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bmp200
10 Posts |
 Posted - 12/28/2014 : 21:34:14
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Hi, I've had a GMC-200 for about 2 years, and it's been working fine until recently.
I had it in one location for a long time - plugged in using the power supply (and with rechargeable battery inside as well) and a USB cable - but then a couple of days ago I had to move it to a different room.
Suddenly it's giving really high readings - like 200-300 CPM or more. When I cross-check with my other counter (SOEKS), everything is normal (around 20-30 CPM).
The only way I've been able to temporarily restore it to normal is by turning it off, opening the case and closing it again. For some reason it works okay after I do that - but then after a while, it will suddenly start giving high readings again for no apparent reason.
I don't know what to check, or what the problem could be. If anyone has any ideas, please let me know.
Boris. |
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| Reply #1
bmp200
10 Posts |
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| Reply #2
ZLM
1261 Posts |
Posted - 12/31/2014 : 18:35:11
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| It is the tube problem. Your tube needs to be replaced. |
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| Reply #3
bmp200
10 Posts |
Posted - 01/02/2015 : 19:52:34
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| OK. What tubes are available at GQ that are compatible with the GMC-200? Are there any better ones? |
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| Reply #4
ZLM
1261 Posts |
Posted - 01/03/2015 : 19:38:53
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| M4011 is the one used in GMC-200. |
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| Reply #5
Craig
USA
22 Posts |
Posted - 01/04/2015 : 11:34:48
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Hi bmp200.
* Although the quality control on M4011 G-M tubes appears to be marginal, and it might take two tries to get a good one (hopefully under warranty), they're otherwise good, sensitive (to average background gamma and hard beta) and seem to be stable over the long term, or until failure. My replacement GMC-200 runs 16cpm in a 10 uR/hr field and has maintained its performance for over a year --per:
> h**p://radviews.com/upload/14200771351096378412.gif
By the accounts posted in our GQ forums, the failure mode onset appears to be sudden and obvious (thankfully). The expensive LND-7317 G-M tube in my $550 (never bumped or exposed to weather) Medcom "Inspector" also failed suddenly --just stopped dead after running with stable performance for over a year. (When I bought my Inspector, the warrantee on its electronics was for a year, but its G-M tube was exempted with only a 90 day warrantee, so these problems must be common in the industry.)
* I replaced that LND-7317 tube with an old Russian, military surplus SBM-20 (modifying the circuit a bit). Russian technology is tough and goes the distance. Unfortunately, the SBM-20 is too large for a GMC-200, so I suggest that you take a chance on another M4011. (And yes: it's a good idea to keep glass G-M tubes away from sunlight and fluorescent lamp light.) |
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| Reply #6
bmp200
10 Posts |
Posted - 01/05/2015 : 18:07:59
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Thanks for the advice. The M4011 seems to be my best choice.
By the way Craig, are those charts all just exports to spreadsheet - or are they from some data logging software? |
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| Reply #7
Craig
USA
22 Posts |
Posted - 01/06/2015 : 10:40:24
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Since the data logging software I use (the old Geigergraph-4.3) only averages to two places, I divide its (usually about 12 hour) daily total count by the total minutes, then divide that 4 place CPM by the fixed (over a year ago) baseline CPM here to get the percentage of normal. I manually enter that percentage for each day into the "ZGrapher" program (a free download at:
> h**p://www.palamsoft.com/
--which produces the monthly graph and has a GIF (or a BMP) output. Finally, I assemble the GIFs for the year in a graphics program (my Picture Publisher-3.1 is long gone, so try Paint.Net), which I upload to RadViews as a supplementary graphic. |
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| Reply #8
Alchemy2
Canada
89 Posts |
Posted - 01/12/2015 : 15:39:06
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I have actually 2 new 4011 tubes, one has maybe 3h on it, the other maybe 5 minutes never been used I'd be happy to send to you gratis. I have taken them out of my GMC-320+ units and don't use them. You're welcome to them!!!
We can exchange info, if interested.
Alchemy2 |
The more I learn, the more I realize I do not know! |
Edited by - Alchemy2 on 01/12/2015 15:42:33 |
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GMC-200 acting strangely
