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garyt
8 Posts |
 Posted - 10/13/2012 : 22:35:25
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I have been using the '300 for a while now and have noticed that, unlike a professional GC, like the Inspector, the decay rate for Counts Per Minute is so slow that it doesn't settle back to background in a reasonable amount of time after being exposed to a high-rate source.
If I put the GMC300 and an Inspector near a source and until they both read 450cpm. Remove the source and the inspector drops back to the background count of ~18 in a little over a minute. The GMC300, however, still displays 180-200cpm after three minutes of being away from the source. Not only does the firmware not decay the value fast enough, but any pulses that come in add directly to the value, making it decay even slower.
I don't know if you have any control over the microcontroller code, but maybe a decay-rate variable might be a good idea. Or maybe there is simply a bug in the decay time routine.
The long decay time is bad when trying to compare background to a sample. The only way to have it decay quick enough to be useful is to take a measurement - power fail it - and take another measurement.
That's pretty lame. |
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| Reply #1
ZLM
1261 Posts |
Posted - 10/14/2012 : 14:10:23
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That looks not the GMC-300 designed.
There are two possible reasons:
1. the tube voltage is out. You can tune the tube voltage lower and see if it still be the same.
2. the tube is unstable and need to be replaced. If the tube keeps same after step 1, then you can write to support@gqelectronicsllc.com for the help.
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| Reply #2
garyt
8 Posts |
Posted - 10/14/2012 : 20:07:38
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It's not the tube or tube power supply. If you have a test radiation source, try running the CPM number up to a high value (like a few hundred CPM) and then remove the GMC300 from the source and take note as to how long it takes the CPM display to drop back to background. If your unit is running the same firmware, you'll see that it takes an abnormally long time to settle back to the background value.
I would like to know how long it takes for your unit to settle back to background.
Thanks in advance for your help! |
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| Reply #3
phgphd
USA
12 Posts |
Posted - 10/15/2012 : 18:48:06
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As Gary suggested I have performed the decay experiment. I ran up the GMC300 to 1150 cpm. It decayed monotonically to 60 cpm at 1 minute. But after that the cpm actually started to increase back to 100 cpm and bounced around 70-100 for the next 50 seconds and finally reached background at 3 minutes. These results would strongly suggest that GQ software is using a N-tap impulse filter with a decay constant equivalent to one minute with a programming error built in.
I also ran up the GMC300 to 470 cpm and then followed the decay. This also exihibited a one minute decay with an initial monotonic (ie, uniformily decreasing) decay, but at 60 seconds an anomalous bump was seen before finally settling to the background level. |
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| Reply #4
ZLM
1261 Posts |
Posted - 10/19/2012 : 21:30:58
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| I think we can change to another tube for different decay time. |
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| Reply #5
and_kom
Ukraine
24 Posts |
Posted - 10/22/2012 : 03:11:31
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| I can confirm the same issue with SBM-20. |
Young and radioactive |
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| Reply #6
garyt
8 Posts |
Posted - 10/23/2012 : 23:26:14
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quote:
Originally posted by ZLM
I think we can change to another tube for different decay time.
Wow! I must have an overly sensitive tube. I don't really know why it manifests as a strangely slow decay rate.
How does the Calibration function work on the GMC-300? |
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| Reply #7
ZLM
1261 Posts |
Posted - 10/24/2012 : 20:24:50
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As I know, higher sensitivity tube, has slower decay rate.
The calibration needs a standard source or meter. There is a section for calibratin in user guide. |
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| Reply #8
CPM
7 Posts |
Posted - 11/16/2012 : 23:09:53
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I see a similar behaviour that garyt and phgphd have seen...Compared to the other geiger counters that I tried, GMC-300 takes
far more longer time to get back to the background reading after getting separated from the source. It takes about 2-3
minutes to get back to the original background level CPM reading. During that time I also observe an increase that phgphd
had seen, and I agree with him that this might be related with the algorithm that is used and the decay constant/parameter that
is chosen.
After getting separated from a source, it should not take 3 minutes for the CPM value to get back/decay to the background reading...my other geiger counters get back to the original background level far more quick.
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| Reply #9
and_kom
Ukraine
24 Posts |
Posted - 01/01/2013 : 12:34:25
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It happens due to poor CPM averaging algorithm.
I consider GMC-300 uses Simple Moving Average (http://en.wikipedia.org/wiki/Moving_average) algorithm with averaging interval 1 minute or similar.
See my chart below for better understanding the issue:
Image Insert:
53.91 KB |
Young and radioactive |
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| Reply #10
ZLM
1261 Posts |
Posted - 01/01/2013 : 21:34:32
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No.
GMC-300 does not manipulate the CPM. It displayes the real true count from the tube. That means the CPM displayed on the GMC-300 is true count of last minute from the tube, not the averaged CPM.
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GMC300 CPM decay rate too long.
