Sciencemadness Discussion Board - How much vacuum can an aspirator pull?

How much vacuum can an aspirator pull?

Mark Van Adium - 11-8-2011 at 10:37

Theroetically they can pull down to the vapour pressure of the liquid, but I find it hard to believe I can pull down to less than 10mmhg with a vacuum aspirator and some <10*C water.

How much can they actually manage with water?

blogfast25 - 11-8-2011 at 12:40

Sounds like a question for google, really...

bahamuth - 11-8-2011 at 15:58

Remember I found a old old physics kit at work, like the stuff one used at school 20 years ago, which had a chrome/nickel coated brass aspirator which was stated to produce down to 15mmHg with tap water..

Tested one type, Brand (Plastibrand) plastic aspirator with a electronic vacumeter, and IIRC it gave around 150 mmHg at full juice of the tap. (do not qoute me on that value, might be way off).

Magpie - 11-8-2011 at 16:51

I get 50-60mmHg with cold tap water using a chrome plated brass aspirator.

blogfast25 - 12-8-2011 at 05:26

Quote: Originally posted by Magpie

I get 50-60mmHg with cold tap water using a chrome plated brass aspirator.

Any pics at all? I've been in the market for one of those or been tempted to get injured in the process of cobbling one together.

Magpie - 12-8-2011 at 09:21

Quote: Originally posted by blogfast25

Any pics at all?

Here's my aspirator as installed. The brass fittings are adaptors from a hardware store. The valve/hose attatchment at the suction port is something I added to facilitate vacuum breaking and attaching a vacuum hose (Buchner filtrations, etc). Only the steel looking item is the actual aspirator. IIRC it cost around $20.

Mark Van Adium - 12-8-2011 at 10:53

Well damn, thanks guys, digging on google revealed claims from 28inhg to the theoretical limit. I think I'll be picking up one of these!

Edit, do they have any other names, I'm really struggling to find them in britain.

[Edited on 12-8-2011 by Mark Van Adium]

blogfast25 - 12-8-2011 at 12:04

Very nice, Magpie.

I second Mark Van Adam: the core of the aspirator may be harder to find here in Old Blighty...

Magpie - 12-8-2011 at 12:48

Here's the one I bought:

http://www.carolina.com/product/filter+pump+or+aspirator.do?...

I also bought a plastic one for $11 but have never used it. It's a NALGENE Vacuum Pump, cat no. 6140-0010. The product info lists a telephone number in Brussels, so perhaps you can find it in Old Blighty.

http://www.carolina.com/product/vacuum+filter+pump%2C+nalgen...

[Edited on 12-8-2011 by Magpie]

Mark Van Adium - 12-8-2011 at 13:05

Here's the one I found, I'm looking forward to converting that NPT thread to BSP. Oh for international standards.

[Edited on 12-8-2011 by Mark Van Adium]

Magpie - 12-8-2011 at 14:22

That looks like the same one I have.

Humbolt is a good lab supply company. Their equipment is heavy-duty, professional grade. I have a ring and ringstand, as well as a Meker burner, made by Humbolt.

blogfast25 - 13-8-2011 at 03:37

Decent price too!

Phthalic Acid - 13-8-2011 at 03:45

Mark Van Adium, they are called water aspirators, hydro aspirators and eductor jets pumps. You cam buy them on eBay from Avagadro Lab Supply for about $28USD. Hope this helps you out =)

blogfast25 - 13-8-2011 at 04:05

Quote: Originally posted by Phthalic Acid

Mark Van Adium, they are called water aspirators, hydro aspirators and eductor jets pumps. You cam buy them on eBay from Avagadro Lab Supply for about $28USD. Hope this helps you out =)

...plus, erm... shipping

. Many US eBayers want silly money for getting the stuff across the pond. And unless it's listed in international listings I don't think we can buy it here anyway. Not w/o going round eBay's back.

[Edited on 13-8-2011 by blogfast25]

Mark Van Adium - 14-8-2011 at 01:46

Yeah, ebay looks to be very expensive, but this place seems fairly reasonable considering I can only find one place with them in the UK, and it's the same price for a plastic one.

Arthur Dent - 14-8-2011 at 04:19

Wow. Thanks Magpie! I'll investigate that place of yours because right now, i'm stuck with a cheap plastic aspirator and it doesn't seem to pull much vacuum, even at max cold water output... and worse, I paid for my plastic thingie what you paid for your metal one! :mad:

Robert

peach - 16-8-2011 at 12:38

The theoretical maximum vacuum an aspirator can pull is the vapour pressure of the water when it boils at that temperature. If you imagine the aspirator is a rearranged kettle, and it's boiling away, the water in it is not going to pull any vacuum because it's happily boiling away at atmospheric pressure. Practical aspirators lag slightly behind the theoretical maximum.

Another way to imagine it is to think about it practically. The water is exposed to the vacuum it's creating. As the pressure drops around it, it will try to boil, spraying gas back into the vacuum it's trying to create. The two balance out, so this is the limit of the difference it can keep up between the two.

At 25C, that maximum will be.... 32 mbar (atmospheric is a bit over a 1000).

That is 24 torr, so 24 mmHg (1 atm = 760), 0.46psi (counting down from atmospheric, 14.5) and finally, 3200 pascals, with 1 atm being 101, 325 pascals.

If you are a young scientist, you should be going for Pascals, as they are SI units and so the most constant and scientifically worthwhile.

Torrs and mmHg persist in chemistry due to the noble nature of mercury and the few points at which a mercury gauge can go wrong. However, the mercury is not a favourite even in those laboratories due to the toxicity.

If you cool the water, it's boiling point will drop and so the pressure it can achieve will drop until it reaches the limit of what's called the knudsen flow domain. This is when gases stop being 'sticky' and suckable, and instead enter 'molecular flow', when they are no longer behaving like a loose solid but rather as a individual molecules. In this region, at ^-4mBar, rotaries also begin to cut out and a different concept is required to 'knock' the molecules through the pump.

32mbar means that over 96% of the atmosphere has been removed, so for filtration, the vast majority of the pressure is already on.

It is also good, even excessive, for removing volatile solvents.

It is helpful / poor for distilling high boilers, like oils boiling over 250C at 1atm. High boilers really need a rotary pump, even if it's a window AC compressor. For extremely sensitive work, it needs to be a dual stage, involve dry ice and liquid nitrogen and be well maintained; other issues at this stage include baking the glassware, sodium, molecular sieve and hydride drying.

The aspirator region of pressure is the same achieved by a piston style fridge compressor, down to 10mbar.

Quote: Originally posted by blogfast25

Quote: Originally posted by Phthalic Acid

Mark Van Adium, they are called water aspirators, hydro aspirators and eductor jets pumps. You cam buy them on eBay from Avagadro Lab Supply for about $28USD. Hope this helps you out =)

...plus, erm... shipping

Boooom, he hits.

Yep.

Since USPS have dropped their 6-8 week shipping, I have lost interest in many items from the US as the flat rate is usually about $36 for any item, no matter how light (unless it's paper).

That is absolutely stupid.

But it's not the US guys themselves, it's the export mechanisms they're forced to use.

The US, and UK, and everyone, is struggling with money at the moment. I want to give some money to the US, but spending four times as much as the item on the postage is not part of the plan. Some US guys also have major issues with shipping things out of the US; when they'll actually MAKE money for the US by doing so.

The same works in reverse. Our VAT system is fairly monged up and it's darn annoying trying to get her majesty's special boys to support small exports. I have had numerous arguments with them over strange loophole issues in the system, and it's getting quite fun now. I was getting ready to take HM C&E (the Queens boarder patrol, for the none UK guys) to court over £9 recently (about $10-15). Not for the money or because I don't like them (I love the post), but simply because the reason for the £9 is malevolent and private exploitation by a member of the public; the owners of the Royal Mail. I won that arguement. In that they gave me my post when I demanded it from them; only paying the tax and not the surcharge.

[Edited on 16-8-2011 by peach]

Argyros - 11-1-2015 at 14:26

I'm trying to get hold of an aspirator and the only ones I can find are on eBay (US) and the shipping is more then the aspirator.

Magpie - 11-1-2015 at 15:25

You're not giving us much to work with.

http://www.carolina.com/science-lab-filtration/filter-pump-o...

Argyros - 11-1-2015 at 16:40

Quote: Originally posted by Magpie

You're not giving us much to work with.

http://www.carolina.com/science-lab-filtration/filter-pump-o...

That is what I'm looking for. But I need to find a seller in EU/UK.

Fyndium - 12-3-2021 at 13:18

I have a garden pump that can supply 3.6bar of pressure and 50L/min of flow. I was wondering if I could save a lot of water by circulating a reservoir with this pump through an aspirator? Now that it's winter, it's easy to cool the water down to 0-10C and keep it there.

Is 3.6bar enough to get full benefit of an aspirator?

zed - 12-3-2021 at 17:44

Yes, you could save a lot of water. Once upon a time, I lived where water was practically free. Well, it isn't free where I live now. It costs plenty.

Your water pressure sounds.... Well, OK. Flow rate is equally important. And colder water, is better water.

The aspirator itself is usually pretty effective and inexpensive. You will just have to try it and see how it works.

If you aren't in the US, and you need a pump, you might find one in China. Deschem is offering one, in what looks like Brass. That might be helpful if you chose to make "hard" connections. Threads are not universal, and I had trouble hooking up a Stainless Steel model. Brass, should you decide to thread a connection, will be amenable to machining..

Fyndium - 12-3-2021 at 23:33

I actually possess an aspirator which originates from German firm Carl-Roth. It's plastic, but from mains water pressure it seems to be getting decent vacuum. I haven't used it in years though, so I have to test and calibrate it's effectiveness against my vac pump.

At my summer place I could use lake water with the pump. It's rated 600W so technically it would cost about 6c per hour to run it.

Deschem is my go-to supplier. Been ordering dozens of times from them. I remember seeing a brass-made aspirator from them. There was also a stainless steel version somewhere.

Fery - 13-3-2021 at 07:00

I have 2-stage oil rotary vacuum pump and one old plastic water aspirator pump.
Few months ago I've bought these 2 glass water vacuum aspirators which cost 4 EUR and 10 EUR including VAT:
https://sklep-chemland.pl/en/filtr-wodny.html
https://sklep-chemland.pl/en/pompka-wodna-prozniowa-alvergna...
They will be useful when producing fuming nitric acid sometimes in future.

Fyndium - 13-3-2021 at 12:46

Are glass aspirators better in some aspect compared to steel, brass, plastic, etc?

Fery - 13-3-2021 at 23:59

Hi Fyndium, when distilling out fuming nitric acid from a mixture of azeotropic HNO3 + H2SO4 the glass aspirator should be resistant against HNO3 acid vapor and NOx. Also suitable plastic material should withstand such corrosive attack. I'm not sure with metals, maybe everything corrosive would be diluted and washed out quickly?
I wanted to have some backup equipment if my plastic aspirator would be not working properly. It is very old piece from a long time ago closed down biochemistry lab which I got for free, yellow color changes clearly visible on formerly white plastic. I tested it some time ago when not yet having any device for measuring vacuum level. I evacuated a flask, submersed the flask into water bath, opened the flask so water entered the flask, compared the difference between the volume of water filling the evacuated flask and water filling full flask. This experiment showed that 3/4 of the volume was filled with water. I expected more, like 9/10. The water was colder than 10 C.

[Edited on 14-3-2021 by Fery]

zed - 14-3-2021 at 07:22

No way of measuring vacuum level? A thermometer will do! There are extensive tables for the boiling point of water at various pressures. Pull your hardest vacuum on a flask partially filled with water. Warm the flask, until the water starts to boil. Note the temperature. Reference it on the table. Ummm. Let me see if such tables are available on-line. I'll be back!

OK, not the table I wanted. But, workable.

https://www.engineersedge.com/h2o_boil_pressure.htm

[Edited on 14-3-2021 by zed]

Fery - 14-3-2021 at 09:37

It was some time ago... I already have 2-stage oil rotary vacuum pump and manometer. But that time I haven't so I tried to check how much water enters evacuated flask. It was 3/4 of its volume. Not too much. That water aspirator is really old one. I still have it as it remembers me the old lab where it served. The lab is not there anymore and the building serves for different purposes. I saved few items from the lab. Heating plate with stirrer etc. Valuable items were sold in an auction. Many low value items were condemned to be thrown into waste or at best into recycling (glass/plastic) when I asked for their intended destiny they were given to me for free.
Using water boiling point depression is also very clever idea to check the vacuum capabilities.

Fery - 24-3-2021 at 06:31

So I checked the old water aspirator with a manometer and it is much more powerful than the evacuated flask filled with water showed. The value on manometer is -92 kPa. So strange that the flask was not evacuated to 92%, the difference between an evacuated flask consequently filled with water and flask full of water should be only 8%. I do not suggest to measure the vacuum power using an evacuated flask inverted in water which is then filled with water after opening its neck.