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markd
2 Posts |
 Posted - 09/24/2023 : 02:19:53
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Hi,
Looking for some advice on this device that I purchased from Amazon. I'm getting a very variable background CPM. It sits around 30+ cpm, but then jumps up into the low hundred then back down to 30+. It can on occasion give a reading in the high hundred before dropping back to "normal".
I've tried it in several locations in the house as well as outside, with the same pattern. I've tried it whilst in its case and out, to try and see if light is having an effect. I've even gone so far as trying timed counts whilst in a metal box, and still get the same pattern of normal readings with periodic high readings.
Have done several factory resets to no avail. As such I suspect the device is defective and am planning on returning it, buy would appreciate any advice.
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| Reply #1
ullix
Germany
1171 Posts |
Posted - 09/29/2023 : 00:15:53
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One more thing to check: remove the battery, wait a moment, and reinsert. Problem solved?
If not check the anode voltage. A voltage too high might result in a pattern such as you observe.
But be aware of the problems related to correctly measure the anode voltage. This article "GeigerLog-Radiation-v1.1(CAJOE)-Support-v1.0" has a discussion of it. Find it here: https://sourceforge.net/projects/geigerlog/files/Articles/
If you have no 1 GOhm resistor, you could use just a regular DVM, BUT even if you had a proper anode voltage of, say, 400V, you would measure only something between 120 ... 140V. If you measure anything beyond 140V, it might be an indication that the anode voltage is set too high. Lower it!
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| Reply #2
markd
2 Posts |
Posted - 09/29/2023 : 12:25:53
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That is exactly what I did - reduce the voltage and it's behaving as I would expect. I set it to around 380V. Interestingly, doing a factory reset was taking it back up to 415V.
Have tested with a gas mantle test source and am seeing CPM around 800, but with a "normal" background CPM around 20. |
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| Reply #3
ullix
Germany
1171 Posts |
Posted - 09/29/2023 : 23:03:27
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Next time you experience such problems do measure the anode voltage! It looks that by some glitch this got increased by far too much.
Look up "plateau" for Geiger counters. This is an anode voltage range within which the Geiger tube works. For none of the tubes used in the GMC counters no official spec is given. But it is often assume that a range of 380 ... 450 V will work. So, 415 V is smack in the middle.
Should work, if this is the true voltage. However, you can't fully rely on this being true. Do measure!
When you measure with with a regular DVM, you will see only something between 120 ... 140V, as the anode voltage collapses.
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Advice on GMC 500+
