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 2.GQ Geiger Muller Counter
 Normal readings for 30 mins, then off the chart..
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2 Posts

Posted - 08/03/2017 :  10:56:20  Show Profile  Reply with Quote
So I just received my GMC-320+ v5 yesterday. I tried it for about 5-10 minutes at a time around the house, and everything seemed normal - background radiation showing 20-30 CPM.

So today I decided to take it on my commute to work, which happens to go past Rocky Flats in Colorado - a known site with issues. Strangely enough as I drove by there, the readings were actually very low - about 15-20 CPM.

However, about 30 minutes into my commute, right as I reached Golden, CO, the numbers started continuously climbing.. First 60 CPM, then it hit 100. It all started as I got near a Starbucks in Golden, where I ordered a coffee at the drive through..

By the time I got to my office in Evergreen, CO, the GMC-320 was reading 1500 CPM! I honestly thought that Starbucks had handed me a nuclear coffee..

It didn't matter where I went with the GMC-320, it would read insane readings.. inside the car, left on the ground across the parking lot as I walked away from it.

I have it on my desk in my office now, and I left it powered down for 30 minutes. It's now reading fairly normal when first turned on, but then starts climbing after about 5 minutes of being powered on. About 70 CPM after 5 minutes, 100 CPM after 6-7 minutes.. then it starts to climb almost exponentially, hitting 1500 CPM after about 10 minutes of being on.

Is this a defect, or do I need to call the authorities? :) Kinda sorta kidding.
Reply #1


2 Posts

Posted - 08/03/2017 :  11:17:57  Show Profile  Reply with Quote
Just wanted to add, it seems to be related to how long the unit is on.

If I power it off and right back on, it jumps immediately to 200-300 CPM and keeps climbing.

If I leave it off for a while, the climb is much slower.

EDIT: If I blow on the tube through the vents, the count drops immediately and precipitously.. Like from 1400 CPM right down to a normal 30 CPM, within the course of a minute. Then it starts to climb again.

I saw someone mention this in another post, but I'm not sure what it means - the tube overheating? Overall this behavior seems overheat-related to me, and indicates a defect in the tube. Does this sound right?

Edited by - tcp100 on 08/03/2017 11:38:33
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Reply #2


United Kingdom
35 Posts

Posted - 08/04/2017 :  00:17:49  Show Profile  Reply with Quote
If it's getting hot then there must be a high resistance, try re-seating the tube in its clips because perhaps they have oxidised a little. Power the device off first - don't touch the clips when it is running! (high voltage, and beware the caps may hold voltage for a long time even after powered off).

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Reply #3


39 Posts

Posted - 08/04/2017 :  04:35:10  Show Profile  Reply with Quote
Hey tcp100

Try to open the (turned off) device, and check if the tube is mounted correctly (there's a + - sign on the tube, it should match the to the plus-pole on the board... (attached a photo of my 320v4; the plus-pole (indicated by a picon) is on the left
(if you have to turn the tube, don't touch it with your fingers, wear some gloves)

If this is correct, i suspect the tube is getting too many volts - this would be a task for the

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Edited by - the_mike on 08/04/2017 04:37:49
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Reply #4


1241 Posts

Posted - 08/04/2017 :  22:09:45  Show Profile  Reply with Quote
The unit seems need a new tube. The tube may be leaking.
You can write an email to for the warranty.
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Reply #5


202 Posts

Posted - 08/04/2017 :  23:41:37  Show Profile  Reply with Quote
The high voltage on this device is not harmful. It's a little device with a small battery that runs for month. There is so little amperage that you don't even feel anything.
It's not volts that kill.

GMC-300E+ V4.20 with sbt-11a alpha tube

My statements are "stuff-a-hobbyist-says" and not in any way professional.
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