GQ Electronics Technical Support Forum - GMC-800 CPM=100*uSv/h ?

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GMC-800 CPM=100*uSv/h ?

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piroquai

2 Posts

Posted - 08/30/2026 : 02:16:27

I've just bought GMC-800

CPM measurements are EXACTLY equal to uSv/h multiplied by 100 at all times.

That doesn't seem to be consistent with the guide.

Is it normal? Should I trust CPM or uSv/h ?

Reply #1

ullix

Germany
1259 Posts

Posted - 08/30/2026 : 23:18:54

There is a linear relationship between Doserate in CPM and µSv. The konstant factor is the so called Sensitivity [CPM / (µSv/h)].

Doserate [µSv/h] = Doserate [CPM] / Sensitivity [CPM / (µSv/h)]

Be aware that this Sensitivity is NOT fixed, but depends on the gamma spectrum of the source! And it is INVALID when the source has Beta radiation!

CPM is what you truly measure, no matter what the source. µSv is an interpretation of the user on the CPM results, because the Sensitivity depends on what kind of radioactivity source is used for calibration. Very often Cs-137 is used. To my knowledge, the best Sensitivity ever reported for an M4011 tube and Cs-137 is 96 CPM/(µSv/h). This value is now used in my GeigerLog software.

Reply #2

piroquai

2 Posts

Posted - 08/31/2026 : 08:50:26

Thanks, I was just confused because in the pdf guide to GMC-800 the warning level is described as 100 CPM or 0.4 uSv/h , the plastic card that goes with the package says 100CPm is ~0.65 uSv/h.

Sadly there's no info anywhere on the tube or the calibration source but 96 CPM/(µSv/h) seems to match what I'm seeing

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