I am looking into getting a vacuum pump, the problem is, I know nothing about vacuum pumps. I would be using it for vacuum filtration and maybe for
distillations under reduced pressure. So I ask for your help in choosing a suitable pump. I would also like to know what the various specifications
mean, such as Torr (which is the same as mm Hg right?) and what "strength" I should be looking for.
I was thinking of getting one from these people. Any advice is appreciated.
I someone has another supplier they think is better, please, go ahead and post them. I don't want to spend too much, <100$, so used would be much,
much better.Dave Angel - 11-8-2012 at 07:53
There have been a couple of threads on vacuum pump advice recently, here and here.
Your choice for general lab use (distillation, filtration, desiccation etc.) comes down to either a diaphragm pump, an aspirator to (connect to your
source of running water), or a recirculation water jet pump, which is essentially a self-contained version of the aspirator.
Given your budget of <$100 then I'd go for an aspirator attached to your water supply and see how you get on. This will do the jobs you have in
mind. Later, if you can budget for more, I find my diaphragm pump does 90 % of my jobs due to its sheer convenience.Fossil - 11-8-2012 at 07:58
I already have an aspirator, however it's not too good and I don't like wasting so much water. What are the specs on your pump and what specs should I
be looking for? I'll give those threads a read too, I probably should have done my due diligence before posting.Dave Angel - 11-8-2012 at 13:03
I had the same concern with the waste of water with my first aspirator, and had to feed it with a hose pipe from a tap tens of metres away. I bought a
nice new one to build into a solid recirculator design with cemented plastic pipe, but chanced upon a rotary evaporator with diaphragm pump included
before I got round to the build. A cheap, recirculator build with a well-constructed aspirator might still do it for you on budget.
My diaphragm pump is a 'KNF Neuberger VDE 0530' and I used to be able to get the specs online; either my search skills are failing me or it's not on
the web anymore. It looks similar in pump head configuration (the V-cylinder style) to the E232 model listed here, just a lot older. Looks like it can do about the theoretical vacuum of an aspirator: say we're at 21 °C, vapour pressure of water is 18.8
mmHg => ca. 25 mbar, which is what the KNF pumps are specced for - an aspirator operating this well would be more than adequate.
Generally I've found vacuum pumps in decent condition to keep value. If you look hard enough you might be able to get a good deal - I got a decent
used rotary for £100 from a small vacuum vessel building company, so that could be a good place to start. If you can avoid buying cheap it really is
a good investment for the lab, if the aspirator doesn't do it for you. Based on the KNF specs I'd say 20+ L/min, 20-25 mbar ultimate vacuum would be
the spec to look for. Good hunting!DJF90 - 12-8-2012 at 01:17
I paid just under £200 for mine, and its great. The only thing I will mention is that whilst the ultimate vacuum is spec' at 8mbar, mine (the same
model) pulls to about 25mbar, both directly on a gauge, or attached to a system. Contamination isn't really a problem as they are designed to be
chemically resistant.
Occasionally, distillation at a lower pressure needs to be performed (e.g. 10mbar or less; sometimes 0.1mmHg is required) and for this, you'd need a
"high vacuum" pump, probably of the rotary vane variety. One such example on ebay at the moment is: http://www.ebay.co.uk/itm/300757575760?ssPageName=STRK:MEWAX... Note the BARGAIN price. A single stage pump ought to get to 1mmHg, at least. For
more suck, you'll need a two stage pump. These typically reach 25 micron (0.025mmHg) ultimate vacuum. If you're intending to run a schlenk manifold
off of one, make sure you get one with a sufficient pumping rate (5cfm would be decent but 2cfm will do). It doesnt matter what pressure it reaches if
it takes forever to get there. Again, my personal experience is with an older version of this pump: http://www.dereksairconditioning.co.uk/p/117/javac-cd-300-30... but you'll see the spec and realise that this exceeds what is necessary. I only
got it because it was such a bargain. Remember, if you're buying a mechanical pump you'll need some form of cold trap to keep it protected. An acid
gas scrubber is also highly recommended.
It is worth mentioning however that a rotary pump is OVERKILL for filtration or rotavapping. In fact it is probably safe to say it is NOT a good idea
due to all the vapours that will pass through it.
As a summary, if you want the most bang for your buck, you're probably best off buying a decent aspirator and building a recirculating station.
Another cheap alternative is a fridge pump. Peach has discussed the application of these to the lab environment in explicit detail and you'd do well
to find the appropriate thread. The added benefit is that these are cheap/free to get hold of, and require minimal tweaking. They're very hardy. And
if it were to break, replacing it would be simple and cheap.
EDIT: I attached an excerpt from a vacuum manual. It shows comparitive figures for rotary pumps of different pumping rate.
