Wednesday, September 15, 2010

vacuum system progress

I've been focused on the vacuum system lately. The MSLS is being worked, slowly, in the background, but I've hit a mental snag with it, and I need to walk away from it for a while. It's just one of my mental idiosyncrasies, not a problem with the design that I need help to solve.

So, the vacuum system. It leaks. I have been methodically working my way through every single connection in the system, working to eliminate leaks.

Simply by cleaning up o-ring joints, I got the ultimate pressure from 80 mT down to 20.
Still not low enough to even be able to turn on the diffusion pump, but it is progress.

But! Speaking of the diffusion pump, that's where I am in the process. Over the weekend, I removed the DP from the cold trap and inspected it.  Here's what it looks like, removed but still assembled:

 I was relieved to find that the fluid looks very clean, and the pump as well, for the most part. It has a few bits of black crud left over from the previous owners- stuff I was not able to remove with a glass bead blaster, but which was apparently removed from the valleys of the boiler by boiling action of the fluid. Scrubbing bubbles, as it were.

So that was all well and good, but for the o-ring which seals the DP to the cold trap.

It was flat.

It is not supposed to be flat, it is supposed to be round.

 My theory is that some insufficiently-educated or -supervised student (the previous owner of this pumping station was the University of Colorado) allowed the DP to get too hot, possibly as a result of a burned out cooling fan. I already know they were running Octoil or some other low-grade DP fluid in it, because they burned it up, leaving rock-hard carbon-loaded sludge behind - incredibly difficult to remove. That does not happen with the silicone fluid that this pump is SUPPOSED to have in it. (and which it DOES have in it, ever since I rebuilt it) It's also possible that some bozo used a non-Viton O-ring. That would be stupid, since DPs are designed to get hot. In theory, the top end shouldn't get hot enough to hurt a Viton O-ring, but in theory, the pump shouldn't have been mistreated either.

The good news is that high quality o-rings of nearly any reasonable size are dirt cheap and available from anybody, including my favorite, McMaster-Carr. The size I need came 10 to a bag and cost $10 total. They should arrive by the end of this week.

I'm also concerned that the boiler heater is too hot.  It is supposed to be a 375 watt heater, but the one that is on it now is dissipating 428 watts.  I'm not sure yet how important this is.  I'm reading and I've inquired with a vacuum engineer at Varian, but I haven't heard back from him yet.

Once the DP has been tuned up and dialed-in, the next step will be to inspect the butterfly valve which separates the chamber from the cold trap.

It occurs to me that I could be including more pictures to illustrate what the hell I am talking about, and that it might even make these posts more interesting. Okay, I'll see what I can do. In fact, if you're interested in this post's topic, check back in a few days, I will try to snap some photos and insert them into this post at appropriate points.

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