Sciencemadness Discussion Board

Vacuum Aspirator Problems

Experitist - 11-11-2018 at 16:04

Hi all,

I purchased this aspirator from Deschem on eBay:

I also purchased a 12V diaphragm water pump that claims 70W 6L/min 0.90MPA/130 PSI to use with the aspirator.

All tubing used is 5/8" ID clear vinyl (I had to use several adapters to attach this size tubing to the aspirator inlet, but everything is screwed in quite tight with teflon tape and O-rings). There are no noticeable leaks. I am pumping cool water (15-20C) from a large reservoir through the aspirator with this pump.

I live at somewhat high elevation (5000-6000 feet) if that matters.

When running the aspirator as it was pulling a vacuum, I measured the flow rate of water out of the aspirator as 3.7 L/min.

My issue is that it doesn't seem to pull anything beyond the slightest vacuum. I connected the vacuum port on the aspirator to a hand vacuum pump that as a gauge attached to it so I might measure the vacuum pulled - the gauge read at most 100 mmHg pressure difference. Even feeling the vacuum with my hand it doesn't feel very strong at all.

I've tried various orientations of the aspirator. The tubing on the inlet/outlet/vacuum port are all on the order of a foot-ish long.

Any ideas on how I might troubleshoot this? I'm at a loss since everything involved is so simple and its hard to see what might be going wrong.

Many thanks.

[Edited on 12-11-2018 by Experitist]

JJay - 11-11-2018 at 16:19

I'm having issues with that same aspirator at the moment... I've used it successfully in the past to pull a strong vacuum, but it really needs a long hose at the outlet and a strong water flow. It doesn't function well if the vacuum hose is flooded with water.

I've found that it pulls a great vacuum with a 1/2 horsepower pump.

S.C. Wack - 11-11-2018 at 16:57

Maybe the problem is you bought a Chinese "Humboldt" ripoff instead of one of the genuine made in the US of fuckin-A models.

CharlieA - 11-11-2018 at 17:21

I note from the specifications for your aspirator that the maximum vacuum is 420 mm Hg. This is only about one-half atmosphere, which is not that great of a pressure reduction. Definitely, increasing the flow of cold water through the aspirator should increase its performance.

monolithic - 11-11-2018 at 17:30

Can you test it with a faucet? It might be the diaphragm pump that's causing the issue. Aspirators don't always like the pulsing flow of diaphragm pumps.

Experitist - 11-11-2018 at 20:30

Stupid question perhaps, but is there a simple way I can connect it to a faucet? That doesn't sound too easy given how hard it was to even get it connected to 5/8" ID vinyl tubing (lots of searching around for different size fittings, dealing with different types of threading standards etc). Does it simply not matter if there's a good seal on the inlet?

JJay - 12-11-2018 at 00:01

When I bought mine, it was called a "copper" aspirator instead of a Humbolt aspirator. It seems to be constructed almost entirely of stainless steel.

I connect the top of mine to a short section of garden hose with a hose clamp and place a garden hose adapter on the faucet or pump outlet.

I've pulled a vacuum of around 700 mm Hg with it from a faucet, but if I need a strong vacuum I usually just use a vacuum pump. I think a water aspirator is generally better for distilling strong acids, though....

andy1988 - 12-11-2018 at 02:26

Quote: Originally posted by Experitist  
Hi all,

I purchased this aspirator from Deschem on eBay

I have that same model. It works well, the vacuum evaporates/boils warm water in a jar IIRC (~20mbar). My setup:
20181112_014818.jpg - 49kB

The sink is about 18" deep, ignore the green hose. The faucet is threaded, I used a hose clincher (my local hardware store sells them).

You might not see it too well but on the vacuum port I used beeswax to seal some vinyl tubing, to move to a smaller diameter. Useful for sealing things.

Heat up the vinyl tubing in water, it expands, much easier to fit around the aspirator. When cooled it makes a good seal, but I found it would blast off due to the pressure, so I had to use a hose clamp in addition.


I also purchased a 12V diaphragm water pump that claims 70W 6L/min 0.90MPA/130 PSI to use with the aspirator.

For this pump 130 PSI is the maximum rating.

My issue is that it doesn't seem to pull anything beyond the slightest vacuum.

I suspect the issue is that the loop should have a minimum pressure? I don't believe the diaphragm water pump will raise pressure by much. See this thread for pressure/flow discussion. Look up the pressure your municipality puts your water supply at (also you may find different water pressure between say, 1st floor and 3rd floor of your building).

Though I suspect the issue has to do with minimum pressure, I will detail issues I had:

  1. Small hard rock in funnel. Didn't even see until the second time I disassembled it when it wasn't working. Shine a light through it to make sure it's clear. EDIT: Actually it was a beetle I think.
  2. When outlet tube curved more (not so straight) vacuum not nearly as good. I expect it would be better/faster if it were straighter/longer.
  3. Black O-ring on aspirator had to be re-seated, it was leaking. Being screwed too tightly pushes the o-ring out of place and made it leak.
  4. I couldn't get a shorter sink with non-threaded faucet to work.

The theoretical max vacuum you can get is the vapor pressure of the liquid; at which point it'd evaporate into a vapor. Look up any chart on the vapor pressure of water depending on temperature.

[Edited on 12-11-2018 by andy1988]

Ubya - 12-11-2018 at 02:30

i have the same aspirator, with the same diaphragm pump, at the beginning i couldn't get a vacuum at all, the reason? you need a long piece of tubing at the outlet, and the water needs to adhere to the sides(i used a tubing a bit too large so the tube would not "fill"), if you have the same problem try to put underwater the last 5cm or so of tubing

VSEPR_VOID - 12-11-2018 at 06:30

Sometimes remove the exit tubing helps

Dr.Bob - 12-11-2018 at 08:26

That aspirator looks very small and short which will not produce as much vacuum as a good one. I would buy a better quality one first, and then try to optimize it. They are not expensive, so why "save" money by buying a cheap one. His glassware may be OK, but that piece of hardware is overpriced and not very good design. Buy a better aspirator and most of the problem will be solved.

happyfooddance - 12-11-2018 at 10:43

I have two of these and they work great, pull a better vacuum than advertised, about 25mmHg. Water boils at room temp with cold water through the aspirator. For $15 you can't go wrong.

Sulaiman - 12-11-2018 at 12:07

I've never owned a vacuum aspirator so this is pure conjecture :

It seems to me that water-powered vacuum pumps operate on two totally different principles or a combination of both;

1) Venturi pumps rely on the drop in pressure due to expansion and velocity.

2) Sprengel pumps rely on trapping 'bubbles' of gas in liquid flowing down a long drop tube, pulled by gravity and momentum.

A pump relying on the Venturi effect alone should have NO drop tube that may cause 'back-pressure'
A Sprengel pump should have a drop tube longer than the pressure drop required
e.g. inches of mercury, or in this case feet/metres of water etc.

So if vacuum is not as expected then either make the drop tube as short as possible, or as long as possible,
or somewhere inbetween :P

Experitist - 12-11-2018 at 18:03

Update: I connected the thing to my garden hose and it worked. Well, it felt like a much stronger vacuum and also maxed out the gauge on my hand pump. So I suppose it is in fact that the pump I have isn't supplying a high enough pressure. Weird because NurdRage showed great success with a similar looking diaphragm pump ( that had slightly lower pressure/flow ratings than mine.

Can anybody recommend a water pump that will supply enough pressure? Ideally something fairly inexpensive.

An alternative approach

Sulaiman - 13-11-2018 at 02:12

I do not have an aspirator because I use one of these
Over the years I have used it for reduced pressure distillation, innumerable vacuum filtrations and as an air pump.
At times I have left it running continuously for days.

I can't really say that it is ideal for distillations but it is perfect for vacuum filtration.
The pump head can be removed and the internals cleaned easily if an accident should occur.
(as the pump is cheap I usually do not use a trap, so the pump has been 'tested' with many corrosive vapours and mists)

I'm not saying buy this model, there are many options available for small pumps, but :
. a small vacuum pump is much more convenient than a recirculating water aspirator,
. I do not believe that a water-recirculating aspirator can be left un-attended for hours without heating the water hence reducing vacuum
. any vapours will be continuously added to the recirculating water increasing concentration AND off-gassing whatever is being pumped.

As I mentioned, I've never owned an aspirator so maybe it has some advantage that I'm unaware of
but owning this little pump has meant that I've never felt the need for an aspirator.
I also have a two-stage rotary vacuum pump but rarely use it.
(for various reasons I also have a bunch of peristaltic pumps, water pumps etc.)

So I suggest that you buy a small vacuum pump for vacuum filtration etc.
. keep your aspirator for when really nasty vapours need to be evacuated and 'hosepipe' water is available
. your water pump can be used for recirculating condenser cooling water using ice-water or other water-cooling methods.

P.S. the above mentioned pump has been in use for the last few days intermittently pumping air through an acetic acid fermentation.
(running at 3 Vdc gives a slow bubbling at depth)

P.P.S. 5000 to 6000 feet altitude makes quite a difference,
a 128 to 152 mm Hg difference relative to 760 mm Hg at sea level.
I live 119m (c390 ft). above sea level so for rough work I ignore the effects of atmospheric pressure, (760mm - 11mm Hg)
what temperature does water actually boil at where you are ?

Using this tool
I estimate 94.9 to 93.2 oC

For me, water boils at 99oC +/- local weather.

[Edited on 13-11-2018 by Sulaiman]

markx - 13-11-2018 at 04:55

Sulaiman.....that plastic piston pump is quite a nifty gadget!
Cheap, compact and relatively reliable. A good find! It definitely beats an aspirator system for general use in filtering and simple distilling.
Aspirators require a serious amount of flow to pull a good pressure and without a recirculating system the prolonged use becomes very unprofitable in terms of water consumption. Even with a recirculating system one still needs a good volume pump and cooling of the system becomes an issue....not to mention that the lot gets really bulky and is not easily transferred between work stations.

CharlieA - 13-11-2018 at 17:14

Ok Sulaimon, you sold me! I just ordered one from ebay-US for ~20USD. I figure at this price, it is a (almost) disposable item. Even at a short useful like if it works reasonably well for vacuum filtrations it will beat setting up an aspirator/water reservoir system.

JJay - 13-11-2018 at 19:01

This pump will work nicely with that aspirator:

I have a quick-connect on my faucet, so setting up a water aspirator for filtrations takes less than two seconds. The water pressure here isn't high enough to pull a really strong vacuum, though, so I'll be using a recirculating system as soon as I feel like assembling it (I already have all of the parts purchased and laid out).

I'd expect automotive vacuum pumps to be rather robust and easy to find at any auto parts store. I doubt they pull a strong enough vacuum for distillations, though.

Sulaiman - 14-11-2018 at 03:59

Quote: Originally posted by CharlieA  
Ok Sulaimon, you sold me! I just ordered one from ebay-US for ~20USD. I figure at this price, it is a (almost) disposable item. Even at a short useful like if it works reasonably well for vacuum filtrations it will beat setting up an aspirator/water reservoir system.

I actually dislike recommending subjective stuff in case I give poor advice,
but I'm sure that you will not regret buying this particular pump even if only for filtrations.

If I plan to use the pump for a long period I add a drop of oil to the piston rod bearing on the crankshaft pinion.
I think it helps - but I do not know.

Most of the noise of this pump comes from the flapping shut of the silicone valves,
so adding a long length of exhaust tubing pipes away from you any fumes AND the bulk of the noise.

I just checked my pump ... -75 kPa, from -80 kPa four years ago.
(it can be as bad as -70 kPa when in need of a clean)

Experitist - 29-3-2019 at 22:17

An update from me since I've finally resolved my problems after taking a break for awhile.

tl;dr: deschem's metal aspirators are crap, or at least some of them are. Buy a Nalgene 6140-0010 aspirator on Amazon for $20 instead.

I started off buying progressively more powerful pumps, including the eBay 1/2 HP one mentioned above. I had lots of problems with these pumps and they were quite bulky and relatively expensive. The 1/2 HP one seemed like it may have been powerful enough to get deschem's aspirator to pull a good vacuum, but the first pump arrived DOA and the second arrived with a major leak that made it unusable. This is when I took a break.

Recently I went back on eBay and grabbed another micro-diaphragm water pump. This time one rated for 12V 100W 8L/min 160Psi. I hooked it up to deschems aspirator and it was actually able to pull a bit of a vacuum, but not a very strong one. I got water to boil at 70C.

At this point I figured deschem's aspirator may just be crap, and I went on Amazon and grabbed a Nalgene 6140-0010 aspirator for $20. I just got it, hooked it up to my new micro-diaphragm pump, and what do you know... it pulled a vacuum strong enough to boil water at ~20C. Success!

So it was the aspirator that was the culprit all along. Hopefully this information saves people some frustration.

[Edited on 30-3-2019 by Experitist]

S.C. Wack - 30-3-2019 at 04:36

Since no one takes my hints (e.g. buying an actual "Humboldt", which is a bit bigger than a Nalgene), BTW one can have the water pump and the aspirator together; with that fresh toxic chemical smell that burned out used pumps on ebay just don't have. There must be 30 slightly different brands at Chinese sites, probably all shit, but at least they don't rip off a trademark outright on them like the "Humboldt". These also have a stubby style different from the familiar Taiwanese Sibata/Brinkmann/Buchi/Heidolph, and it looks like the jets have some kind of weird muffler outlet. The aspirators on the old Sibatas are very similar to those made by Humboldt.

happyfooddance - 30-3-2019 at 09:59

Quote: Originally posted by happyfooddance

I have two of these and they work great, pull a better vacuum than advertised, about 25mmHg. Water boils at room temp with cold water through the aspirator. For $15 you can't go wrong.

This is the same aspirator, fyi. Sometimes they are on sale, pretty much always cheaper than amazon.