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fnfunkin
1 Posts |
 Posted - 02/26/2026 : 19:33:42
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Hi everyone,
I recently picked up a GQ GMC-500+ and have been really pleased with how solid it seems… except for one issue I can’t quite figure out.
When I first got it, readings were pretty stable — I was getting consistent CPM values when measuring background radiation in my home (~18–22 CPM). But over the past week, I’ve noticed some very inconsistent spikes and drops that don’t make any sense based on my environment.
Here’s what I’m observing:
With the detector sitting in the exact same spot on my desk (no movement), I’ll see readings suddenly jump from around 20 CPM to 90–120 CPM for several seconds, then drop right back down again.
If I wave the device slowly in the air, the spikes are even more erratic — sometimes it jumps 3–4 times higher than I would expect.
I’ve tried shielding with a small lead block and the spikes still occur (albeit slightly dampened), so I don’t think it’s just background variation.
Battery level is good, and I’m running the latest firmware (v1.05) available from the website.
I haven’t changed tubes — it’s still using the stock SBM-20 tube.
A few questions I have:
Is anyone else seeing this sort of large CPM fluctuation on otherwise normal background readings?
Could this be a hardware calibration issue or a failing tube?
Should I try a firmware downgrade/upgrade to debug this?
Are there settings I should check (e.g., debounce or sample time) that might smooth readings?
I have photo/video logs of the CPM graph over time if that helps. Appreciate any insight — definitely don’t want to jump to conclusions if it’s something simple I overlooked!
Thanks in advance! |
https://fnf.lol |
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| Reply #1
3z_phoenix
11 Posts |
Posted - 02/27/2026 : 01:18:21
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hi fnfunkin,
i have a GMC-500+ too, now good close to 2years in use, running firmware version 2.42,
until now with no such behavior.
You say "running latest firmware availible on website", thats some tricky thing bcs.
EmfDev often told here in forum:
""the firmware is bound to the serial-number of the device""
""You can email support@gqelectronicsllc.com with your serial-number to check for updates"
I not know exactly my tube-type bcs. it have a dark plastic shrink hose, to prevent desturbing light-photons.
I not know if this is related to M4011 tube only or the SMB-20 too.
It can never be bad idea to prevent desturbing light onto glas-tubes.
Ullix told here often there is a wrong calculation in
"fast estimate time", for not become effective it should be set to 60 seconds.
From my view, in normal radiation background, there is no need to keep respect of the deadtime-value of the tube.
The resulting error from this is too low, i switch it off... as long as i know this should be also the default.
Life-time of the tube, sure there is a possible degradation over the time, nothing is build lasting forever.
Also the tube can fail before reach the manufacture expected lifetime.
In normal radiation background for me that expected maximum count is close to a human life.
I also still use the default calibration values, bcs. in compare with
https://odlinfo.bfs.de/ODL/EN/topics/location-of-measuring-stations/map/map_node.html
my counter follows realy close the shown values of local actually natural background radiation there.
Still bound in the 5...10% error of the device.
For make sure to use the manufacture-defaults first a factory reset can be a good start.
check if fast estimate is set to 60 seconds, and not use deadtime of the tube.
The tube should also be covert with this dark plastic sleeve, only slide over the tube, not use hot air for shrinking.
Digital counters can also be triggered by high electromagnetic fields.
First i would only use the local batt at the device for power up.
Also the counter should not be close to any switching powersupply or something like that...
most unlikely, but still to keep in mind...
Aaaah, and there can be changeing levels of Radon-gas, mostly can happen in cellar-rooms, or with change of weatherconditions, different levels of radon escape the earth.
Let us know if anything helps to come closer in found the reason of the strange behavior of your counter...
kind regards, 3z_phoenix
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[first rules from my survival-handbook]
1. ...the difference between democracy and anarchy are 3 canceled main meals ~ONLY~
2. ...better have it - but not need, as need it - but not have!
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Inconsistent CPM readings on new GMC-500+ — possib
