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 SI3BG tube conversion factor
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15 Posts

Posted - 03/01/2022 :  17:36:26  Show Profile  Reply with Quote
Some time back I found i think here info about the 500+ and the conversion factors for the high and low tubes. I recall a thread with someone named Ullix and Stargazer and getting these numbers. I wrote an app to pull data and in track /store info, but now it has been a couple of years and I cant find the thread where I ever got these numbers - and now I have some questions;

for the 4011 tube: 153.84 and for the Si3BG 2.024291498 - I take the count divided by the number to get uSv/hr or apply various math formulas for mSv or R etc.. per hour minute second month etc..

but now I cant remember a few key things.

1: are these numbers matching to the number In the unit (e.g. on screen).

2: if not, what are the right numbers

3: in my app I pull both values separately and track separately, but i have always wondered, if both tubes are on how does the unit itself decide how to blend the data into one reading?

4: unrelated, but I wanted to buy another 500+ but I noticed on amazon the gmc store stated "2017 model". i recall the original models didnt have a separate power supply for each tube which was then updated. Are these old stock or is that just identifying the design was set forth in 2017 to differentiate from oh say the 1955 victoreen units?

thanks in advance, I really appreciate it.
Reply #1


2145 Posts

Posted - 03/01/2022 :  18:10:37  Show Profile  Reply with Quote
Hi skydiverman, check this stickied thread:

If not that one, maybe another with similar title.

1,2. If you mean the calibration data, the default is (M4011) 1 cpm -> 0.0065 uSv/h and (SI-3BG) 1 cpm-> 0.00468 uSv/h.

3. Threshold calibration 3 for tube 3. if CPM reaches that value, it will try to calculate uSv/h with both tubes.

4. 2017 model means that GMC-500+ model came out in 2017. There has been a lot of hardware/software changes but the model is still the same. But the firmware and hardware revision should be different
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Reply #2


15 Posts

Posted - 03/01/2022 :  19:07:14  Show Profile  Reply with Quote
Originally posted by EmfDev

1,2. If you mean the calibration data, the default is (M4011) 1 cpm -> 0.0065 uSv/h and (SI-3BG) 1 cpm-> 0.00468 uSv/h.

Thanks for all the good info above.. your number on the 4011 matches (although in reverse format) but regarding item 1,2 - reading the tacky thread, i found the following from Ikerrg

"But the SI-3BG @ 0.48 uSv/h/CPM looks definitely the best candidate for the next firmware! "

now this is a lot closer to the .494 (2.0) and even the 1.135 for gamma only, (mixing the rate of CPM to uSv and uSv/CPM a bit here) - but regardless you quoted .00468 above which seems off by a couple of units. am I reading that right?

Edited by - skydiverman on 03/01/2022 19:14:34
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Reply #3


1109 Posts

Posted - 03/02/2022 :  03:23:19  Show Profile  Reply with Quote
The concept behind the conversion from the dose-rate measured as counts of a counter to the dose-rate in units of uSv/h is anything but trivial, and there is no single conversion value, which is correct at all conditions!

A lot of discussion took place here on the forum:

I tried to put this all in perspective in the chapter "Appendix G – Calibration", which you find in the GeigerLog Manual, download here:

There are a few things which I want to emphasize:

The conversion factor is a single value. I call it Sensitivity and it has the the units: CPM / (uSv/h). It is easy to comprehend: a tube with Sensitivity 154 is more sensitive than one with Sensitivity 2. GQ is using Calibration, which is the inverse of Sensitivity (compare 0.0065 with 0.495 in units of uSv/h/CPM).

The conversion factor is determined for a true Gamma source, which itself has to undergo a very elaborate calibration - only a few institutions can do it. Thus the "conversion factor", "Sensitivity", "Calibration" is ONLY EVER valid for Gamma sources! When you use a radioactive source with a significant Beta component (Potassium, Thorium, ...) the conversion factor is MEANINGLESS and thus the use of uSv/h for your results is MEANINGLESS! When using it nevertheless, you only indicate that you do not really understand what you are doing!

There are only 3 types of radioactive sources used for the calibration: Cs-137, Ra-226, Co-60. They have significantly different Gamma spectra. Since the sensitivity of a Geiger tube is gamma-energy dependent, the consequence is that the correction factors will be different, depending on what source is used for calibration.

It follows that you, as the experimenter, must decide, which gamma source spectral profile is closest to your current situation and pick that for conversion factor. It thus is an INTERPRETATION of the experimenter; it may not be scientifically valid! (more in the GeigerLog manual)

Given all the uncertainty, it also follows that generally there are no more than 2 significant digits needed for the conversion factor. Using 3 like "154" is already pretentious.

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Reply #4


15 Posts

Posted - 03/02/2022 :  06:31:19  Show Profile  Reply with Quote

Thank you for your info and also the links, which I will read shortly.

You are correct that i am not an expert on any of this! The device itself presents as its default readout in uSv/hr which I believe is an imperfect representation of the radiation, as you are saying, for a beta source vs a gamma source the conversion factor may be different and then it gets complex when there are perhaps multiple kinds of sources at once.

And yet the device presents the number nonetheless, and I am ultimately trying to confirm the conversion factors used, as I couldnt find the original thread in which the numbers were disclosed. your statement "compare 0.0065 with 0.495 in units of uSv/h/CPM" I think however is all that I need to confirm that the factor 0.495 is the current accepted number for the SI3BG for that purpose, and not .00468 as EMFDEV provided above (by which I dont mean that there is great value in the different between 495 and 468 but rather the location of the decimal point!) THe value i had from the thread was some time back and was .494, so I think that gets me what I need for this simple purpose. Accepting of course, that depending on the source the reading in count should perhaps be interpreted differently.

thank you!
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Reply #5


1109 Posts

Posted - 03/02/2022 :  09:35:02  Show Profile  Reply with Quote
It is easy enough to look up the conversion factor currently used in your counter by yourself when you run GeigerLog.

For my 300E+ counter I get (1 tube, no WiFi; sorry for the large pics, but the numbers get blurry otherwise):

For my 500+ counter (2 tubes and WiFi) I get:

For the 300 you see 3 "Calibration Points", which all three resolve to Sensitivity=154. Why there are 3 points is a different story - you never need more than a single one!

For the 500 the first 2 of the points resolve to 154, which the counter applies to the first tube, the 3rd point resolves to 5.155 and is applied to the 2nd tube (thereby confirming that a single value is enough).

Mind you, these are not my numbers, but GQ's numbers as they were put by them into the firmware of the counter! And it is what the counter uses to calculate and display uSv/h results.

In GeigerLog you can modify these values directly in the dialog boxes shown. Once you have done the mods via the counter's menus, you will appreciate what GeigerLog offers ;-)!

GeigerLog never uses any of these numbers from the counter, but defines its own defaults. Which of course can be changed by the user.

By the lack of anything better, I also use 154 for the first tube. I use 2.08 for the 2nd tube, after lengthy discussions and experiments, some of which also told in the manual.

Confused enough? I have more ...
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Reply #6


15 Posts

Posted - 03/02/2022 :  15:33:57  Show Profile  Reply with Quote

May I ask one more question regarding the SI3BG?

I found a datasheet (in russian) but the translator claims the working range of the tube is up to 300 R/hr

however GQ claims the dose rate for the GMC500+ is only 10x that of the 300+

"Range of dose rate indications, ľSv/h 0.00 to ~4250
(0.00 to ~42500 on GMC-500+)"

and if my math is correct 42500 uSv is 4.5 R

it would seem to me that if the ratio of sensitivity is really 77x then the top end for the SI3BG would be more like at least 35 R/hr. if we take the max CPM from the user guide (0 to 982980) that would put us more like 200 R/hr

Yes we're in the land of nuclear war not samples, but at least conceptually am I right on the above? that the upper range would be in at least 10's of R/hr ? i assume nobody had really tested for this, but it seems the obvious use of the 500+, as a replacement for the old CDV715 / 700 high/low range.

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Reply #7


15 Posts

Posted - 03/02/2022 :  19:06:58  Show Profile  Reply with Quote
I find myself with another question now, having read the threads...

if I was to take a reading of an unknown source;

if i was to read CPM1 and then put a blocker like a piece of steel which would be a 100% beta blocker, and read CPM2, could I then do something like

CPM1 - CPM2 * BetaConversionFactor = Beta uSV
CPM2 * GammaConversionFactor = Gamma uSV
= total uSv

which would be more accurate than simply using the approximate conversion factor * CPM without doing this process?
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Reply #8


1109 Posts

Posted - 03/03/2022 :  01:13:21  Show Profile  Reply with Quote
your 2nd question first:

The 500+ can be asked for CPM, CPM1st, and CPM2nd (and the corresponding CPS). 1st means the 1st tube, supposedly a M4011, 2nd means the 2nd tube, supposedly a SI3BG.

CPM: returns the sum(!) of counts of 1st and 2nd tube.
CPM1st: returns the counts of the 1st tube
CPM2nd: returns the counts of the 2nd tube

CPM is completely meaningless information and should not be used for anything!

Holding a plate of steel between source and counter would certainly absorb all betas, but, depending on the circumstances, you may be up for a surprise with respect to the countrate - it may increase! You could be generating bremsstrahlung, which the counter can detect.

Read chapter "Appendix – Range of Electrons in Matter and some History" in my "Potty Training for your Geiger counter" article Therefore you may have to use Plastic-Metal-Sandwiches instead.

But let's assume whatever you use, it does a complete job of eliminating betas, and does not produce anything else.

If there were any betas initially, both tubes would now show lower count rate. Why would there be any discriminatory effect between the tubes?

Furthermore, the conversion factor is based on Gamma. Pure Gamma. And there is no conversion factor for Beta for a Geiger counter. To my knowledge, nobody has ever attempted that. For good reason; I don't know how such a thing could be done given the properties of beta (energy spectrum, range in matter).

In summary, I don't see a way to measure total uSv/h with a Geiger counter on beta sources.

Edited by - ullix on 03/03/2022 01:13:46
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Reply #9


1109 Posts

Posted - 03/03/2022 :  07:19:37  Show Profile  Reply with Quote
your first question:

First off: if anyone speaks Russian, please take a go at the translation of this Russian data sheet of a SI-3BG Geiger tube: I am not fully convinced the presented translation is correct.

But, using it as given, I'll comment one by one the elements of the data sheet relevant to us:

Own Background: 0,2 Pulses/s
Working Dose Range: 300R/h (very high!)
Sensitivity to Gamma Radiation: 188 – 235 Pulses/s/R/h

Own Background: 0,2 Pulses/s

Meaning, the tube will produce CPS=0.2 or CPM=12 countrate even without any background or whatsoever radiation. I think GQ is also giving this value for every tube they use.

But: in a 500+ counter, the maximum I have ever seen is 1/10th of that. In this moment running on a 500+: CPM=0.28. Why is that? Is the electronics improperly tuned? If it is improper here, why would it be proper on the other tube?

Working Dose Range: 300R/h (very high!)

Since we are talking about Gamma radiation, we can equate 1 R = 1 Rem (not quite right, but close). And we know that 1 Rem = 10000 mikro-Sv by definition.

Then 300R/h = 300 * 10000 => 3 million uSv/h = 3 Sv/h. That is a lot!

To put this into perspective: Under US regulatory conditions, a Nuclear Power station worker would receive his lifelong exposure limit within 47 minutes at 3 Sv/h! German workers are always a bit more feeble, and would get their lifelong limit within 8 min! Details in the GeigerLog manual under "Occupational Radiation Limits".

Sensitivity to Gamma Radiation: 188 – 235 Pulses/s/R/h

I find it always frustrating seeing science oriented sites can't even get their presented formulas right: This does not make sense: Pulses/s/R/h. What they may have meant is Pulses/s/(R/h), or Pulses/s/R*h. If not, something else is badly wrong.

Assuming my correction, the Sensitivity converted to our units is then:

188 Pulses/s/(R/h) = (188 * 60 CPM / (10000 uSv /h) = 1.13 CPM/(uSv/h)
235 Pulses/s/(R/h) = (235 * 60 CPM / (10000 uSv /h) = 1.41 CPM/(uSv/h)
Average: 1.27 CPM/(uSv/h)

At first glance, given that I am using 2.08 CPM/(uSv/h) in GeigerLog, I am at least in the right ballpark. Further, a user has had access to a synchrotron, a nuclear machine producing pure Gamma (and lots of it, the SI-3BG really had a field day!), and determined the Sensitivity to 2.38 CPM/(uSv/h).

Close enough, except: this value is taken relative to 154 for the first tube. If the Russian numbers are right, then the 154 would have to corrected down to 154 * 1.27 / 2.38 = 82 CPM/(uSv/h). Thus only about half as sensitive as currently claimed!

I have no idea which one is right, but you see again how brittle this Sensitivity (or Calibration) factor discussion is!

With respect to the upper level measurable with the tube, I'd trust the Russian values (hopefully we get translation confirmation).

Whether this can be measured with a GMC-500 counter is a different question. In a old thread we had discussed the power, which the High-Voltage source in the counter can deliver, and found that it would collapse and render the whole counter unusable before it reached the upper count rate end.

Honestly, I consider the 2nd tube more a nuisance than a value. Unless you want to play with the counter and replace the tube with something else; then it is good value ;-).

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Reply #10


15 Posts

Posted - 03/03/2022 :  13:27:51  Show Profile  Reply with Quote
Ullix, thanks - that makes a lot more sense. And I appreciate the time spent on a very thorough replies!

in regard to my statement;
"if i was to read CPM1 and then put a blocker like a piece of steel which would be a 100% beta blocker, and read CPM2, could I then do something like..."

I was meaning a first reading without the blocker and a 2nd reading with the blocker but with the same tube.. i realize in your reply that with the 2 tube 500+ the terminology is CPM1 and 2 which I should have avoided. but the question is moot in that there is no beta conversion factor, so that neatly answers the question.

It seems that the true top number is not known but is somewhere at least 4.5R (per the 42500 uSv stated) and below 300R/hr (and probably quite a bit below that).

I see in your screen image the factor of .194 / 5.155 for the SI3BG but in your text the number 2.0202 another reference to 1.35 in one of the threads (which seems close to the Russian numbers 1.27), and 2.08 in your most recent message. This continues to confuse me somewhat as while there clearly there is some context to the number - it seems there should be some consistency at least for gamma. I dont expect to use tube 2 except in a nuclear war, but given recent events that seems somewhat more of a likely scenario. in that case the difference between 1.3, 2, and 5.5 is quite a bit. perhaps I am asking for something that does not exists, but at least for a pure gamma is 2.38 the best number out there for a high range gamma source? that works for me if so. and again, thanks for all the great info.

Edited by - skydiverman on 03/03/2022 13:33:13
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Reply #11


1109 Posts

Posted - 03/04/2022 :  01:50:01  Show Profile  Reply with Quote
but at least for a pure gamma is 2.38 the best number out there for a high range gamma source?
How would I know? How would anybody know?

Because nobody has ever done an experiment to determine the sensitivity of a M4011 tube; it is just taken as the average of two Russian SBM20 tube values (see GL manual).

Likewise, for the SI-3BG tube, to my knowledge, nothing exists beyond the Russian certificate with perhaps questionable translation.

If you want a Geiger counter with a truly measured sensitivity, plus a legally valid certificate on it, get a GammaScout counter (see GL manual)! And even they have the certificate only for a Cs-137 scenario.

But, please, don't blame me for numbers I am not responsible for! The image numbers .194 / 5.155 are copyright GQ Electronics, embedded in the firmware of the counter, as read out by GeigerLog. They also have changed over time, they were e.g. 0.39 / 2.56, which, another hard to believe thing, is an exactly 2.0 fold difference. Users have done experiments, and concluded on 1.135 / 0.88 even lower than the Russian numbers (Reply #13 in

In GeigerLog I ignore all GQ numbers, even though I am able to read them from the counter. And, by the way, I can change them in GeigerLog and save them back to the counter. Good luck trying this via the counter's menu :-)).

My approach is as follows, and it is what I put into GeigerLog as default:

For the M4011 tube I use Sensitivity = 154 CPM / (uSv/h). Not that I think the number is good, but because I have nothing better.

For the SI-3BG tube I use Sensitivity = 2.08 CPM / (uSv/h). This is based on experiments, and while I know their weaknesses, this is what I have chosen. And remember, all is basically taken as relative to the M4011 value. How close is that to reality? No idea. And, by the way, to which reality: Cs-137, Ra-226, Co-60, synchrotron, your home?

The synchrotron so far is the only pure-gamma source ever used. But since the energy spectrum of its radiation is unknown, the 2.38 derived from it may be the worst number of all. Nobody knows.

My suggestion: live with the uncertainty and confusion ;-).

And, better yet, avoid using the uSv numbers! In particular don't mention them in a legal situation: the Sensitivities used have NO basis in science!

As an aside: has your user name anything to do with your hobby? Next time take a counter with you and record height also; I am very curious!

Edited by - ullix on 03/04/2022 01:52:46
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Reply #12


15 Posts

Posted - 03/04/2022 :  06:23:02  Show Profile  Reply with Quote
That would be a good test, actually - as the ride up and down are at different rates. But I have not jumped in a while, and have basically hung it up while raising my daughter, so i cannot test that at the moment. i have run the GMC500+ on an aircraft trip a few times, enough to notice differences for example traveling over the rocky mountains vs other segments over flat lands for example.
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Reply #13


1109 Posts

Posted - 03/04/2022 :  08:22:24  Show Profile  Reply with Quote
After a Russian translation was found some updates can be given:

The (very high) "Working Dose Range: 300R/h" is confirmed.

The "Own Background: 0.2 Pulses/s" needs slight correction.

This is not from the datasheet (at least even on the new site I found nothing about it) but it is from an experiment, which they have run themselves, and I'd say incorrectly reported the data. Their experiment's graph show CPM=0.19 (counts per MINUTE, not second), which is close to what I see: CPM=0.28 (see Reply #9 above). Looks now ok to me.

Sensitivity to Gamma Radiation: 188 – 235 Pulses/s/R/h

This needs a correction as 235 is the midpoint, while the upper end is actually 282:

Converting the Sensitivity to our units is now:
188 Pulses/s/(R/h) = (188 * 60 CPM / (10000 uSv /h) = 1.13 CPM/(uSv/h)
235 Pulses/s/(R/h) = (235 * 60 CPM / (10000 uSv /h) = 1.41 CPM/(uSv/h)
282 Pulses/s/(R/h) = (282 * 60 CPM / (10000 uSv /h) = 1.69 CPM/(uSv/h)

With the midpoint now at 1.41 CPM/(uSv/h) it has moved a little bit closer to the 2.08 as used in GeigerLog. Not too bad. And the M4011 looks a little bit less bad than before.

Still, don't overvalue uSv/h !

re diving: GeigerLog has long-distance flight data as default in its data directory. Just load them as CSV into GeigerLog.

Edited by - ullix on 03/04/2022 08:24:29
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