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 2.GQ Geiger Muller Counter
 False high readings
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LS1M

3 Posts

Posted - 03/19/2022 :  05:21:47  Show Profile  Reply with Quote
What can cause a GMC-600+ to falsely read high? Last weekend, it went from 30-50 CPM to over 4000 for several hours. Powering it off & on again didn't help. I got so worried I called 911, but a hazmat team confirmed there was no radiation. Customer support recommended a factory reset, which I did, but I figured if the sensor was broken it would read zero, not high? How am I supposed to know if a high reading like that is real or not?

Thanks
-Mark
Reply #1

LS1M

3 Posts

Posted - 03/19/2022 :  05:50:41  Show Profile  Reply with Quote
Maybe the tube voltage? If I go to main menu -> init setup -> Tube settings -> tube voltage, I get 572 volts. What is the normal range?
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Reply #2

ullix

Germany
775 Posts

Posted - 03/19/2022 :  06:12:42  Show Profile  Reply with Quote
@LS1M: Overvoltage could indeed have the consequences of very high readings. But while 572 V is a bit on the high side, it would still meet the specs of the manufacturer (475-675V, https://www.lndinc.com/products/geiger-mueller-tubes/7317/)

However, the reading might be completely off. There were multiple reports of problems in this forum recently that were either proven or likely to be caused by over-voltage. This is one: http://www.gqelectronicsllc.com/forum/topic.asp?TOPIC_ID=9817

Read it and find the instructions on how to measure anode voltage and do that.
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Reply #3

LS1M

3 Posts

Posted - 03/19/2022 :  06:39:27  Show Profile  Reply with Quote
I did try measuring the voltage, but it showed something like 59 volts on my DVM, which apparently is because of the low DVM resistance. Mine is 1 Mohm. It also triggered a high CPM count until I disconnected. According to your pdf, looks like I need a 1GOHM resistor. Would this one work?

https://www.mouser.com/ProductDetail/Ohmite/MOX-400231007FE?qs=sGAEpiMZZMtlubZbdhIBIPfbcHfecTmFJc4aggPOobo%3D
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Reply #4

ullix

Germany
775 Posts

Posted - 03/19/2022 :  07:34:12  Show Profile  Reply with Quote
Yes, 1MOhm is too low; 10 MOhm is standard.
That resistor would work; also good that it is 1%.

Edited by - ullix on 03/19/2022 07:35:17
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Reply #5

Damien68

France
695 Posts

Posted - 03/19/2022 :  08:06:12  Show Profile  Reply with Quote
with a 1Mohms DVM and 1Gohms add resistor, this divide the signal by (1000 + 1) / 1 = 1001 so you must read 0.57v.
it's okay if you have a fairly low caliber like 2 volts available in your DVM. otherwise it's not excellent.

To check the impedance of your multimeter, you can either:
- check it by measuring it with a second multimeter.

- you can also measure the voltage of a battery and measure it again with adding a resistor in series. and compare the results
if you add the same resistance in series (in your case 1Mohms) the measurement must be divided by 2

I can't be sure about what I'm going to write below but I believe that:
if your tube is totally broken it will not give any counts because the gas/air pressure (1 Bar) is too high.
the problem is if it's simply corrupted for example by micro cracks because at this time small quantity of air can enter in the tube and the gaz pressure will remain low. At this time there are risk of auto-ionization and false counts.

it's important to avoid placing a hands under the meter and also avoid placing the underside of your meter too close to electric cables or metal parts especially if its USB connector is connected to a power supply or a computer. this to avoid electromagnetic interference.

Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 03/19/2022 09:12:52
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Reply #6

EmfDev

1700 Posts

Posted - 03/21/2022 :  10:48:00  Show Profile  Reply with Quote
Hi Mark, check if the reading is repeatable. Meaning that the counter reads high everywhere. If not then it may be faulty sensor and it might need to be replaced. ***OR check if the connections are good and check if the wires are too close to the inductors after you attach the back case, make sure they are not too close.
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