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Pheckphul
8 Posts |
 Posted - 04/08/2014 : 15:07:56
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I visited Trinity Site during the now once-a-year public access day last Saturday (5th). The literature stated that the measured radiation at the site is 0.5 mR/hr. However, my GMC-320+ measured ~0.05 mR/hr. I also compared the GMC-320+'s measurements to those of two Geiger counters (both of which were calibrated within the last year) that were being used in the public information booth to demonstrate the level of radiation in items that they had on display, and likewise the GMC-320+'s readings were dramatically different. The difference wasn't exactly a factor of ten as was the measurement I got at ground zero, and unfortunately I don't recall the exact difference as I was a bit sleep-deprived.
This experience has left me wondering as to the accuracy of the unit. Based upon the calibration section of the user guide, I assume you have calibration sources that you use to calibrate the units before shipping them out. Perhaps my unit somehow missed being calibrated?
Lastly, from using the data viewer, it appears that the GMC-320+ stores the data as CPM. How does one go about converting CPM to mR/hr? From what I've read it depends upon the source and the unit doing the measuring. |
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| Reply #1
ZLM
1261 Posts |
Posted - 04/09/2014 : 15:00:46
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The regular background(not Trinity Site) reading should around 0.05uSv/h.
The calibrated data are stored in the unit. You can check the conversion rate from CPM to uSv/h OR mR/h. From unit main menu -> init setup -> Calibrate
Edited on Apr-9th-2014 |
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| Reply #2
Pheckphul
8 Posts |
Posted - 04/09/2014 : 18:22:10
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quote:
Originally posted by ZLM
The regular background reading should around 0.05 mR/h, not the 0.5mR/h
The calibrated data are stored in the unit. You can check the conversion rate from CPM to uSv/h OR mR/h. From unit main menu -> init setup -> Calibrate
I hope I'm not being too annoying, but can you explain the three calibration points? The values of mine are as follows:
1: CPM 60 uSv 0.3
2: CPM 240 uSv 1.2
3: CPM 1000 uSv 5.0
To me all three values look to be the same. i.e. all multiples of 60 CPM equating to uSv 0.3. Are these just defaults for m4011 tubes?
Lastly, can you please explain the comment that the background radiation at Trinity Site should register as 0.05 mR/hr on my 320+ instead of 0.5 mR/hr? mR/hr is a concrete measurement and should be measured equally no matter what Geiger counter is performing the measurement assuming the GM tube configuration is the same for the type of radiation being measured (alpha vs beta vs gamma). CPM is the only variable measurement. Am I not correct? |
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| Reply #3
ZLM
1261 Posts |
Posted - 04/09/2014 : 20:31:34
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Sorry. I did not write correctly. I edited my original post.
It seems your GMC-320+ detected the background radiation level at 0.05mR/h, but it is not as high as it should be. One of the factor may be the Alpha detection.
Yes. You are right, all Geiger Counters should get same level from a same source.
The calibration data in your unit is for generic measurement. Basically it uses only one factor for three data. You can change it to match your own testing with different factors.
It is good if you got CPM recorded. You can use that data to calibrate your unit to the mR/h value you think it should be. If you do not know how to do it, you can either post the file here or send it to support@gqelectronicsllc.com for the further assistant.
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GMC-320+ accuracy?
