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DeivF
USA
2 Posts |
 Posted - 11/15/2025 : 15:32:34
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I recently purchased a new GMC-600 Pro directly from GQ Electronics. I already have a GMC-600+ and a Black Cat Systems GM-45. Just as with both the GMC-600+ and GMC-600 Pro, the GM-45 also uses the LND7317 "pancake" detector.
Soon after acquiring the GMC-600 Pro, I noticed that it seemed to be reading anomalously low. Just to check, I logged all three detectors continuously for three days. I placed all three detectors side-by-side on a table with the LND7317 windows facing up and nothing (except the table) for several feet in any direction from the detectors. Over the three day period, the GMC-600+ recorded an average of 47.2 CPM, the RM-45 recorded an average of 48.2 CPM, while the GMC-600 Pro recorded and average of 34.0 CPM.
In other words, over the three-day period, the GMC-600+ and the RM-45 recorded averages within 2.3% of each other, while the new GMC-600 Pro recorded approximately 28% less (it recorded a three-day average that was only 72% of the other two detectors).
Soon after, I took the GMC-600+ and the GMC-600 Pro on a commercial aircraft flight from California to Chicago and back. I placed both detectors side-by-side in my nylon-sided carry-on bag which I placed in the overhead storage. Both units had the LND7317 windows facing up and had nothing except one thickness of nylon (the top of the bag) over them. The one-hour running average (OHRA) readings from the GMC-600 Pro, both on the ground and at altitude and on both flights, were consistently only 70% to 74% of the OHRA readings from the GMC-600+.
While I don't have any calibration sources, I assume that since the readings from both the GMC-600+ and the RM-45 have historically been similar (within a few percent of each other), the new GMC-600 Pro is giving anomalously low readings.
Can this be corrected? Or do I need to return the GMC-600 Pro to GQ Electronics?
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Edited by - DeivF on 11/15/2025 15:38:27
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| Reply #1
ullix
Germany
1224 Posts |
Posted - 11/16/2025 : 03:35:06
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Your question relates to multiple issues.
First, on any GMC counter the first thing you should do is to switch off the dreaded FET option - this may ruin all your data. It is switched off when set to 60!
Second, no two counters give the same CPM values, even when of the very same make! Comparing the uSv/h (micro-Sievert) values is the only valid way for comparison! But note 2 things: first, this not possible unless each counter has been correctly calibrated. And I am almost sure that neither counter was, because it costs quite a few extra bucks to do it in a reputable lab. And second, this calibration is valid only, O N L Y when your radioactive sample is a pure Gamma emitter. When it also emits betas - most samples around hobbyist do - a uSv reading is INVALID and can never be compared between counters!
Third, the radioactive process can be described by a Poisson distribution. Human beings cannot tell whether any of the data scatter has a Poisson property. But software can tell. To my knowledge, the only one useable is my GeigerLog software. You find it here (it is open source): https://sourceforge.net/projects/geigerlog/
Load your data, then click "Poiss" to run a Poisson test. Data are ok when you get such a result, no matter how weird the data may look like;
And lastly, nice flight data! I'am collecting such stuff for hosting on my GeigerLog site. When you are ok with, please post the raw data!
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600 Pro reading anomalously low
