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Author: Subject: How much vacuum can an aspirator pull?
Mark Van Adium
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[*] posted on 11-8-2011 at 10:37
How much vacuum can an aspirator pull?


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?
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blogfast25
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[*] posted on 11-8-2011 at 12:40


Sounds like a question for google, really...
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bahamuth
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[*] posted on 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).




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Magpie
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[*] posted on 11-8-2011 at 16:51


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



Knowing that I can buy good quality NaOH, HCl, and H2SO4 locally gives me great peace of mind.
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blogfast25
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[*] posted on 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.
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Magpie
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[*] posted on 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.

aspirator.JPG - 98kB




Knowing that I can buy good quality NaOH, HCl, and H2SO4 locally gives me great peace of mind.
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Mark Van Adium
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[*] posted on 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]
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blogfast25
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[*] posted on 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...
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Magpie
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[*] posted on 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]




Knowing that I can buy good quality NaOH, HCl, and H2SO4 locally gives me great peace of mind.
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Mark Van Adium
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[*] posted on 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]
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Magpie
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[*] posted on 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.




Knowing that I can buy good quality NaOH, HCl, and H2SO4 locally gives me great peace of mind.
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blogfast25
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[*] posted on 13-8-2011 at 03:37


Decent price too!
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Phthalic Acid
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[*] posted on 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 =)
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blogfast25
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[*] posted on 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 :o. 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]
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Mark Van Adium
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[*] posted on 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.
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Arthur Dent
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[*] posted on 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




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[*] posted on 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 :o. 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]


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]




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