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Erwin55
Germany
41 Posts |
 Posted - 09/21/2022 : 08:24:52
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Dear all,
not a big issue, just a stupid question: How does the (default-) calibration data of the counter correlates with the data mentioned on the card shipped with the GMC? I'm only a litte bit confuse.
Regards Erwin |
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| Reply #1
ullix
Germany
1171 Posts |
Posted - 09/22/2022 : 00:28:09
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Easy answer: It does not correlate ;-)
I have never paid attention to this card, and thus never realized it, but you are right, it is nonsense to universally equate CPMs with uSv/h. And within the card details the nonsense goes even deeper:
The point of a calibration is to find the factor which allows the conversion of CPM to µSv/h. For example, a M4011 tube uses a sensitivity of 154 CPM/(µSv/h), a SI3BG tube (the 2nd one in a GMC-500+) uses a sensitivity of 2.08 CPM/(µSv/h), and the pancake tube LND7317 (in the GMC-600) uses a sensitivity of 348 CPM/(µSv/h).
So, when in a radiating environment of 1 µSv/h, you get:
with a LND7317 tube: CPM= 348
with a M4011 tube: CPM= 154
with a SI3BG tube: CPM= 2
So, one and the same thing would cover all first three Action level lines on the card!
Then the card states: CPM: 51 - 99 => µSv/h: 0.10 - 1.00. But 51 and 0.10 implies an underlying sensitivity of 51/0.10 = 510 CPM/(µSv/h), while the upper end gives a sensitivity of 99/1.00 = 99 CPM/(µSv/h). That is a 5 fold difference in the sensitivity!
Incredible.
As an aside, the relationship 1 mR/h = 10 µSv/h is true by definition, HOWEVER ONLY when pure Gamma radiation is in play! The unit mR (milli-Röntgen) is outdated and should not be used.
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Edited by - ullix on 09/22/2022 00:29:42 |
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| Reply #2
Erwin55
Germany
41 Posts |
Posted - 09/24/2022 : 10:48:41
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Thanx Ullix,
I like your precise answeres, especially line 1. :D
I was confused and now I'm fine. |
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| Reply #3
J
Ireland
2 Posts |
Posted - 11/06/2022 : 15:28:48
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Excuse me jumping in, but in summary would you advocate referring in broad terms only to an increase by percentage of CPM to indicate varying degrees of exposure risk (with duration also considered) ?
Otherwise, micro severt readings on the card are presumably reliable as stated ?
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Theres nothing to fear, but fear itself
.apart from ionising radiation in some circumstances. |
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| Reply #4
ullix
Germany
1171 Posts |
Posted - 11/07/2022 : 06:15:34
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If you are interested in the medical effect of a certain radioactivity dose, then the only acceptable unit is "Sievert". (The unit "Röntgen" is largely equivalent, but is a deprecated unit, which should no longer be used).
The challenge is to find a conversion from the dose-rate as Geiger-counts by a tube per time into the dose-rate in µSv/h.
While this is only a single, simple factor, called Sensitivity, it is not easy at all to determine this factor. For a M4011 tube a Sensitivity of 154 CPM/(µSv/h) is used, though there are no data that such a calibration had ever been done.
This is an important topic, and you find more on it in chapter "Calibration" in the GeigerLog manual (https://sourceforge.net/projects/geigerlog/files/).
This "card" has so much BS on it, that I suggest to not use it all. Use official sources; you find links in chapter "Occupational Radiation Limits" in the manual. You also find my comments on the limits in chapter "On what grounds do we set the radiation safety levels?". The latter explains the limits implemented in GeigerLog.
Please, also note that the conversion factor is valid ONLY for the situation, in which it was calibrated in. All calibrations known to me were ALWAYS done in a pure Gamma situation. Thus, when you have a source emitting strong Beta radiation, this calibration is NOT valid!
Unfortunately, almost all source which Geiger hobbyists can use are strong beta emitters! Then applying any conversion produces only more BS!
The exception might be background radiation, provided you measure far enough (>2 m) from any stone wall or tiles. But then you have to take into account the additional complication due to counts from electronic noise. GQ specifies them as "up to CPM=12". Now what? "To subtract or not to subtract", as Hamlet used to say ;-).
For your own environment I'd suggest to use changes of CPM counts, as you seem to be suggesting already. Establish the background at various locations (on your desk, the dinner table, the kitchen counter, the bathroom, the floor in the hallway, the cellar, outside, ..., each position measured for an hour) and watch for changes.
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