quote:

---

Originally posted by Stargazer 40

It would be kinda fun to set up a meter with your numbers and send it in for calibration. Would it come back with anything changed saying the tubes and electronics are pretty consistent. If so, GQ could ship all GMC-800 meters with a much better calibration set. (Or just send it in as is and see how it comes back. Both ways would work.)

---

quote:

---

Originally posted by Stargazer 40

It would be kinda fun to set up a meter with your numbers and send it in for calibration. Would it come back with anything changed saying the tubes and electronics are pretty consistent. If so, GQ could ship all GMC-800 meters with a much better calibration set. (Or just send it in as is and see how it comes back. Both ways would work.)

---

quote:

---

Originally posted by Stargazer 40

Here are the numbers I calculated using DT delay of 90 usec and the initial input of 1538 CPM

Cal 1 - .998 to 1535 Tony 960 Chg 1.6X
Cal 2 - .988 to 15198 Tony 8991 1.7X
Cal 3 - .976 to 30039 Tony 17115 1.8X
Cal 4 - .940 to 72274 Tony 40079 1.8X
Cal 5 - .880 to 135268 Tony 78848 1.7X
Cal 6 - .760 to 233739 Tony 144614 1.6X

So my initial number is high by 1.6X, and average is going to be that my readings are 1.7X higher. DT is not in the shipped meter numbers and the 90 usec DT delay is a pretty good number for the tube. Just have to drop the initial number and this is a good match. I'll do that in a heartbeat.

Thank you for sharing!

---

quote:

---

Originally posted by Tony81269

quote:

---

Originally posted by Stargazer 40

If you look at the 'Type' line it says pretty clearly 'QA/Production'. Doesn't say anything about NRC calibration and obviously is only meant to show it passed basic operational testing prior to being shipped. All the stuff reported here is most likely conforming to design standards. Since they don't make this a part of what's delivered (I didn't get this form with my GMC-800) I can only assume it was supplied as a result of a request?

If GQ had done an NRC type calibration it would look a lot different. But as ullix has stated in another thread, all the calibration points are linear out of the box. GQ felt the GM tube was 'close enough' to linear that they didn't need to add more detailed numbers for the vast majority of those purchasing this device. And they provide access to a means to meet NRC calibration certification and even recommend a third party to do it. They have no involvement in that but have provided the user a meter that can be NRC certified. I have calculated the numbers that should be in each of the calibration spots taking into account DT. I'll post those after Tony posts the actuals from his meter. That should lead to a very interesting discussion.

---

Ok guys, I got my meter back from Applied Health Physics. Attached is the calibration sheet. In my next post I will show the calibration points on the meter and CPM values

---

quote:

---

Originally posted by Stargazer 40

If you look at the 'Type' line it says pretty clearly 'QA/Production'. Doesn't say anything about NRC calibration and obviously is only meant to show it passed basic operational testing prior to being shipped. All the stuff reported here is most likely conforming to design standards. Since they don't make this a part of what's delivered (I didn't get this form with my GMC-800) I can only assume it was supplied as a result of a request?

If GQ had done an NRC type calibration it would look a lot different. But as ullix has stated in another thread, all the calibration points are linear out of the box. GQ felt the GM tube was 'close enough' to linear that they didn't need to add more detailed numbers for the vast majority of those purchasing this device. And they provide access to a means to meet NRC calibration certification and even recommend a third party to do it. They have no involvement in that but have provided the user a meter that can be NRC certified. I have calculated the numbers that should be in each of the calibration spots taking into account DT. I'll post those after Tony posts the actuals from his meter. That should lead to a very interesting discussion.

---

quote:

---

Originally posted by EmfDev

@Tony81269 Once you receive your counter, we advice that you go to the calibration menu and make a record of the CPM : µSv/h for each of the 6 calibration points as a factory reset will clear the values.

---

quote:

---

Originally posted by Stargazer 40

If you look at the 'Type' line it says pretty clearly 'QA/Production'. Doesn't say anything about NRC calibration and obviously is only meant to show it passed basic operational testing prior to being shipped. All the stuff reported here is most likely conforming to design standards. Since they don't make this a part of what's delivered (I didn't get this form with my GMC-800) I can only assume it was supplied as a result of a request?

If GQ had done an NRC type calibration it would look a lot different. But as ullix has stated in another thread, all the calibration points are linear out of the box. GQ felt the GM tube was 'close enough' to linear that they didn't need to add more detailed numbers for the vast majority of those purchasing this device. And they provide access to a means to meet NRC calibration certification and even recommend a third party to do it. They have no involvement in that but have provided the user a meter that can be NRC certified. I have calculated the numbers that should be in each of the calibration spots taking into account DT. I'll post those after Tony posts the actuals from his meter. That should lead to a very interesting discussion.

---

quote:

---

Originally posted by ullix

Am-241 is a bit of an unexpected radioactive source, but as this Pseudo-Certificate has absolutely nothing to do with "Calibration" I guess this is just used for convenience to create "some counts".

Am-241 is used in ionizing smoke detectors. Those devices look like this:


Putting one of them on the top side of a GMC-300 counter I get some CPM=100, strongly position dependent, like shown in this GeigerLog run:


Am-241 is an alpha emitter. It is this property, which is used in smoke detectors. The decay is rather complex, having multiple alpha energies, even fission, and it also comes with multiple gamma emission. The gammas are of low-energy (<100keV), about on a par with medial-X-ray energies.

If you want to use then at home you can buy such encapsulated units on AliExpress for $3.40
https://www.aliexpress.com/item/1005007315035011.html

But I very much suggest to keep them assembled to avoid releasing Am-241 to your environment! I would not want having any radioactive emitters, and in particular alpha emitters, floating in the air I breathe!!!

---

quote:

---

Originally posted by GTS

More fuel to the BS bonfire. If I am not mistaken, Am-241 is mostly an alpha emitter with a little gamma so the GMC-800 is not going to detect this source very well given its geiger tube type. The clicks should be just above background with that slight gamma emanation.

---

quote:

---

Originally posted by ullix

A "Certificate of Calibration" ? Well, this is hilarious. Doesn't GQ feel embarrassed by presenting such a gibberish as "Certificate"? Let alone as "Calibration"?

There is not even a single line which qualifies as calibration!

At best this is a QA check at the end of production. The sloppiness is already visible in the making of this form, as under the heading "Energy MeV" none(!) of the 3 entries has anything to do with energy!

Further, when you do have a CPM value of 50, then as a trivial consequence of the Poisson law is the Readout Deviation 15%. This is not a separate check!

And then note that the background is given as a range of 15 to 50 CPM. This implies that the AM241 measured count rate of 50 is within the background. In other words: they have not gotten any signal at all!

This thingy of paper is the ultimate of BS!

---

quote:

---

Originally posted by Stargazer 40

Thanks. It doesn't sound like he made any adjustments to the calibration points to get it to be 'spot on' or did he say after adjustments it was 'spot on'?

---

quote:

---

Originally posted by Stargazer 40

Thanks. It doesn't sound like he made any adjustments to the calibration points to get it to be 'spot on' or did he say after adjustments it was 'spot on'?

---

quote:

---

Originally posted by Stargazer 40

Do they still have your meter? Or is guy taking a long vacation?

---

quote:

---

Originally posted by EmfDev

GQ Geiger counters are advertised for general use and not for professional use. However, some models like the GMC-800/500/+/600+ can be calibrated with 1-2% accuracy and certified with the NRC / NIST via 3rd party.

---

quote:

---

Originally posted by Stargazer 40

OR it could be six points because that's what the NRC certificate needs to say to imply it's fully calibrated. Deadtime is real and the meter processor can do the calculations based on counts and deadtime for that tube and increase the exposure from the lower CPM deadtime creates. But you can also do a linear interpolation between each of the actual readings with six calibrations that cover the range of the tube. I estimate at 2000uSv point 6 that the CPM needs to be raised about 33% because of deadtime of 90 usec for the J321 tube. Your analog meters can't do deadtime calcs. So the six points of calibration for the Ludlum is likely just interpolating between them OR each range on the switch (.1X, 1X, 10X, 100X) is calibrated separately as four point instead of six for this GMC-800. So likely the GMC doesn't have deadtime computed by processor because many calibration points describe the curve. If your numbers are little changed from factory, then likely deadtime being incorporated. If the CPM is falling off of linear then deadtime is not computed, ie., the lower CPM because of deadtime misses needs to indicate higher exposure rate.

---

quote:

---

Originally posted by Tony81269

quote:

---

Originally posted by Bobakman

Ok NP I know a guy in Houston Texas.
Below is his eBay page get in touch with him message him on Ebay and he can set you up with a good deal on a reconditioned Ludlum for a lot less than a brand new one.He has guy that calibrates them for him as well. This forum wouldn't let me attach his eBay link directly
Simply replace the 2 astrisks with 2 "t"'s, and it'll take you right there.

Bob

h**p://ebay.com/str/mybrainfart

---

I looked through your friends ebay store. We wouldn't be interested in any ludlum 3's or other alpha/beta/gamma types. I did see he has a model 12-4 neutron detector. We aren't required to have neutron detectors but I would like one anyway. A couple things about this one is the price is $4000. I do not know what a new one would cost. Maybe I should find out. Another thing is that is says it is new/2024 and that it is seller refurbished. That seems strange to me. Not saying it isn't true but where would you get an almost brand new 12-4 that you had to refurbish. I would really have to know the story behind that one. Not saying this guy is shady or anything at all. And finally there are my bosses that will say "why would we pay for something we don't really need?". As future RSO I would like to have one because we do have Ambe sources but are not required to have meters that measure neutrons

---

quote:

---

Originally posted by Stargazer 40

OR it could be six points because that's what the NRC certificate needs to say to imply it's fully calibrated. Deadtime is real and the meter processor can do the calculations based on counts and deadtime for that tube and increase the exposure from the lower CPM deadtime creates. But you can also do a linear interpolation between each of the actual readings with six calibrations that cover the range of the tube. I estimate at 2000uSv point 6 that the CPM needs to be raised about 33% because of deadtime of 90 usec for the J321 tube. Your analog meters can't do deadtime calcs. So the six points of calibration for the Ludlum is likely just interpolating between them OR each range on the switch (.1X, 1X, 10X, 100X) is calibrated separately as four point instead of six for this GMC-800. So likely the GMC doesn't have deadtime computed by processor because many calibration points describe the curve. If your numbers are little changed from factory, then likely deadtime being incorporated. If the CPM is falling off of linear then deadtime is not computed, ie., the lower CPM because of deadtime misses needs to indicate higher exposure rate.

---

quote:

---

Originally posted by Stargazer 40

In earlier reviews of the GMC-800 there was a question about why GQ went to six calibration points instead of the three they have been using. Now that we see this certification form it's pretty obvious that these points pretty much cover the bases as to needed coverage for most uses. These aren't survey meters that would have much higher exposure coverage.

I think given what you show in the pics above that the pill is likely about in the middle of the large threads. You have circled the retaining lug. At the same time, a half inch or inch would well be within a margin of statistical error.

I am most interested in the CPM for each of the exposure amounts on the form. Thanks for sharing those when this gets back from testing (if you don't get the form back then they can be read from the calibration points in the config section). Then of course how it compares to the Ludlum after calibration, especially the high end.

---

quote:

---

Originally posted by Stargazer 40

In earlier reviews of the GMC-800 there was a question about why GQ went to six calibration points instead of the three they have been using. Now that we see this certification form it's pretty obvious that these points pretty much cover the bases as to needed coverage for most uses. These aren't survey meters that would have much higher exposure coverage.

I think given what you show in the pics above that the pill is likely about in the middle of the large threads. You have circled the retaining lug. At the same time, a half inch or inch would well be within a margin of statistical error.

I am most interested in the CPM for each of the exposure amounts on the form. Thanks for sharing those when this gets back from testing (if you don't get the form back then they can be read from the calibration points in the config section). Then of course how it compares to the Ludlum after calibration, especially the high end.

---

quote:

---

Originally posted by Stargazer 40

No idea of the certifier's setup but CoPilot GPT4 says your meter is reading a little low compared to theoretical. To get perfect reading for your estimated source strength at 19 feet it should be 2.20 mR/h. Here's calcs for your measure and that would provide calibration counts for the GQ table:

To determine the distances required to achieve specific exposure rates from a 222.6 mCi Cs-137 source, we can use the inverse square law for radiation. The exposure rate constant for Cs-137 is approximately 0.33 mR/hr per mCi at 1 meter.
Given a 222.6 mCi Cs-137 source, we can calculate the distances for the desired exposure rates:
1. 10 µSv/h (1 mR/h): d = sqrt {0.33 * 222.6}/{1}} approx 8.57 meters or 28.1 feet
2. 15.8 uSv/h (1.58 mR/h) d = sqrt {0.33 * 222.6}/{1.58}} approx 6.82 meters or 22.4 feet
3. 22.0 uSv/h (2.20 mR/h) d = sqrt {0.33 * 222.6}/{2.20}} approx 5.78 meters or 19.0 feet
4. 100 µSv/h (10 mR/h): d = sqrt {0.33 * 222.6}/{10}} approx 2.71 meters or 8.9 feet
5. 200 µSv/h (20 mR/h): d = sqrt {0.33 * 222.6}/{20}} approx 1.92 meters or 6.3 feet
6. 500 µSv/h (50 mR/h): d = sqrt {0.33 * 222.6}/{50}} approx 1.21 meters or 4.0 feet
7. 1000 µSv/h (100 mR/h): d = sqrt {0.33 * 222.6}/{100}} approx .86 meters or 2.8 feet
8. 2000 µSv/h (200 mR/h): d = sqrt {0.33 * 222.6}/{200}} approx .61 meters or 2.0 feet
9. These calculations assume no shielding and ideal conditions.

---

quote:

---

Originally posted by Stargazer 40

No idea of the certifier's setup but CoPilot GPT4 says your meter is reading a little low compared to theoretical. To get perfect reading for your estimated source strength at 19 feet it should be 2.20 mR/h. Here's calcs for your measure and that would provide calibration counts for the GQ table:

To determine the distances required to achieve specific exposure rates from a 222.6 mCi Cs-137 source, we can use the inverse square law for radiation. The exposure rate constant for Cs-137 is approximately 0.33 mR/hr per mCi at 1 meter.
Given a 222.6 mCi Cs-137 source, we can calculate the distances for the desired exposure rates:
1. 10 µSv/h (1 mR/h): d = sqrt {0.33 * 222.6}/{1}} approx 8.57 meters or 28.1 feet
2. 15.8 uSv/h (1.58 mR/h) d = sqrt {0.33 * 222.6}/{1.58}} approx 6.82 meters or 22.4 feet
3. 22.0 uSv/h (2.20 mR/h) d = sqrt {0.33 * 222.6}/{2.20}} approx 5.78 meters or 19.0 feet
4. 100 µSv/h (10 mR/h): d = sqrt {0.33 * 222.6}/{10}} approx 2.71 meters or 8.9 feet
5. 200 µSv/h (20 mR/h): d = sqrt {0.33 * 222.6}/{20}} approx 1.92 meters or 6.3 feet
6. 500 µSv/h (50 mR/h): d = sqrt {0.33 * 222.6}/{50}} approx 1.21 meters or 4.0 feet
7. 1000 µSv/h (100 mR/h): d = sqrt {0.33 * 222.6}/{100}} approx .86 meters or 2.8 feet
8. 2000 µSv/h (200 mR/h): d = sqrt {0.33 * 222.6}/{200}} approx .61 meters or 2.0 feet
9. These calculations assume no shielding and ideal conditions.

---

quote:

---

Originally posted by Bobakman

Ok NP I know a guy in Houston Texas.
Below is his eBay page get in touch with him message him on Ebay and he can set you up with a good deal on a reconditioned Ludlum for a lot less than a brand new one.He has guy that calibrates them for him as well. This forum wouldn't let me attach his eBay link directly
Simply replace the 2 astrisks with 2 "t"'s, and it'll take you right there.

Bob

h**p://ebay.com/str/mybrainfart

---

quote:

---

Originally posted by Stargazer 40

Nuclear physics in college was a long, long time ago. Don't remember a lab like optical physics where we played with these things. My calibration work on these GQ meters (plus some other Russian GM kits and Mazur) have been wonderfully enlightening. I am very interested in the differences you get when this is NRC calibrated. The GM tubes are pretty well understood. That the GMC-800 is so close to the Ludlum says any factory calibration will likely not be far off. But the cert will be the important part as discussed previously. Thanks for digging in on this.

---

quote:

---

Originally posted by Bobakman

Ok NP I know a guy in Houston Texas.
Below is his eBay page get in touch with him message him on Ebay and he can set you up with a good deal on a reconditioned Ludlum for a lot less than a brand new one.He has guy that calibrates them for him as well. This forum wouldn't let me attach his eBay link directly
Simply replace the 2 astrisks with 2 "t"'s, and it'll take you right there.

Bob

h**p://ebay.com/str/mybrainfart

---

quote:

---

Originally posted by Tony81269

You know what? I'm not exactly sure when that source was born. Let me get back to you on that.

But the part about the reading at 6ft and 19ft is true. I did the math after I had taken the measurements and was surprised. I should have done it before the 19ft

---

quote:

---

Originally posted by Stargazer 40

oops, that was six feet from source. Whole different problem.

And for CS-137 the answer is much closer to what you got

For Cesium-137 (Cs-137), the specific gamma-ray constant (#915;) is approximately 0.33 R·m˛/hr·Ci1.
Let’s use this value to calculate the dose rate at 6 feet from a 300 mCi Cs-137 source.
First, convert the activity to curies (Ci):
300 mCi=0.3 Ci
Next, calculate the dose rate at 1 meter (3.28 feet):
I1#8203;=#915;×Activity=0.33×0.3=0.099 R/hr
Convert R/hr to mR/hr:
0.099 R/hr=99 mR/hr
Now, apply the inverse square law to find the dose rate at 6 feet (1.83 meters):
I2#8203;=99(63.28#8203;)2#8776;30 mR/hr
So, a GM meter would read approximately 30 mR/hr when 6 feet away from a 300 mCi Cs-137 source.

Again from CoPilot GPT4

---

quote:

---

Originally posted by Tony81269

GMC-800 Reading at 6ft from 300 mCi Cs-137

So, GMC-800 reading 15.76 mR/h and Ludlum 3 reading 16 mR/h. To me, that is really close considering the GMC has not had a NIST calibration.

---

quote:

---

Originally posted by Tony81269

quote:

---

Originally posted by Bobakman

@Tony81269

I sent you a PM you should’ve got it in your email. Let me know if you did.
Bob

---

Not sure if I'm set up to get emails from people on here. I'll look at my settings. If you want to know anything just ask right here. I'll tell you

---

quote:

---

Originally posted by Bobakman

@Tony81269

I sent you a PM you should’ve got it in your email. Let me know if you did.
Bob

---

quote:

---

Originally posted by Stargazer 40

No, from contact to sending in to time it takes to any interaction with them to return time and payment types they use. Nice to have that process documented. They may contact GQ to get adjustment entry points. Be nice to know that has taken place if needed.

---

quote:

---

Originally posted by Stargazer 40

No, from contact to sending in to time it takes to any interaction with them to return time and payment types they use. Nice to have that process documented. They may contact GQ to get adjustment entry points. Be nice to know that has taken place if needed.

---

quote:

---

Originally posted by Stargazer 40

To document that process would be really great!

---

quote:

---

Originally posted by Stargazer 40

LIABILITY

Certification carries weight with decisions made as a result of measurements taken. It's that simple. Many people can drive cars or even fly airplanes. Without a drivers license or pilots license you simply aren't counted as qualified to do so. With ham radios, I can have a perfectly wonderful and useful radio that allows me to talk to people on say a federally managed fire. But the FCC can fine or put me in jail if that radio is not Part 90 of some FCC code certified.

---

quote:

---

Originally posted by Stargazer 40

Tony, I think EmfDev means that they build them and establish deadtime and tube voltage values that are within the range of the components as far as design, and if you want them further tweaked to certification standards then that's a step you can take because it's possible to effect the necessary accuracy of certification.

---

quote:

---

Originally posted by EmfDev

GQ Geiger counters are advertised for general use and not for professional use. However, some models like the GMC-800/500/+/600+ can be calibrated with 1-2% accuracy and certified with the NRC / NIST via 3rd party.

---