Removing air from water using vacuum

[TommyO]

Ok guys, I need help.

I work for a Contract Manufacturer that is working to solve a problem for a customer of ours. The customer has designed a snowglobe (it's actually in a jar) with a cool little Christmas tree inside it. It's a great idea, but there are a ton of nooks and crannies that trap air inside them really really well when you insert the tree into the jar of solution (96.9% Distilled water, 3% glycerin, .1% IPA).

What we're trying to do is find a way to dislodge the air from the Tree as the customer advertises them as being high quality and air free. Smacking the jar around dislodges some, but man those air bubbles can be stubborn!

The customer has suggested using a vacuum system as their previous Chinese vendor had done (yes, we stole work back from China!) but the Chinese vendor is reluctant to disclose details of their methods.

So we've tried various methods, but we have had no success. The customer has suggested placing the tree in the jar filled with the solution then placing that jar in a larger tank of solution and subjecting the whole thing to vacuum. The idea is that the vacuum will remove the air from the tree and the larger tank will keep air from re-entering while we remove the vacuum and complete the seal between the figure and the jar.

I think they're crazy, but they insist that the Chinese vendor did it this way.

Initial trials have achieved 0% success. The gauge on the pump maxes out at 30 mHg and I've left the container (a 1' tall, 8" diameter jar filled 75% with solution) under vacuum for as much as an hour (I've just been informed that the vacuum pump is now emitting smoke so I should probably hustle to check it out...).

That being said, I'm appealing to those who might have a better understanding of what we're trying to achieve as the theory behind our goal is just beyond my understanding of the water/air/vacuum relationship. Does anyone have advice or can provide insight on how best to remove the air from the tree? This is an interesting issue and obviously there are financial implications for my company and our customer. We'd like to prove that we're as capable as our Chinese cometition and win one for the good guys!

Please feel free to ask any questions that might clarify the situation. I cannot express how appreciating I am of anyone that would like to help!

 

[NascentOxygen]

You're placing a dry piece of plastic tree into a jar of water, and its dragging air along trapped in the feathery leaves? If you had agitated the tree in a large bath of water just before doing this, so the water displaced the trapped air, it would take water along with it, instead. Try a drop of detergent in the water to lower the surface tension, but not so much that you have froth forming. If you use water that has been boiled to fill the jar, then there will be almost no dissolved air to form bubbles in the jar.

 

[johnbbahm]

This is a common problem with investment casting. Does the tree start to boil as you reduce the pressure. You may need to only get to 1/2 atsm to usefully remove air bubbles.
look at the machines in this link http://gemstone.smfforfree4.com/index.php?topic=4809.0
There are also debubbling solutions that help a lot. In your case, if you want to keep your .1% IPA in your solution, you need to be careful how low of a vacuum you use.

 

[harrylin]

I suppose you mean 30 mm Hg... that's too much. And you'd better start with already degassed water in the first place, before you put the tree inside. Even a simple water Venturi pump may be better (and it won't start smoking!), although it's not a good idea for mass production. In fact, standard oil pumps don't like water vapour, there's a gas ballast valve to be used for that.

 

[TommyO]

Actually Harry, I was way wrong, the gauge reads inHg, sorry.

I've shelved the vacuum until I can learn a bit more and decided to explore solving the problem at the source. If I can get the air out to begin with or try NO's suggestion of detergent (initial tests are promising btw, thanks!) to keep the air from sticking to the tree, we'll be in good shape.

I'm not finished with the vacuum though as we still need to remove the air from the jar after the tree has been inserted. Also, it suits our production needs (mechanical, repeatable) and keeps the production within customer spec (solution ratio, fall rate of the snow), I just need to learn a bit more about it. You guys have been a big help so far, thanks again!

@johnbbahm No, it does not start boiling, it does nothing.

 

[cmb]

As suggested, the addition of a surfactant to improve wetting appears to be the solution [no pun!] you require.

If you are not going to improve wetting by altering the solution, then you need to cause the surfaces on the tree (and inside, wherever) more hydrophilic. At the moment, I would guess that the surfaces are quite rough and hydrophobic, hence an increased wetting angle compared with smoother surfaces that increases forces at the 3 phase wetting line to the point that they are, clearly, too great at the moment for simple mechanical or thermal (diffusion) processes to dislodge the gases.

(PS, if that answer helps you earn the cash from your contract, feel free to PM me with a sub!)

 

[cmb]

Incidentally, there is one other way I am totally confident will fix the problem, but I don't think there is a point mentioning it because it'd not be useful in production volumes. You have to come up with a solution fit for a production line, and this is a case of fixing material/supply issues. I reckon you'll find the tree is too rough and hydrophobic.

 

[johnbbahm]

"@johnbbahm No, it does not start boiling, it does nothing."
I am thinking something is wrong with your vacuum system. At .5" Hg water will boil itself into ICE. (fun to watch). In vacuuming the air out of slurry mixtures, you have to ease down the vacuum, so the rapid release of gas does not splash the slurry all over the place.
You have to watch it, I suspect that is one of the reasons the bell jars are clear.

 

[harrylin]

Actually Harry, I was way wrong, the gauge reads inHg, sorry.
[..]

And I may have exaggerated a bit. I often use a little water venturi pump to degas water, and while it can in principle reach lower pressures than 30 mm Hg (amazingly it's even possible to get the water to boil at room temperature), I guess that it usually doesn't go that low - but still good enough for my purpose. It depends on the application.

However 30 inHg is about 1 bar (assuming it means inches of mercury). That corresponds to you saying "maxes out": I guess that it starts out at 0 and reaches 30 inHg. If so, it measures the pressure difference between the environment and the inside. Thus it doesn't directly measure the vacuum pressure which is low (near to zero), but you don't know exactly how low!

It may be that your vacuum pumping system will work if you combine it with one of the other tricks.

 

[TommyO]

I am thinking something is wrong with your vacuum system. At .5" Hg water will boil itself into ICE. (fun to watch). In vacuuming the air out of slurry mixtures, you have to ease down the vacuum, so the rapid release of gas does not splash the slurry all over the place.
You have to watch it, I suspect that is one of the reasons the bell jars are clear.

I really don't like admitting this, but I've got some egg on my face. I found the pump vent valve partially opened and after I closed it air started to surface.

My newest issue is that it has not completely purged from the inside of the jar. If you remember, I'm placing the tree inside a small jar, partially sealed, then placing that into a larger jar (my "chamber") filled with water.
The outer tank was under vacuum for 1.5 hrs and there were bubbles still forming on the inside of the interior jar. As long as I can prove that the technology works we'll invest in a better system. (Right now I've got a piece of aluminum that I cut out of a sheet pan with some rubber gasket material glued to the bottom placed over a glass flower pot. Yup, that's my chamber!)

I had seen the ice form in other tests I had done, wicked cool, but no idea why it's happeneing.

 

[Low-Q]

Put the jar in the microwave oven and make the water boil. When the water is still at 100 degrees celcius, seal the jar and cool it down. Now there will be pretty much vacuum inside as the steam turns into water. Everything that once was air is replaced by water or vacuum. Maybe you will have a small bubble inside, but that makes it easier to shake the snow off the ground. But no more small bubbles on the tree.

Vidar

 

[RocketSci5KN]

Be extremely careful of any containers not designed for vacuum. They can violently implode and cause injury.

 

[klimatos]

What we're trying to do is find a way to dislodge the air from the Tree as the customer advertises them as being high quality and air free. Smacking the jar around dislodges some, but man those air bubbles can be stubborn!

Have you considered ultrasound, such as used in cleaning jewelry and fine mechanisms?

 

[cmb]

Have you considered ultrasound, such as used in cleaning jewelry and fine mechanisms?

That was the impractical idea I didn't bother to mention.

You guys are nuts! Add some wetting agent/detergent!

 

[johnbbahm]

One thing that might be worth trying is to dip the tree into Alcohol, The alcohol may keep the bubbles away from the tight areas of the tree, and then boil away.

 

[mrspeedybob]

Put the tree in the jar, pull vacuum on the jar with tree already in it, and then let the vacuum pull water into the jar. In otherwords, remove the air from the tree before adding the water.

 

[sophiecentaur]

Curses. I read this thread from the beginning and someone else seems to have made all the useful comments that I could think of! Some good, inventive minds at work here.
I agree that thorough cleaning of the tree, with alcohol could help.
Just a thought - but I can't think of a specific solution. When they go for the final low vacuum stage in the manufacture of radio valves, they use a 'getter' which mops up the residual few atoms in the envelope (after it's been sealed) by blasting them against the side of the tube with a small 'exploding' piece of metal - heated with RF, i believe. There could be an equivalent to this if you flushed all air out of the system, first, with CO2, or some other gas. Then, after doing the best job that you can, to remove gas bubbles, some absorbing pellet could mop up any dissolved flushing gas after the bulb has been sealed.

Btw, has anyone suggested using already-boiled, distilled water, which should be much more free of dissolved gas, in the first place?

 

[cmb]

Btw, has anyone suggested using already-boiled, distilled water, which should be much more free of dissolved gas, in the first place?

I don't think anyone in the thread is actually looking for a simple, practicable idea like that.

 

[harrylin]

[..] Btw, has anyone suggested using already-boiled, distilled water, which should be much more free of dissolved gas, in the first place?

I made approximately the same suggestion in post #4, although I'd do everything by vacuum pumping for simplicity. And I did not think of the difference between distilled water and tap water - are you sure that distilled water holds less gas than tap water?

 

[sophiecentaur]

If it's been boiled recently, it's bound to have less dissolved gas.

 

[NascentOxygen]

I read this thread from the beginning

Btw, has anyone suggested using already-boiled, distilled water, which should be much more free of dissolved gas, in the first place?

In reply #1 in this thread:

use water that has been boiled to fill the jar, then there will be almost no dissolved air to form bubbles in the jar.

 

[sophiecentaur]

It was clearly not in bright red flashing capitals or I would have noticed it. Lol

 

[turbos4audi]

Hello all,
My name is Chris and I work with Tom.
A few things to note: We ARE using bottled distilled water for these snow globes. Our gauge is a negative pressure gauge, not absolute and reads from 1-30 inHg. Our pump is a http://www.welchvacuum.com/products/welch/welch_vacuum_products/laboratory/belt-drive_high/small.jsp".
So our current scenario: We have our chamber, which is a large glass jar. Maybe, 3 gallons. We fill it with our water/glycerin/IPA solution (96.9%/3%/.1%). We then put the tree into the chamber water. We start the pump. It gets to about 30 inHg and we leave it there for 10 min. In that time, bubbles form on the side of the tree and chamber walls and expand as the pressure drops. But they never leave the sides of either unless you bang the chamber on the table (which is a little unnerving). I assume this is because of surface tension. We turn the pump off after 10min and take a mason jar full of solution and snow and insert it into the chamber solution with the tree. We then insert the tree into the Mason jar and seal it. All of this is done under water. When we are done there are almost always a few lingering bubbles that we have to “burp” through the seal. This is not an issue. Occasionally there are a lot of “micro bubbles” and these are the issue. They cling to the snow and when you try to burp them through the seal the snow gets stuck and compromises the seal. I feel like we need to get MORE air out of the camber solution before we assemble the globes.
Is there a vacuum pressure that would over power the surface tension of water?
I was reading a little about sparging with a gas like helium. Is this a practical option? Would it even work?

Thanks guys.

-Chris

 

[sophiecentaur]

I may have missed it but do you actually boil the water before all this? Boiling will significantly reduce the amount of dissolved gas.
You need to keep the boiled water in a full, sealed jar before using it, to avoid more gas getting in.

 

[turbos4audi]

No we do not boil it. We could try that today.

Do any of you guys have any experience/knowldge with ultrasonic degassing? I found THIS site showing ultrasonic degassing of a beaker of water (about 1/3 the way down the page). It looks very effective. Maybe my dentist will let me borrow his ultrasonic cleaner.

 

[NascentOxygen]

I'm not sure whether this "tree" is a snip off a living specimen, or some faux green thing of plastic, but whatever it is, can't you swish it around in a bucket of water (with a few drops of detergent) before the bottling process so that its entire surface has already been wetted? The foliage won't carry in much air if it is thoroughly wet, and it's easier to dislodge the trapped air when you can swish it around. (If it's a biological specimen, then there are probably microbubbles that will emerge from foliage pores as you lower the pressure before sealing.)

 

[turbos4audi]

The tree is a plastic casting that has been painted. We have tried the detergent method. We have also tried soaking them in Glycerin, Alchohol and shaking them and beating them it all sorts of ways including a paint shaker.

I boiled some water yesterday and built a glode out of it. The results we not so good. Almost all the snow floated to the top. It was pretty cool when I built it too, close to room temperature, maybe a little bit warmer.

I tried sticking a quantity of snow in jar full of water into the vacuum chamber and it just caused a lot of bubbles to get stuck in the top layer of snow.

 

[sophiecentaur]

I boiled some water yesterday and built a glode out of it. The results we not so good. Almost all the snow floated to the top. It was pretty cool when I built it too, close to room temperature, maybe a little bit warmer.

I don't understand how the snow floated on boiled water but not on unboiled water. What could be the difference? My suggestion to use boiled water obviously implied that all other conditions should be the same. You would still need a wetting agent.

It clearly makes sense to de-gas the water before you put it in the globe because you can be more 'violent' with it without damaging the globe and its contents.

Did you consider using a light oil instead of water? Or maybe it wouldn't be clear enough. It would definitely cost more!

I wonder whether 'washing' the fake snow first could help.

 

[turbos4audi]

All the snow get washed and filtered first. The snow is small chunks of plastic and some of it is dusty. We wash it, then filter it and then let it sit on water until it is needed (days).

 

[sophiecentaur]

But it "floated"?
It seems that it was just not wetted. If it is more dense than water, nothing but surface tension would stop it sinking.

 

[NascentOxygen]

So it sounds like the plastic is less dense than the water. Have you got a large chunk so you can demonstrate that it will sink? Lots of plastics would be unsuitable for snow because of the fact they are too light to sink.

 

[turbos4audi]

It seems that way. But if you look at the snow through the glass with a microscope you can see tiny little bubble attached to the snow. This brings me back to trying to degass in one way or another. Unfortunately I do not have a big block of it. It come from China somewhere I think.

 

[turbos4audi]

But it "floated"?
It seems that it was just not wetted. If it is more dense than water, nothing but surface tension would stop it sinking.

So the snow I used may not have been soaked as long as it should have been. I did the test again this morning with 4 different jars. And while most of the snow sank there were still a couple flakes that were floating because they had a tiny bubble attached to them. This is my current issue. THere weren't even any trees in those 4 jars. Just the solution, snow and a rubber plug to seal it.

 

[sophiecentaur]

What does the snow do when you mix it up well with water and detergent? If any of it sinks then you are OK. Else you will need to find a less dense fluid or heavier flakes. I should have thought that filings from something that you have to hand would do.
Have you never tried them until now?

 

[turbos4audi]

What does the snow do when you mix it up well with water and detergent? If any of it sinks then you are OK. Else you will need to find a less dense fluid or heavier flakes. I should have thought that filings from something that you have to hand would do.
Have you never tried them until now?

We have used these flakes many times thus far with no problems. The only difference now is that we are subjecting the water and tree to a vacuum. We never did this with the snowmen.