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vargr
4 Posts |
 Posted - 11/09/2017 : 00:28:14
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I've just acquired a GMC-500+, and on the whole I'm pleased with the device...
...but there seems to be an issue with the reported dose when the counter is in a high radiation environment. I have access to a portable X ray generator, and the GMC will quite happily report CPM of over 50,000 (more if I hold the trigger down for longer!), but the 'uS/hr' numbers get all screwy when the count exceeds about 10K/min.
video: https://www.youtube.com/watch?v=Bh5paVstB-E&feature=youtu.be
Anyone else seen anything similar, or can explain what's going on? |
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| Reply #1
ullix
Germany
1107 Posts |
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| Reply #2
vargr
4 Posts |
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| Reply #3
vargr
4 Posts |
Posted - 11/12/2017 : 12:52:48
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| I see the log file just contains CPM data. Unfortunate. |
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| Reply #4
ullix
Germany
1107 Posts |
Posted - 11/13/2017 : 01:26:54
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quote:
Originally posted by vargr
I see the log file just contains CPM data. Unfortunate.
???
What are you talking about: the Geiger counter device can record as either CPM or CPS, and GeigerLog can record as either CPM or CPS. For consistency the latter stores the log files as CPM by multiplying the original CPS count with 60.
Obviously you can go back by dividing by 60 ;-) |
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| Reply #5
vargr
4 Posts |
Posted - 11/15/2017 : 12:36:16
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Well, yes, obviously.
'Unfortunate' because I really wanted a record of the dose (uS/hr), not the CPM/CPS. I'm having no issues with CPM; I can get the thing to read >50K CPM. It's the dose that isn't calculated correctly.
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| Reply #6
ullix
Germany
1107 Posts |
Posted - 11/16/2017 : 00:28:12
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I am not sure I can follow. You go from CPM to CPS by dividing by 60. You go from CPM to µSv/h by multiplying CPM with 0.0065. That is the way it is done in the GQ counters - whether this is appropriate for your conditions is a different matter.
There are more issues with the "calibration". You can read more about it in the GeigerLog Manual in "Appendix G Calibration" (obtain from here https://sourceforge.net/projects/geigerlog/files/ )
GeigerLog draws all graphs in units of your choice, CPM, CPS, or µSv/h. So, record your counts in the counter, then download the history into GeigerLog, and show the time course in the preferred units. |
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| Reply #7
ullix
Germany
1107 Posts |
Posted - 11/16/2017 : 05:19:58
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After sending my last post I remembered that I had once found a bug in the GMC500 firmware, that was reported to GQ, but apparently is still not fixed.
Using GeigerLog to read out what GQ calls "3 point calibration" from an GMC300 series device, you get :
Device Calibration:
Calibration Point 1: 60 CPM = 0.39 µSv/h (0.00650 µSv/h / CPM)
Calibration Point 2: 240 CPM = 1.56 µSv/h (0.00650 µSv/h / CPM)
Calibration Point 3: 1000 CPM = 6.50 µSv/h (0.00650 µSv/h / CPM)
So that 3-point thingy really is a constant factor for the whole counting range of the device.
Doing this with a GMC500, however, gives this:
Device Calibration:
Calibration Point 1: 60 CPM = 0.39 µSv/h (0.0065 µSv/h / CPM)
Calibration Point 2: 10000 CPM = 65.00 µSv/h (0.0065 µSv/h / CPM)
Calibration Point 3: 25 CPM = 9.75 µSv/h (0.3900 µSv/h / CPM)
The third point is obviously completely wrong. What I figured out is that up to CalPoint 1 the CPM data are multiplied with the first value (0.0065), up to CalPoint2 the next factor (which is identical to the first), and for all the rest beyond CalPoint 2 the factor of CalPoint 3 is used, i.e. 0.39. This, however, is wide off the mark!
Since you specifically mentioned 10k/min as the upper limit for meaningful readout, this bug is likely the reason.
And, by the way, this senseless value is also found in the GMC500 simulation software!
You should be able to correct this by changing the CalPoint 3 to: 25000 & 162.5. Or change all 3 points to those values used in the 300series, it does not matter. Neither setting have anything to do with saturation at high count rate, again I refer you to the Appendix G in my manual of Geigerlog.
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GMC-500+ reported dose at high count rate
