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 GMC-600 Review

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T O P I C    R E V I E W
Stargazer 40 Posted - 10/21/2018 : 15:19:27
When I first made the decision to acquire a GM meter it had been many years since I had given any thought to nuclear theory. I looked primarily at features and what friends in the First Responder field were using for personal dosimeters. I also read reviews and ranking articles. With a desire to finally get a hold of something, I took the most fully featured meter I could find that provided the best value. I bought the GMC-500+. The two different sensitivity tubes really do deliver a broad range energy readings, covering not only background or experimental levels, but incident levels as well.

That turned out to be not only the right meter for getting back into this field of study, but the perfect time as well. In the last two and a half months in this forum we have tackled many of the shortcomings of what I thought was a pretty good purchase in the first place, and the value of the meter to me has increased many times.

With the addition of the 500+V2 I had two meters with different features that looked pretty much the same. And because they were inexpensive enough (relatively speaking) I really wasn't too worried about taking them apart and looking at their innards (and making modifications). I learned a lot, but also because of the willingness of GQ's staff to interact on this forum, and because meter users out here have a lot of knowledge to share (and strong opinions). We continue to see improvement in utility as we all learn a lot more about these meters.

So why in the world do I need one of the 600 series meters? Why not go try something else. First and foremost I want tools that have the potential to be usable in a First Responder incident. Don't expect to ever participate in one where they would be needed, but I would like to know that they will give me a pretty good idea of what's present that I can't see. I know pretty well how these meters work and I have seen a willingness (and follow through) on GQ's part to make them the leader in personal dosimeters. I like this rather unique learning environment. And to be honest, whenever I look closely at the competition I don't see the value I presently am getting with these products. The depth of menu based adaptability and flexibility is huge in these GMC meters compared to others. Secondly, I don't want to relearn how something works and it's limitations. The meters, from the nifty demo software available for each one, show good similarity of use. Thirdly, and this does relate to the use of the 600, is that I wanted a meter that is responsive to alphas. These two 600 meters are the top of the line for GQ and I wanted to see the differences in them versus the very solid value in the 500+. Finally there have been a number of posts on the GMC-600, but not much in the way of a review. So hopefully this will turn out to be helpful to those considering as well as those that already own them.

Which 600 to buy - 600 or 600+. I have already stated that I like LND GM tubes. Made in USA and I can call the company and chat with technical folks. That makes the 7317 tube in the 600+ a strong contender. At the same time, the SBT-11 tube in the 600, while it has a following out there, has little information available by comparison. It also is one of the Cold War Surplus tubes, and there are a limited number of them available. Then there is the question of SBT-11 or SBT-11A.

Both the 7317 and the SBT-11/11A are pancake mica window tubes. They are capable of measuring alpha radiation through the mica windows. I adapted an end window LND 712 to one of the 500+ meters and have seen the sensitivity the mica window provides. Don't have an alpha source yet, but have one on the way. I wanted a larger surface measurement area though than the end window tube, as one aspect of First Responder use is to wave the wand/meter slowly over a possibly contaminated surface as a 'frisker'; to see if some local contamination exists. Both the 7317 and the SBT-11 had the necessary surface area and sensitivity to make that viable. And from the video that ZLM posted in a previous thread, the 7317 tube does appear to work okay with the GMC power supplies (more on that later). So I would have preferred the LND tube in the 600+ initially. That large surface area mica window has handling problem for me however. I live at 5800 ft (1,770 meters) above sea level. The mica could rupture above 8000 ft (2,440 meters). Even shutting a door too fast can cause a rupture that close to the limit. So I took the more conservative route and acquired the 600. It arrived a couple days ago.

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Comparing with the glossy white 550+, it is more of a flat gray. The case is the same length and width, but about 1.5 times thicker than the 500+.

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The menu buttons and their functions are identical with the 500 meters.

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Since the case supports both the 7317 and the SBT-11, the grilled openings in the case back are selected by covering the one not used with a thicker GQ label. So the SBT-11 opening is toward the top of the meter and the 7317 opening is more in the middle.

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6   L A T E S T    R E P L I E S    (Newest First)
Stargazer 40 Posted - 10/25/2018 : 09:07:05
While related to 600+'s GM tube and not the 600's, still nice to get a feel for how directionality affects these pancake tubes. I received this chart today on the 7317 LND GM pancake tube from the company. It applies to the side/backside response of the 7317 compared to the mica window. As expected, but nice to see differences.

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Stargazer 40 Posted - 10/25/2018 : 08:53:46
We haven't looked at alphas yet. Here's an Americium 241 source still in its smoke detector frame. Label says <.9uCi. Provides about 10mm of standoff from end window detector. Let's first look at the non-alphas by placing a piece of paper over the source blocking alphas - 1.451uSv/h - 508CPM

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And then with paper removed - 30,000 CPM - 85.791uSv/h

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60 times the CPM and dose rate.
Stargazer 40 Posted - 10/24/2018 : 16:18:28
I recently added an additional higher activity source to those that I can test these meters on, a 10 uCi Cs137 disk from Spectrum Technologies. GQ has posted the following data on the webpage for the GMC-600.

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My 600 power supply is set for 19% or 390VDC, which is a third of the way into the plateau. This is what I found with meter case placed against source. First is a couple of bags of Potassium Salt - 205CPM - .586uSv/h

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Then my stack of Thorium gas mantles (note the 60% drop in dose rate from the post above with corrected conversion factor) - 9,791CPM - 27.974uSv/h

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Then the 5 uCi Cs137 disk shielded with lead sheet for gamma only - 7,028CPM - 20.080uSv/h

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Same 5 uCi disk with shield removed - 87,284CPM - 249.38uSv/h

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Next the 10 uCi Cs137 disk shielded with lead sheet for gamma only - 14,388CPM - 41.109uSv/h

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Then unshielded 10 uCi source - 105,895CPM - 302.56uSv/h

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The last seemed low to me by about 200uSv/h. Likely the source was not centered on the GM tube. Especially so because when I placed both the 5uCi and 10uCi sources under the Gm tube I got - 255,943CPM - 731.27uSv/h.

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These source disks are very small point sources. Placement is really critical to getting good counts. I'm seeing what I expected for the most part. When I finally run this latest source addition through the other tube types it will be interesting to put all these in a table.

Stargazer 40 Posted - 10/22/2018 : 07:21:15
I turned the 600 meter on and compared background overnight to the two 500+ meters. One with M4011 on and the other modified for use with the LND 712 wand. These two are very similar background from 15-35 uSv/hr. The M4011 conversion factor is 154 CPM/uSv/h and the LND712 conversion factor is 108 CPM/uSv/h. GQ uses the same conversion factor for the SBT-11/11A tubes as the 7317 - 300 CPM/uSv/h (LND specifies gamma sensitivity of 350 CPM/uSv/h for the 7317). Background for the 600 was half of the amounts of the 500+ meters. This made me think before verifying whether 11 or 11A that I had an SBT-11. I adjusted the conversion factor down to 150 CPM/uSv/h and all three meters showed essentially the same background. Turned out to not be a good idea. I tested each of the meters on my stack of gas mantles.


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LND 712

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The 600 showed about 2.5 times the dose rate of the M4011 tube'd 500+. This says the conversion is much closer to the 350 CPM/uSv/h of the LND specs for the 7317 for these gas mantles, and confirms some comments I've read for the SBT-11/11A on the internet as being almost as sensitive as the 7317. So I set it for 350CPM/uSv/h in the Calibrate menu and will start there with I do testing with the Cs137 sources. The lower reading than you might expect from the LND 712 is from a stainless steel enclosed tube compared to glass for the M4011 and mica for the SBT-11A.
Stargazer 40 Posted - 10/22/2018 : 06:40:32
Let's take a closer look at he SBT-11/11A GM tube. It is a rectangular pancake tube with a mica window.

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Here's a pic of schematic

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The four anodes are parallel and each is positively charged by separate pins on the back of the tube. It appears to be a single gas chamber so the anode distribution seems to be present to provide a more rapid/even response to an ionization event. This is similar to a round pancake tube like this SI8B where a single anode is comprised of a densely ringed plate under the mica window.

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During the manufacturing process the SBT-11/11A tubes are tested for thickness of the mica window. Thinner sheets of mica allow more alpha particles to pass, so at some thickness amount the tubes that are thinner are labeled 11A. They are considered more sensitive, but are also more fragile. In this picture you can see a fuzzy label on the back of an SBT-11 tube where the '11' is not followed by an 'A'.

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Since both 11 and 11A tubes are alpha capable, and likely very similar response to beta and gamma radiation, GQ does not differentiate between them in building the 600. The tubes are no longer manufactured and are Cold War NOS, so when they are gone, the 600 will no longer be produced. That's not to say another less expensive pancake GM tube that fits the GQ power supply capability won't be added to a new model. Only that the SBT-11/11A version will go away.

The tube in this GMC-600 is an 11A.

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Stargazer 40 Posted - 10/22/2018 : 04:41:16
One of the things I was most interested in from the discussion accompanying the 500+ review was the Fast Estimate Time menu selection under User Option menu. User06 was comparing response to his Terra-P meter, and for areas he was working in, waiting around for a minute for CPM estimate didn't make sweeping or 'frisking' for high energy objects very efficient. The function works very nicely, but unless in a very active field, the lower 5, 10 second averages are very erratic as you might expect. As well, this meter has version 2.01 of firmware installed. And if you're using a recently purchased 600/600+ you need to make sure you request an upgrade from 2.00 version, where there were some issues with this function. When set to 60 sec, the Fast Estimate Time is essentially 'Off'.

Where else have we seen firmware 2.0X? It is in the 500+V2. In going through the menu structure I looked at Init Setup. There under Calibrate was a Tube Voltage menu item. Firmware controlled tube voltages are available in the 500+V2, while the older V1 requires adjustment of a tiny pot on the PCB. Now I was pretty excited as I had asked about the dual nature of the new PCB in the 500+V2 Review ('GMC500/600' labeling and '20180903' dated). We now see some insight into GQ's multi-use approach to product design. Create a broadly capable PCB, where component adds allow for diverse hardware adaptation and power supply capability, and vary operational control with firmware.

The basic PCB is the same for the 500/500+V2 and the 600/600+ (is this 600 then also a V2?). But power supply components and detector connections (and of course the cases) vary with model. So although the firmware is different for different models, changes to a very consistent set of menu functions can apply to many different models that make use of the same PCB. When you have a company that is as responsive as GQ has shown itself to me to be over the last three months, it is another reason to stay the course. Let's take a look inside.

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The reason for the case thickness increase is to support the bracket housing the SBT-11 tube (shown on the left). You can see the grated circle that is used for the mounting bracket for the 7317 model that is covered by the large GQ label. Power supply connections are different, allowing leads from the GM tube socket to be attached to the PCB. For this single detector model the components for the tube2 power supply are absent, and firmware precludes adding them.

So for each of the 500/500+/600/600+ meters GQ has paired a dual counter and dual power supply capable PCB with different combinations of detector(s) and cases to meet different customer interests. With each GM dosimeter in the series you get essentially the same use experience.

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