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 2.GQ Geiger Muller Counter
 GQ GMC 500+ Assistance with CPS spikes

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T O P I C    R E V I E W
Volts Posted - 01/19/2022 : 13:02:39
Hi all,
I am after some help to determine if my GQ GMC 500+ is faulty Ė Iíve had the device since September and never had anything but low background readings (20-30 average).

Starting around two weeks ago, I started to notice the device was alarming for a few seconds at a time, maybe 5-10 times a day.

The device is stored in a cupboard, on a shelf and never moves. Its plugged into a power supply so remains 100% charged. Temperatures in the cupboard are very stable, around 19 degrees Celsius and thereís no drafts or windows so no UV exposure to the tubes.

Since the alarms started, I have been downloading the data to see if I can make sense of it. I appreciate that the Fast Estimate Time setting as Dynamic can spike the CPM, providing artificially high readings so when the alarms happen sometimes the CPM is showing as 1045 for a second, then subsides.

The data extracted though shows a more accurate picture with very low CPS, apart from some small blips that are triggering the alarms.

Its hard to see what is happening on the tube count during an alarm as its only on the display for 2 seconds, but I can see that CPM1 has most of the counts but CPM2 does start to register counts too.

Iíve extracted the data to Excel and applied some formatting to help see the CPS blips Ė the example below shows 110 CPM, driven by increased CPS for 3 seconds.

How do I determine if this is a fault?

Thanks in advance,
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ullix Posted - 01/21/2022 : 01:03:23
I just remember, the newest 500s may allow to see and set the anode voltage via the menus. Does your device allow this, and if so, what is the current anode voltage?

Reason for my question: if the anode voltage is grossly mis-adjusted and too high, then the tube might discharge every now and then. A fluorescent tube gives light by doing this permanently.

The anode voltage reading is not very accurate; maybe plus/minus 10%, but it can serve as a relative value. I would suggest to then reduce the voltage by 30V, and observe the readings for a few days and look for those outliers.

However, should the bad readings be gone, it might still be an indication for a bad tube and you need to contact service anyway.

Volts Posted - 01/20/2022 : 05:10:04

Thank you, will try GeigerLog test and contact support.
ullix Posted - 01/20/2022 : 00:33:05
One important way to test Geiger data is to run a Poisson test. Load the data into GeigerLog and click the button.

Though, I sense it won't pass; there are too many big outliers. Unless you have big machinery running nearby (elevator, air condition, old fridge, vacuum cleaner, radio transmitter, ...) I'd say this counter is faulty.

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