Help me find the right vacuum pump for my bell jar

I managed to pick up a cheap bell jar and platform w/rubber gasket. Now I’m looking for a suitable pump to evacuate it. I have access to some general purpose air pumps but not sure how much of a vacuum they’ll be able to pull, and they don’t have gauges. I think they are the type that HVAC guys use to evacuate refrigerant from heat pumps. I’m also worried that they might be too strong - is it possible for a vacuum pump to be strong enough to cause the bell jar to implode?

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How much of a vacuum do you really want? A good oil pump can do better than a good oil-less pump, but it really depends on the pressure you’re looking for. This Gast diaphragm pump will do 25 inches, while a more expensive two-stage from Fisher will get you down to 0.24 inches, and something like this oil pump gets you down to 1E-4 Torr, which isn’t even easily expressed in inHg.

For fun with a bell jar, you don’t need much vacuum to seal it. You can always put a vacuum gauge in-line so you have some idea of what the pressure is. And yes, be careful, as it is possible to implode glassware, although it generally needs to have a chip or some other flaw in the glass.

Of course, that’s probably way more than you want to pay for a vacuum pump. I’d start with a cheap vacuum pump, as that’ll do 0.001 inches based on the 5 Pa label.

That’s cheaper than the one I found for $275: The primary difference seems to be that this one has a gauge. It’s rated at 10Pa, although the one review it has claims it couldn’t boil water at room temperature. If I am reading the phase diagram for water right, 10Pa should be way more than sufficient to boil room temperature water.

I poked around locally and found this one, which has a gauge and claims to be able to get to 10Pa.

And the cookie jar w/pump is a good idea. If a cookie jar can hold up to that kind of punishment, then I guess I shouldn’t be too worried about cracking a bell jar.

You shouldn’t be worried about getting a vacuum so strong that it breaks the jar. Consider, for instance, a pump that can go down to 0.1 atm. As vacuums go, that’s not very impressive, and doesn’t seem nearly as strong as something that can go down to, say, 10^-6 atm. But as far as pressure difference across the glass goes, that first pump is still doing 90% as much as the second one.

It is, of course, possible to break a vacuum jar. But if that happens, it’s because the jar is defective, not because of the pump.

[quote=“astro, post:2, topic:778885”]

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Grant Thompson is lots of fun, but I’d trust this guy a bit more for something like this:

There’s vacuum and then there’s volume of gases removed. It’s possible that the 10Pa rating was with the chamber empty and that with the container of water evaporating in the chamber the vacuum pump couldn’t keep up with the volume of water vapor produced. You’d reach an equilibrium state where the pump was only removing as much water vapor as was being produced.

Maybe look at Harbor Freight for a refrigerant vacuum pump and a set of gauges. You can hook up the pump through the gauges, and then close off the jar once the vacuum you want is achieved. You can get the setup I have for cars for around 100 with a coupon.

I have this pump and it works well. It does get close to its specified rating (of ~5 Pa). That’s way more of a vacuum than is needed to boil water and do other fun things, but isn’t close to a hard vacuum, so you’ll need more equipment for your fusion experiments.

You’ll want to change the oil every so often, particularly if you’re pumping down moist objects–the water will contaminate the oil and reduce performance. So keep a few spare quarts handy.

I found vacuum grease to come in very handy as well. Apply a bit to the fitting threads to reduce losses. I also use a thin film on the bottom seal for my bell jar.

You may be able to find a used vacuum pump on eBay for a reasonable price.

the compressor from any discarded window AC or fridge makes a good vacuum pump
depending on how much you want to pull?

you could get a cheap harbor freight auto AC vacuum pulldown pump too