Baratron

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
David Housley
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Baratron

Post by David Housley » Wed May 02, 2012 10:15 pm

Hi, Group
I work with Carl Greniger with his fusor and the group of teens who run it. As was posted in the announcements forum, we recently upgraded from our bell jar to a very nice stainless steel chamber. A big thanks goes out to Robert Tubbs who has provided advice, parts, and a beautiful TIG weld to allow the chamber to be adapted to the present vacuum system, as well as finding the chamber on ebay in the first place. As mentioned, we are now able to acheive a much better vacuum than ever before, so it is time to start looking at upgrading the vaccum instrumentation to match. I did a search following Robert's advice on the best system, which appears to be a Baratron. The main reason is the gas insensitivity to the capacitive type of vaccum measurement. I did some research on my own, both on here, and elsewhere, and agree for this use it is the gold standard. I have a few questions specific to its use in fusors that I hope the experience of this group can answer. I appreciate any and all advice along these lines, as most of my hi-vac work has been with cold cathode gauges for measuring the high vacuum areas so I admit to being ignorant to the subtleties of the Baratron. So, here goes:

1) Since the "sweet spot" for fusion seems to be around the 12 micron range, it seems logical the full scale capability should include this range.

2) The ranges I have seen are usually in increments of 10 Torr, so 0.1 Torr full scale (FS), 1 Torr FS, 10 Torr FS, etc.

3) Even thought the precision is rated to four decades, the accuracy suffers a lot with a larger range, thus, if you have a 100 Torr FS sensor, you can't reasonably expect it to be accurate reading 0.1 Torr


Ok, down to the $64 question: What is the optimum range, in your experience, for a Baratron gauge for use in a fusor? Is there any real value in having multiple range gauges for the fusor work? Obviously, even used ones can be pretty pricey, so cost is always a factor, but not necessarily the most important factor.

I know there is some excellant professional experience on this board, and I appreciate the opportunity to tap it to make the best choice. Thank you for your time and consideration in responding.

David H.

SteveHansen
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Re: Baratron

Post by SteveHansen » Wed May 02, 2012 11:33 pm

Baratron is a trademark of MKS Instruments so I presume you are talking about the MKS product.

The standard Baratrons have a resolution of 1:10,000. Thus a 1 Torr full scale instrument will resolve to 0.1 microns. The accuracy is specified as percent of reading and is typically 1/2 percent or better. There are some caveats.

Baratrons (and all capacitance gauges) require zeroing as zeros will move with temperature changes. To zero the instrument you need to pump the gauge to (ideally) at least 1 decade below the resolution. That would be 0.01 micron or lower for a 1 Torr instrument.

There are other low range errors that can creep in (hysteresis, etc.) so it is recommended that the practical reading range be limited to about 3 decades and if the device is used in a process control system, about 2 decades below full scale.

If you have a 1 Torr FS instrument and your fusor runs at 15 microns, your reading should be 15.0 microns with an expected error not to exceed 0.1 microns.

In looking at Baratrons, the least expensive are the general purpose instruments. These are 1/2 percent and are not temperature regulated. The next step up are temp regulated units with a standard heater shell running at 45 degrees C. These have accuracies of down to 0.15 % of reading. Higher temperature units are generally used where there are high vapor pressure residuals in the system.

For something like a fusor, a 1 Torr FS instrument is a good choice, 0.1 Torr FS is better but you will pay more. A very good 1 Torr model is the 722.

If you buy new, no issues. Used units may have contaminants on the innards (fairly easy to tell gross contamination by looking in the port to see what's on the tube and baffle) or may have gone out of calibration.

With regard to calibration, the factory would like you to calibrate once per year. If you are running thousands of dollars of product through a process tool or trying to maintain your FDA certification, you do that. However, mostly what you are looking at is decent accuracy but most of all repeatability. I have several Baratrons that are between 10 and 15 years old and are still within 1 percent of reading accuracy.

David Housley
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Re: Baratron

Post by David Housley » Thu May 03, 2012 1:56 am

Thanks, Steve for your reply. I apologize, I should have referenced that Baratron is an MKS tradmark, and that they are considered the high end diaphram capacitance gauge. The fact that you work for them and contribute to this board has great sway towards an MKS Baratron as well.
From what I read, a 0.1 Torr gauge will almost certainly be heated. So, if given a choice, a 0.1 Torr gauge would be your choice for a fusor? Also, do you know roughly what the cost of a new 0.1 Torr baratron would be? It would be great to start with a known calibrated unit if we can afford it, but we also have to think carefuly about the best use of available funds. I appreciate any additional insight you can give alongthose lines.

Thanks!
David H.

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Chris Bradley
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Re: Baratron

Post by Chris Bradley » Thu May 03, 2012 9:30 am

Great tech response from Steve.

However, I might add that in terms of 'what is best to use', I think the answer from the point of view of amateur plasma discharge work [where pressure metrology is just a means to an end] is 'the cheapest, working unit you can get'!!!

As you explore the parameter space (especially with your new set-up that will likely provide more consistent behaviours) you will come to know what happens at what pressure, and you may well simply stop referring to the gauge at all as you become intrinsically familiar with the pressure by viewing the plasma response. In fact, observing the plasma discharge may prove to be a more accurate way to determine where you are in the 'pressure' parameter!

The point at which you may want to know an exact figure is when you come to disseminate your results so others can comprehend what pressures you are working at. However, even that may be only of some limited value as the pressures that one shape/construction of chamber does 'its thing' at may be different to another.

Add in the measurement uncertainty/calibration issues, and it will all add up to you reporting what may essentially be a fairly arbitrary figure in terms of someone else's comprehension/ability to reproduce the result. The key service for a pressure gauge *during your* experimentation is so that you can perform repeatable and consistent runs. In that regard, if the gauge is always wrong by, even, an order of magnitude, so long as it is always wrong by that amount then it serves its experimental purpose!

That being said, exploring and perfecting vacuum pressure metrology is a technical goal in itself, and is therefore especially worth the educational pursuit if that is the objective.

Wilfried Heil
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Re: Baratron

Post by Wilfried Heil » Thu May 03, 2012 10:22 am

There is an alternative, also from MKS. The MicroPirani 925 Series is a micromachined Pirani gauge that can measure in the lower millitorr region. The advantage is that it will work from atmosphere down to fusion pressure. It has just the right pressure range for a fusor, where the working pressure is on the order of 10-25 millitorr. The one I'm using is a MicroPirani 925C.

SteveHansen
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Re: Baratron

Post by SteveHansen » Thu May 03, 2012 12:01 pm

The microPirani is an excellent gauge and is also very repeatable if you check the zero every now and then (zero pressure is <10^-5 Torr). I have a couple of these and also Baratrons in ranges from 1 to 1000 Torr (and a couple in the 1000 psi range).

The price for a new 626 Baratron (unheated) is about $1080. There is a 0.1 Torr version but it's not popular. The 627 (heated to 45 degrees) is about $1800. If you look at the data sheets you can get the zero temp coefficient and from there you can calculate the amount of zero drift per degree C change in the ambient. In most situations you won't see a significant drift with an unheated unit.

On the other hand, the 925C is around $400.

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Richard Hull
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Re: Baratron

Post by Richard Hull » Thu May 03, 2012 2:14 pm

Steve Hansen is the man to listen carefully to on these matters as he is an old "vacuum head". (which is a high compliment, of course).

Chris is correct and recognizes the realities of operation. The experienced fusor operator develops an instinctive "feel" (artifice) for the operational range of pressure and voltage. As such, the finest of gauges will ultimately act as a data source rather than a crutch or "set point" device.

The above being said, I use a Tylan General ("baratron") that is, indeed, heated and is rated at 0.1 torr. For me, it is highly valuable only as an indicator that I am "bottomed out" to the degree needed to start admitting deuterium gas and carefully setting my vacuum throttle valve to start the fusion process and hunt for "the sweet spot". The actual fusion operating pressure reading has no real value once I am in search of the sweet spot! It is what it is and will wander over several microns during a one or two hour operating session. I use the gauge reading only when taking data down for recording purposes in my operational log book for the fusor. The sweet spot is best found using the voltmeter, ammeter and neutron counter as your guide along with the visual indication of the inner grid and plasma. This involves co-jointly jockeying the variac on the power supply and the vacuum throttle and or gas needle valve.

I lucked onto my Tylan gauge back in 2002 at a "Teslathon" in Rochester NY. A good friend sold it to me for $25.00! He had several Tylan gauges from dismantled semi fab lines that were very clean. I bought most of them.

The common gauge most often seen surplus is 10 torr, but if you see a 1 torr snap it up! Steve was correct in that the 1 torr Baratron type device is certainly your cheapest new or used option in this type of gauge for fusor use.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

David Housley
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Re: Baratron

Post by David Housley » Thu May 03, 2012 11:00 pm

Thank you everyone, for your experienced, well versed responses--I am taking careful note of them all! The Piranni gauges are available for reasonable prices, are robust, and easy to use. The only downside I can see is that their calibration is gas sensitive, whereas the Baratron is not.
Our setup has a slightly different goal than most fusors--it is mainly a teaching/learning platform, a tool for learnng and doing real science, and for scientific experimentation. To that end, being able to have good instrumentation is vital to establishing baseline data, and measuring experimental results. Obviously, "tuning" is required during operation to maintain output. One long term goal is to also interface the fusor to computer control. This is also mainly as a learning project in computer controls, software, interfacing, etc. To this end, reliable instrumentation is also vital. We are targeting making this the best platform possible for learning and experimenting with the students, and are making the investments necessary towards that end. We know we have a long way to go, and much to learn yet, but that process is the most important part for the students.
Please know I have the highest respect for the experts on this board, and I am grateful for the information and experience you share. I guess ultimately I am looking for the "best" instrumentation, then see if we have a chance to afford it; if not, then what is second best, and down the line. Steve, I know you are the defacto expert here, and I will take your word as gospel for the final decision of which instrument to use, so, will a Pirrani gauge give us the required accuracy with hydrogen? Is the Baratron ultimately the gold standard? I guess I just want to "do what's best" given the setup and ultimate goals. Thank you all again for your input and help!

David H.

Chris Roberts
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Re: Baratron

Post by Chris Roberts » Thu May 03, 2012 11:03 pm

The only thing that I would mention is the heated Baratrons take hours to warm up and fully stabilize. (4 hours seems to be the norm) The unheated ones are fully ready to go in about 15 minutes. So that depends a lot on how you run your fusor. If you want to have a group come in, power the thing up, and be on your way immediately, I would stick with an unheated one. If you plan on letting the fusor pump overnight before a run and really want that extra stability, then a heated one might be useful.

Personally I would not go with a heated one. Hope this helps.

-Chris

David Housley
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Re: Baratron

Post by David Housley » Thu May 03, 2012 11:50 pm

Thanks, Chris, this is definately useful advice. I was thinking along the same lines, i.e., a non-heated baratron---given how the sessions run, long warm up times don't fit the model well. Any other thoughts or ideas are very welcome.
Thanks,
David H.

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