Interest in RGAs (possibly a candidate for an FAQ?)

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ian_krase
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Interest in RGAs (possibly a candidate for an FAQ?)

Post by ian_krase »

Spinning off a bit from Joe Gayo's post: I feel like it would be good to have some writing about the use of RGAs from someone experienced in their use.

(I, uh, may possibly have just recently lost my chance in an Ebay auction to buy a complete RGA cheaply.)

As far as I can tell, the cheapest Residual Gas Analyzers on the market today cost just under $4000 new, and one finds a fair number on Ebay for a lot less -- though commonly the mechanical / probe portion is sold separately from the electronics box.

So I'm curious if someone who has actually done this has any practical advice on buying and using an RGA.

(It seems to me like there's possibly going to be a trend towards miniature low-end RGAs replacing the bayard-alpert gauge in the next 20-30 years.)
Jerry Biehler
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Jerry Biehler »

I have a couple Dymaxion RGAs and a Inficon Quadrex 200. Outside of leak finding they are not going to be too terribly useful on a fusor.

If you get a electronics unit and probe separate you will probably have to recalibrate it. The software for the dymaxion has that all built in. I think the quadrex has to be done manually.

Another thing to watch out for is the channeltron used on the high sensitivity ones do not like to be exposed to air. The one in my quadrex seems to be still pretty happy but I have a brand new channeltron waiting to go into it once I have it in a semipermanent installation. It's stored in a 6" section of stainless piping witch conflats and a copper tabulation that it was evacuated with. This was how it came from inficon.

Filaments are another issue, they are not cheap to replace.

Before you do buy one make sure that you can get the software to run it. No software means you have a paperweight.

I will be really surprised if small RGAs replace ion gauges. Way too complex, BA tube goes out and that's like $60-200 to replace and when you do you get all new internals. Contaminate a RGA and kill the filament and you get to spend at least several hundred for the filament assy and then you got to figure out how to clean the pole pieces and worse if you have a channeltron or MCP.
ian_krase
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by ian_krase »

FWIW, I don't actually do fusors -- my interests run more towards higher vacuum and a bit to developing cheap ways of doing higher vacuum, so "is this outgassing or not and in what specific way" is a more interesting question to me.

The cheapest RGAs use a plain faraday cup, such as the Extorr ones -- do the ones with electron multipliers have to use exclusively the electron multiplier?

It seems like the filament issue should be tractable. Extorr filament replacements (coated iridium) cost about $100. Their entire ionizer costs only $200, and their ionizer actually functions as a BA gauge as well. And it seems like things could get better. (Specifically, I'm thinking of multilayer ceramic or mica printed circuits to replace the complicated assemblies of spot-welded sheet metal and ceramic standoffs.)

Can you elaborate on calibration -- what are used as standards?
Dan Knapp
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Dan Knapp »

Scientific Instrument Services (sisweb.com) does filament repairs at a very reasonable cost (e.g. I noted one RGA filament repair priced at $43). If they don’t list your particular filament, you can send them a burned out one for a quote.
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Richard Hull
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Richard Hull »

RGAs are definitely not for the average would-be fusioneer. They are really useful to the committed vacuumist who looks at 10e-5 torr as almost atmospheric! For those who strive to move as deep and "clean" as 10e-7 torr the RGA become a really useful tool.

The average fusioneer need pump no deeper than 10e-4 torr, especially with the new small CF cross fusors with operating pressures near 25 microns.
As noted above, not everyone is on a sole mission to build a fusor. Scrupulously clean Deuterium atmospheres in a fusor are not needed to do fusion.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jerry Biehler
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Jerry Biehler »

Not all filaments can be rebuilt. The ones on my dymaxions are two rods with a loop of filament wire. You cut the support off after you install it so once it's installed it pretty much over with. I did manage to move a good filament from my 100amu one with issues to my 200amu by making a fixture to hold it all. It's a $300 filament: https://www.sisweb.com/ms/replacement-rga-filament.htm

You can turn the electron multiplier on and off but it is still in play, it just acts like a faraday cup at that point.

You're probably not going to reinvent the rga here. These things need to be able to be baked out to 200c usually and often they are cleaned in a hydrogen furnace when they get really bad. That means ceramic and stainless. There are some RGA with significantly smaller probes like this MKS one. https://www.mksinst.com/f/hpq3-high-pre ... s-analyzer

The company I work for was just bought out my MKS this year. I wonder if I get a discount. Lol.

Atmosphere is used as standards. You find known mass peaks at a reasonably distance apart and then it does an auto cal that goes through and tunes everything to get nice mass separation and scale and span adjusted. Hopefully everything is close before you start so it can find the peaks. I usually use something like mass 16 for h20 and N2 at 28 to do an initial run since they will be the most prominent and then mass 2 for hydrogen (from the water) and 32 for o2 to fine tune.
ian_krase
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by ian_krase »

MKS also makes the 835 Vacuum Quality Monitor, which is even smaller -- it's notable in so much as that it is not a quadrupole. Rather, it's a trap that ionizes, traps, and then releases a charge of gas.

(unfortunately, it's dependent on an electron multiplier. And still somewhat more expensive than the cheapest known RGA.)



That filament looks a lot like the filament for the Extorr RGA, except three times as expensive. What's the point in including the ceramic insulator if you're not going to make it able to be removed in one piece?!

(When building electron sources, my usual practice is to spot-weld the filament into place in-situ. Actually, one thinks about it... many industrial bandsaws have a bandsaw blade welder built right into the machine.)

I hadn't heard about a "hydrogen furnace" process for cleaning before -- I was under the impression that the cleaning process involved lots of Alconox and fiddly little brushes.



What about for the person who considers 10E-5 "A sorta-OK vacuum, but I should be doing much better"? Because that's kind of where I am right now. Of course I think that the real problem is that half my stuff is rusty, half is covered with silicone grease, and there's only one non-rubber seal in it.
Dan Knapp
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Dan Knapp »

The predecessor of the VQM 835 is the VQM 830. I’ve seen 830’s (electronics module only) on eBay. I bought a spare for my 830. The actual trap itself I’ve never seen on eBay. I think when systems are surplused, the trap (sensor head) gets left on the vacuum system, and the surplus dealers don’t recognize what it is. In the original publication of the autoresonant ion trap, they built the trap from Kimball Physics eV parts (the physicists’ Erector Set), so the amateur could build one. The trap doesn’t require the high precision construction that a quadrupole does. You could also build the electronics. In the original paper, they used an analog devices frequency generator chip with the AD evaluation board. Unlike a quadrupole, the trap will only work at better than 10-6 Torr. For monitoring fusor vacuums, one has to differentially pump the trap.
If anyone is interested, I can dig up the reference to the original publication; or google “autoresonant ion trap” with first author Ermakov.
Last edited by Dan Knapp on Fri Aug 02, 2019 10:43 pm, edited 1 time in total.
Jerry Biehler
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Jerry Biehler »

The ceramic part comes off, it is probably there for shipping to support the ends. But yeah, its simple but expensive and it needs to be to spec otherwise the head unit will detect something wrong because it measures current. Its just one of the things to watch out for when buying a used RGA.

There is zero issue getting down to -7 to -8 with o-rings. I do it on both my systems and it is all nitrile or viton seals. Get rid of the grease as a starter. At most I put a thin coat of Krytox LVP on the rings.

Yeah, you put hydrogen into a vacuum furnace and it can be used to clean things by burning off everything that is not metal and ceramic.

And yep, often the RGA probes get left on the machine thinking they are just a ion gauge or something. Oh well.
ian_krase
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by ian_krase »

How did they handle the amplification? I proved to be unequal to the task of building the electronics for my own quadrupole...

I took a look at Kimball Physics. That's some extremely expensive stuff! A far cry from C. J. Strong's blobs of wax, wire, and glass.
Dan Knapp
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Dan Knapp »

The original publication is:
An electrostatic autoresonant ion trap mass spectrometer
A. V. Ermakov, and B. J. Hinch
Review of Scientific Instruments 81, 013107 (2010); https://doi.org/10.1063/1.3276686
It includes the circuit diagrams.
The Kimball Physics eV parts are indeed quite expensive. An amateur with access to metalworking shop facilities could make the needed parts. There is no way around buying an electron multiplier, which would then be the most expensive component. Never buy a used multiplier on eBay; the reason it’s there is that it was replaced.
ian_krase
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by ian_krase »

Can't you make your own discrete dynode electron multipliers? AFAIK all you need is beryllium copper -- or perhaps "silver-magnesium alloy".
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Jerry Biehler »

Just use a faraday cup. You really don't need the extra sensitivity of a EM for most things. I suppose you could also take the guts out of a small PMT and use that too. Or salvage the MCP out of an old image intensifier tube.

A friend and I were looking at what it would take to rejuvenate a channeltron. It might not be too hard, they are one of the simpler designs out there.

But in the end just keep an eye out for a used RGA on ebay. I have picked all mine up for under $200 which is way cheaper than you could even think about building one for. Just keep and eye out for the gotchas. The old inficon quadrex units are still good and you dont have to worry about software.
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Justin Fozzard »

Here is a link to get a free copy of the Ermakov and Hinch paper on Sci-Hub:

https://sci-hub.se/10.1063/1.3276686

Here is a link to locate Sci-Hub when it moves around:
https://whereisscihub.now.sh/

Or use this to go direct to the home page:
https://whereisscihub.now.sh/go

I often find useful papers on Sci-Hub and they can usually be downloaded without a paywall.
Dan Knapp
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Dan Knapp »

In regard to the EM discussion, I think an EM will be required for a home built ion trap RGA. I don’t think one could get enough ion current out of the trap to detect with just a Faraday cup.
There have been reports of rejuvenating continuous dynode multipliers (e.g. Channeltrons), but I was never successful in bringing one back to life. It would be interesting to hear if anyone else has been successful. I have a drawer full of dead ones.
Last edited by Dan Knapp on Thu Aug 08, 2019 9:37 pm, edited 1 time in total.
Roberto Ferrari
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Roberto Ferrari »

With regard to EM, my experience servicing mass spectrometers is as follows:
Washing them with isopropyl alcohol (IPA) and drying it with clean, dry N2 usually permits to extend lifetime.
Scrubbing with a cotton swab and extensive washing with IPA sometimes recovered a rebel one.
Manufacturers prevent about exposure in air but in my experience someones left by a whole year exposed to air worked fine when returned to work.
Dan Knapp
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Dan Knapp »

Roberto, are you referring to continuous dynode or discrete dynode multipliers. I have successfully rejuvenated discrete dynode (beryllium copper) multipliers by a variation of the method you describe, but not channeltrons. For discrete dynode multipliers, the process is essentially a cleaning. Indeed, years ago we routinely washed and baked the multiplier every time we opened a mass spec. We would install the multiplier hot and try to get it under vacuum before it cooled. The continuous dynode multipliers undergo a chemical change on the surface, and rejuvenation attempts to restore the original chemical composition of the surface. A continuous dynode multiplier that is simply surface contaminated can be successfully cleaned with solvent followed by baking, but one that has lost gain by surface chemistry change requires a chemical treatment to restore function. I’ve never been able to make the chemical rejuvenation work.
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Jerry Biehler »

The preamp and amp circuits for the faraday cup designs are out there. I have the schematics for the quadrex RGAs, should be easy enough to copy. But I still would never want to build a RGA. I have other things I would rather be not doing. lol

Yeah, we were looking at possibly just recoating the channeltron. I found details on the coating but I cant remember what that is right now, id have to go look again.
Roberto Ferrari
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Re: Interest in RGAs (possibly a candidate for an FAQ?)

Post by Roberto Ferrari »

Dan, I was talking of continuous EM.
My guess is that succesful intervent cleaned contaminated surfaces. May be partially but good enough to recover function.
Probably those that never recovered got chemical changes.
All in all, I recommend to give a try.
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