Friday, October 29, 2010

in which our hero accomplishes very small but significant progress in a project designed to create very small but significant pressures

In theory, I've been working on the vacuum system.  I've been interrupted in various ways, in fact, I had to reassemble the system partially just to make it safe from spilled beers and the like while the Denver Mad Scientists Club invaded my sanctum, well, communio to tell the truth.  But I digress again.

 Various stuff be happening, and not much of it vacuum work.

 BUT!  I has a plan.  And I am making glacial, but non-zero progress.  I have the "stack" partially disassembled, and I need to finish that.  I need to take apart one of the valves to get its o-ring sizes (seats and joints) so I can order all new O-rings.

By the by, I don't know how common the term "stack" is in the general vacuum community...

I lie.  I just checked the VacuumX Yahoo Group and it's a commonly-used term.

Okay good, cuz I was just about to define it for those reading who might not be familiar with it.



 To wit: the "stack" on a vacuum system is most often a vertically arranged sequence -- I'll go from top to bottom -- consisting of:

1. chamber (or a baseplate for a bell jar)

2. big-ass gate or butterfly isolation valve, labeled "Hi-Vac Valve" in the diagram

3. a cold trap and accompanying "optically opaque" baffle, usually only needed if the high vacuum pump is a diffusion pump

4. and at the bottom, the high vacuum pump such as a diffusion pump, which in turn is "backed" by the mechanical pump.

Somewhat amusingly, the mechanical pump is the part everybody notices, because it's big and makes lots of noise. But it is a simple thing, unfinicky, and the least sophisticated element of the system. Normally, it will give the operator no trouble if it treated properly. The diffusion pump and cold trap are where all the action is.

Now if you're wealthy or you're a university or a big company or a government lab, you don't use a diffusion pump, you use a turbo or a cryo or some other high vacuum pump.

(mumble Vonnegut mumble)
HERE IS A PICTURE OF MY TURBOPUMP*:
And in those cases you probably don't need the cold trap either.  Well maybe.  But that's outside the scope of this entire blog, I think.  I'm thinking I don't want to start trying to teach too much in the way of basic lab techniques like basic high vacuum work, because that's a basic laboratory skill with which every budding (or accomplished) mad scientist should already be familiar.  If not, well, there's plenty of books and web resources for high vacuum work, such as the aforementioned Yahoo group.  Also, check out The Bell Jar.

 So, in my system, I have a 3" oil (DC-704 silicone) diffusion pump (Veeco) backed by an (oversized) two-stage rotary vane pump (Cenco-HyVac).  The top of the diffusion pump is bolted to an LN2 cold trap.  The cold trap keeps DP vapor out of the high vacuum side of the system, and (an occasionally overlooked but important aspect) it keeps water vapor from your process chamber out of the DP.

 Aside: this is why I also have a handy and monstrous dewar I can go fetch large quantities of LN2 in, saving (coronary-inducing) delivery charges, and making it cheaper because of quantity as well.  You pay for a base boil-off fee, and the more you buy, the less you pay per liter.

 Veeco's cold trap is a rather clever coaxial design with optically dense baffles at one end, a cold finger, and a coaxial (toroidal) gas path. I haven't seen anything like it implemented anywhere else.  I will say this: the system appears to be almost overdesigned. It is certainly conservatively designed and if I can ever get it rebuilt, it ought to be a reliable and high performer.
  Moving on...

 The top of the cold trap, at a point offset horizontally from where the DP is connected below, is connected to a butterfly valve, about 5" bore and only 1" thick body - rather nice.  It is a leak-tight high vacuum valve which will hold off an atmosphere against a 5" throat.  Now isn't THAT nice?  Think: you can pump down the entire vacuum system to some ridiculously low pressure.  Open your chamber, insert your process stuff or do whatever it is you gotta do in there, close your chamber, now with the rest of the vacuum system at high vacuum and isolated, you open a roughing valve connecting your process chamber to the roughing pump, but not the the high vacuum parts of the system.

 When the chamber pressure is nice and low, you open the butterfly valve and your chamber pressure falls through the floor.  Otherwise, you'd be sucking the entire process chamber out through your cold trap and your high vacuum pump.  And that, friends and neighbors, is bad design.

 So that's what a fully valved system with a chamber isolation valve does for you, if you're lucky enough to stumble into one for a price you can pay.


That is, if you've been good.

 If you've been bad, you're still trying to eliminate leaks and repair damage caused by years of abuse and neglect by ignorant savages college students working under the poor-to-nonexistent supervision of college professors or TA's or grads or whoever is - or should have been - in charge of such things.

Ahem. But I'm not bitter. I paid $20 US for the thing although it didn't have a roughing pump and the DP was a mess.

 Now where was I?  Oh yeah: O-rings.  I'm gonna find all the sizes of o-rings used anywhere in the system, and I'm gonna replace every damned one of 'em.  And when I replace them, not only am I going to use good Viton O-rings as any fool would, I am going to process them through certain laboratory resources I have at my disposal which will render them extra-good for high vacuum work.  I have read in books that it is possible to build a full-featured system with many O-ring sealed joints that will consistently run at 1x10-8 T.  Normally, we think in terms of routine pressures of 10-6 Torr, maybe 10-7 on a good day with a tailwind. Well, I aim to prove those books right, and see routine pressure in my chamber below 9x10-8.

 Oh guess what I found in a couple of white papers? One of them was a NASA article by people working with big chambers. Just go ahead and put a thin coating of good high vacuum grease ALL OVER your baked-out O-rings before installation. The grease blocks gas molecules better than the Viton, and it keeps the Viton from loading up on water vapor and other stuff after you've baked it out under high vacuum. Heh. Yeah, you need high vacuum in order to build a high vacuum system right.  Don't use vacuum grease if you expect to go much below 5x10-9. But if you're trying for those pressures, you are already familiar with UHV and you don't need my advice. Or you're very foolish.

 So once I have all the o-rings process and bagged and all the parts I think I need to wrap this thing up, I'm hoping I can finish disassembling, re-cleaning, and rebuilding the whole thing in a single weekend, which I probably can if I enlist the help of a good friend who has voiced enthusiasm for said assistance.

I'm just taking my own sweet time leading up to that point.

There, I posted dammit.

*that is not my turbopump station, it belongs to my employer.
Without the stuff I've stuck on it, it costs about $7,000 -- a bit out of my reach.

1 comment:

Gomez said...

Oh, and all of this is happening at glacial speeds because there isn't much money to support it.