Coulter's Smithing Home.
It can be hard to put together a vacuum system on a budget. This is an attempt to do a fairly nice one for not too much money. Earlier systems we have played with taught us to just go ahead and buy a good pump – the surplus ones were surplus for a reason. Sure, most of them will boil water at room temperature, big deal. The system pictured here should be able to do metal evaporation and other nifty things soon.
I'd really wanted to do the standard bell jar thing, and may yet.
Everyone around here really objected to my idea of using cut off
3 liter wine bottles as unsafe. Who knows? Haven't tried that
yet, though I have plenty of cut–off bottles. I think with the
right baseplate to take inward stress off the bottle base, it would work.|
At any rate, here is what I did do. This is made from a fire extinguisher. I cut it off in the lathe, then welded a pipe flange from McMaster-Carr on there. It leaked a little after the first pass, so I welded it again. Dang, those flanges are heavy – that is 1" steel there. Getting the door to reseal is currently a problem. I'm using a gasket I made from Viton sheet, also a McMaster item. It seems one cat hair in there is enough to mess things up. Gotta go get a real torque wrench too as it's possible to induce a leak by over–tightening one bolt in relation to the others. Once that source of leaks is handled, I will start with other penetrations of the basic chamber. I will use WWII era metal vacuum tubes for getting wires in and out. They cut easily with a tubing cutter. The meter is reading the pirani gage we built at .473 volt. At atmospheric it reads about 3.5 volts. The bulb sees half of that. That's not quite the best vacuum we've had, this was probably done while waiting for it to leak to atmospheric again so it could be opened. The next hole I put in was for a tiny, all metal needle valve so I wouldn't have to wait so long. The first time it took days to leak back up enough to open! Obviously, this thing needs a window too. I got a quartz window at McMaster for it, but one thing at a time.
|Here is the pump we bought from McMaster by spec without even checking who made it. They chose alright, it is a very nice one, which unlike the surplus ones I have, does NOT backstream oil when turned off. I'd wanted to do an all–metal construction, but looking at that fitting changed my mind. It has the intake filter and check valve in it, so machining on it is a no–no. So there is one little 1/4" piece of Viton pipe in there doing the connection. Hope that will work out well enough. It does help with vibration. This pump has another nice property. It's quiet when not moving lots of air, and doesn't draw anything like the nameplate power once you have a decent vacuum. Something on the order of 250 watts for a 1/2 hp motor. Even so, we will automate this to run only as needed, using the pirani gage for a control source. This pump cost something like $2600 at McMaster, their number: 8460K72. Ouch! But, we now have a for–real pump, with low vapor pressure oil that's never seen water or anything else nasty. We can now use the surplus other pumps for doing vacuum impregnation, freeze drying and suchlike things that aren't too good for a good pump.|
Once this gets further along, there are plans to build an efficient oil diffusion pump here. We have and love "Proceedures in Experimental Physics" by John Strong, ISBN 0-917917-56-2. There are what seem to be some good designs there, but thermal efficiency was obviously not a concern to the designers. Make a pump out of thick brass pipe and heat one end while trying to cool the other? Come on now. We will modify the design of figure 13 in the vacuum chapter to be more efficient. First, the heating element will be in the oil, which will be in a teflon boiler. Pipe will be steel (nickel plated) for low thermal conductivity along its length, but adequate conductivity through the sides for cooling. Chimney will be copper pipe, and insulated as suggested. Pump will be air cooled. If the first try works out, we will probably try a two stage version. Speaking of expensive, how about a chunk of virgin teflon 2 inches diameter by 3 inches long? Seems a shame to bore most of it out of there to make a cup. We'll use another metal vacuum tube to run the wires in and out, and probably build a gage right into the thing. There will at least be a thermocouple in the oil.
I guess the moral here is that after years of trying to do something on the really–cheap, and failing, even Doug will open up the checkbook. Time to get on with things and have them working instead of being just dreams!
Coulter's Smithing Home
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