measuring activity of a samples. Dose units work only when the meter is calibrated to the specific isotope which is being measured. In other cases, the readings can be lower or higher than in reality.

Congratulations, you have now successfully calibrated your Geiger counter! However if you are a professional and you want the most accurate calibration on your meter, then you should most probably send your meter for calibration to a professional lab but for most hobbyists, this method should be more than enough!

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