So I was browsing around in a new farm/tractor store yesterday to see what their inventory was like, and I saw some vaccum pump oil for milking machines for about $6 for a gallon. I dunno about the suitability of the specific product I saw, but it was labeled as high quality, anti-foaming, and a few other characteristics which I forgot.
I don't know what kinds of pumps are used on milking machines, but this might be a low cost alternative product, compared to the high cost of other oils.
WB
Depends upon how high the vacuum is. The oil could just be some prevacuumed oil that the high volitiles have already been cleaned out of so that the stuff doesn't contaminate the milk.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!
Why not try it? It's unlikely to do any harm to your vacuum pump. As long as you can measure the resultant vacuum, and are willing to flush the pump many times if it doesn't, then what's the harm.
11 to 12 in Hg at the teat...
Merciless mockery in r.c.m?
I wouldn't try it if you have a vacuum system that runs at very low pressures, like less than 1" Hg. And particularly, I wouldn't try it in any system where the mechanical pump backs up a diffusion pump. The problem is that if there are some low pressure volatiles left in it, they may back stream into your diffusion pump or the vacuum vessel itself.
Pete Stanaitis
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Wild Bill wrote:
Maybe. Kind of depends what you think high vacuum is. Microtorr or inches.
I guess I for one did not see any mockery. High vacuum pump oil is expensive because it has to meet a number of specifications, not the least of which is vapor pressure. I think the idea of trying it, and seeing what the vapor pressure is, with a thermocouple gage, is probably the best way (except for reading the manufacturer's specifications, that is!) to find this number out.
Jim
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lol, show me a oil based rotary vacuum pump that can get down to microtorr. I have a welch 1400 that can get to about 2 millitorr (measured on two different TC guages) but have never heard of one getting to microtorr. For that you need to use a diffusion, ion or turbo pump depending on your budget and level of oil contamination in your final product (turbo pump being the very best and most expensive). I have a new small Varian diffusion pump from ebay that is supposed to be able to do 10-6 torr but I haven't found anything to try with it yet.
The ultimate vacuum you get is also highly dependant on the cleanliness and dryness of your system. I worked on an E-beam lithography system years ago. It had a rotory-vane pump for roughing the outlet of a diffusion pump which had a liquid N2 cold trap. There were also 3 ion pumps that would not get turned on until the vacuum reached about 10-5 torr. On a good day, you could approach 10-7 torr.
You can do tolerably well if you cold trap the rough pump. I think with a bit of zeolite and some LN2 you could get down into the high fives, or low fours. Granted that's not just the rough pump.
But hey, one micron is 1000 microtorr.....
Jim
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Depends *alot* on the size and cleanliness of your chamber.
This is pretty standard stuff nowadays. Oil type rotary vane pumps are now replaced by oil-less 'scroll pumps' so there is no hydrocarbon oil to get into the system.
We just turned on a new system at work, with a scroll pump to back a turbopump, for roughing things out. The turbo gets into the low sixes rapidly. The main chamber is pumped by a cryopump, reacheing a good base pressure in the mid 8-s which is pretty good, seeing as the entire thing was shipped up at air, with the valves all open.
Jim
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How are the cryopumps these days in terms of vibration? We were between a rock and a hard place back in the 80's. Either live with the constant fear of burping up diffusion pump oil all through the system or see a quality hit because of the cryopump vibration.
We experimented awhile with turning off the cryopump when the beam was on. On short jobs this was ok, but not a good general fix.
We would always keep the sustem pumped down with the welch pumps. With new oil you could hold 10-5 if nobody put their oily fingerprints inside the bell jar or contaminated it in other ways. If the oil gets contaminated with moisture or dirt you jsut wont be able to pull a good vacuum. We ran oil diffusion pumps and could get down to 10-7 but it wasnt really needed for what we were doing. The bell jar was a piece of work with complex mechanical mechansims to move the substrate for the several depositions that were applied.
John
Pretty quite. Aside from the first stage compressor, which is hose-coupled to the head, things are very quiet.
Jim
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One caveat here is that once the pressure in the system falls below about five microns, the mean free path length for an oil molecule becomes very, very long. In other words, an oil molecule can travel meters before it hits anything else.
So below that pressure there is a tendency for oil to backstream into the experimental chambers. So if one is backing a diff pump or a turbo pump and the main pump shuts down for any reason, the backing (mechanical) pump will contaminate the high vac side of the main pump with oil.
Jim
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This is one hell of a useful lead for me. I've got a piston-type pump (actually from an old dairy) that runs to about 25in Hg that I'll try this stuff out instead. I'm also in the middle of rebuilding a liquid ring pump that'll need a tank with at least 15gal. of oil, and this cuts the cost down pretty impressively. Up til now I've been using non-detergent motor oil in the pump, so this'll be a step up in any case.
Btw, the pumps are being used for vacuum bagging composite hulls and decks - probably not the high-end type application others are looking at. Basically, get 21in Hg or better with enough volume to seal all the leaks.
Regards,
Matt Turner Turner Racing Shells Ltd.
Then there is the alternative of using a sorbtion pump, instead of the cold-trap. Gets essentially all of the oil, and only lets through a little methane. And maybe a bit simpler to operate if you can't get LN2 easily.
I think maybe you are suggesting a 'sublimation' pump.
Sorption pumps are typically big containers full of zeolite sorb, and are immersed in LN2 buckets. Those are great for roughing out systems that need to be ultra clean.
The STM guys always used to use those.
Jim
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