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Michael C Cusumano
USA
2 Posts |
 Posted - 12/07/2023 : 10:52:59
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Hello,
I purchased a GMC-800 and am very skeptical that it's reporting valid measurements.
It occasionally spikes as high as 800 CPM with no stimulation. It seems worse shortly after being turned on but recurs after a few minutes.
I saw similar discussion in a thread about another device.
I am curious if others have experienced and if there's any fix. I am also (lol) curious if anyone can recommend a similar, more reliable device <$100. |
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| Reply #1
UnstableIsotope
USA
67 Posts |
Posted - 12/07/2023 : 14:36:35
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800? I've never even seen a cheap Chinese GC do that randomly and I have two.
What's the normal background CPM in the area you are measuring? |
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| Reply #2
EmfDev
2234 Posts |
Posted - 12/07/2023 : 14:47:46
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| Hi Michael, can you please send me your device s serial number to check if it is updated? |
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| Reply #3
Michael C Cusumano
USA
2 Posts |
Posted - 12/07/2023 : 15:29:40
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Revision 1.08
Serial number xxxx
Background is reporting about 20 cpm or about 0.14 microSv/h, ignoring periods of dramatic spikes. I don't have another device to confirm this is correct. |
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| Reply #4
EmfDev
2234 Posts |
Posted - 12/07/2023 : 18:02:53
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| It seems your device already has the latest firmware update. Please try to do a factory reset and let us know if the occasional spikes still happens. 20CPM average is normal. |
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| Reply #5
ullix
Germany
1162 Posts |
Posted - 12/08/2023 : 01:58:15
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These spikes are very likely all a consequence of the inappropriate use of the "Fast estimate". It is a nuisance and should be switched off by setting it to 60 (sec)!
If you wish you can use GeigerLog to simulate the bad behavior of this setting. Don't use it!
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| Reply #6
UnstableIsotope
USA
67 Posts |
Posted - 12/08/2023 : 07:35:18
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quote:
Originally posted by ullix
These spikes are very likely all a consequence of the inappropriate use of the "Fast estimate". It is a nuisance and should be switched off by setting it to 60 (sec)!
If you wish you can use GeigerLog to simulate the bad behavior of this setting. Don't use it!
A fast estimate window of 5 or 10 seconds can easily hit up around 80 or 100CPM in a 20CPM environment for a fast incoming particle train, but not 800. I think it's something else. Nevertheless, I agree, set the window to 60 if that's possible on the GMC-800. |
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| Reply #7
UnstableIsotope
USA
67 Posts |
Posted - 12/08/2023 : 08:29:59
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I ran my 500+ for a while with a 5s estimating window.
The max estimated CPM was 73. My normal background is 15-20.
For an estimated 800 CPM a 5 sec window would have to see 67 hits. A 10s window would require 134. This is quite unlikely in a normal background.
I think the OP either has a meter with a problem or some real, albeit transient, source nearby. |
Edited by - UnstableIsotope on 12/08/2023 10:56:02 |
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| Reply #8
ullix
Germany
1162 Posts |
Posted - 12/11/2023 : 01:21:40
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Don't forget that radioactive decay is following a Poisson distribution, and this is defined from 0(zero) to plus infinity! Any counts are possible. Remember that joke with the monkeys typing Shakespeare?
I just did a test using a "GMC-300Eplus" counter. This one does NOT offer the FET "correction". So I used GeigerLog to show what a FET correction of the "dynamic" (aka 3 sec) setting would look like, if it were activated.
In dark-blue is the counter's original CPM signal. In light-blue the FET distorted signal, derived from the counter's original CPS signal.
You see that FET makes the signal swing wildly, reaching a max of CPM=240, while the true CPM average is CPM=17.95.
It is helpful to verify proper data with a Poisson test, shown in next pic.
On the left is the check of the original CPM - it doen't get much better than this. On the right is the check for the FET distorted data. The data are ruined and can't be corrected.
So, first thing to do is to set FET=60 to switch it off. If then you observe some spikes which you find unexpected, first check with Poisson. If this tells you those spikes are highly unlikely then you may have a hardware defect in the counter, which warrants further inspection!
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Edited by - ullix on 12/11/2023 01:23:25 |
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Wildly high CPM spikes on GMC-800
