Hexavalent
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Interesting Vacuum Pump
Having toyed with numerous homemade vacuum pumps, reversed air compressors and more I am still not happy; vacuum filtrations, with all kinds of solid
sizes and paper porosities are still extremely slow. Thus, I have decided to invest in a proper lab vacuum pump; what are your thoughts on this one;
http://www.ebay.co.uk/itm/3S-VACUUM-PUMP-1-4-HP-FOR-AIR-COND...
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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watson.fawkes
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The link you cite is a refrigeration service pump. It will draw vacuum, but it's not a "proper lab" piece of equipment, mostly because it's not
designed to pump anything but non-reactive gases, i.e. refrigerants. Using one naively will leading to problems. You can integrate one into a
larger piece of equipment and make it perfectly serviceable, mind.
The main problem for using it for filtration is contamination of the pump lubricant. Condensed water in the pump oil is a problem, and since that's
the default solvent, it's not like you can avoid it. Lubricant doesn't degrade immediately, but you'll need to change it periodically. At the very
least get a pump with a sight glass for the lubricant; it's much easier to tell when it's time for an oil change.
Much easier for maintenance is a diaphragm pump, because the evacuation stream doesn't pass through the lubricant. The fancy lab pumps in this class
are made from PTFE and other rather inert plastics, but there are cheaper ones that will suffice for common solvents.
The "ultimate vacuum" rating for a pump used to drive filtration is a canard. What matters is the ΔP to the atmosphere, not the ultimate
vacuum. Even a cheap pump can get you to 0.01 bar. Nominal atmospheric pressure is 1.01 bar, so that's a ΔP of 1.00 bar. If your cheap pump
only goes to 0.03 bar, ΔP is 0.98 bar, which is only a 2% difference in pressure. There's more than 2% variation in barometric pressure due
to the weather. The pump you've cited goes to 0.03 mbar, three orders of magnitude more than you need for a filtration application.
I recall there's a fellow who built a pressure filtration system out of hardware-store PVC pipe parts. That kind of thing can easily get you six times
the ΔP over atmosphere by using a cheap tire-inflation pump. And it's not like you even have particular materials problems, since you can
integrate a ceramic Buchner funnel into the bottom of such a device. The PVC pipe parts just hold air.
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bob800
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Have you tried a good water aspirator? The one shown in the link has served me faithfully for at least year now, with no signs of wear. As long as your paper size and
pore sizes are big enough, I've never experienced anything but a rapid and clean filtration. However, the more "gelatinous" precipitates
will clog the paper immediately—the only cure is a larger pore size (or perhaps a stronger vacuum, though I've never tried that). Often
times I'll just let the peskier precipitates drain through a big coffee filter overnight.
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DJF90
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Being a "high vacuum" pump, that is unsuitable for your needs. The vapours and corrosives that would pass through it during filtration would make
relatively short work of it. A water aspirator is perfect, if you have a source of running water in the lab. Otherwise you might employ one in a
recirculated setup, or get a diaphragm pump. A good pump will set you back a few quid (the list price for a chemically resistant KNF is about £800),
but you'll find one adequate for your needs on ebay for maybe £100-150).
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Hexavalent
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Thanks for the advice guys, I don't think I'll be buying this one
I have had a metal aspirator before from the school technician, but to get even a small vacuum I needed to go outside and use the water hose at full
blast, the tap fully opened - even then, the filtrations were only marginally faster than gravity filtration.
Was this a dodgy aspirator, and if I get another one, maybe in glass or plastic, is it likely to need this also or can it work from the medium-speed
flow of my water-fountain recirculating pump normally used for a condenser?
If yes, then where in the UK can I get one? I can't see any on eBay and would hate to spend £100+ on a pump if this would do for just the odd,
occasional filtration.
On a side note, what do you think of this?
http://www.ebay.co.uk/itm/Suction-Pump-Aspirator-Emergency-M...
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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DJF90
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Are you sure you hooked up your aspirator properly? Also, check that if it has a check valve on the "vacuum" arm, that this hasn't failed or stuck.
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chemrox
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I have an aspirator that boils water at room temp. It's all glass and has a built-in backflow stop. Don't sell them short. It'll do better than
your refrigerator service pump. If you buy a mechanical pump wait until you find a good deal on a used Welch. Alcatel or one of the other lab-grade 2
stage pumps. An old beat up 2 stage Welch with rusty guts will pull 1 torr if you attend to the connections. Get one of those strap tie tools and
tighten up all the tube connections.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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