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Old 03-12-2015, 08:07 AM   #1
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Physics Teacher, Professor or Professional. A vacuum based question.

Folks,

Strange question of the day, looking for a Physic's professional to try to answer a question about applying vacuum on a fluid and its ability to "draw" air bubbles out of a fluid filed system. Not necessarily degasification (?) looking for the explanation of how it works. This is a dry technique, the fluid is basically stationary for the process.

Thanks,

Gary
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Old 03-12-2015, 08:37 AM   #2
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Not a physics teacher or a professor, and since I retired not a professional either, but once upon a time I was an engineer. I'll try to explain this in layman's terms, so if I mess up the explanation, someone please correct me so I can be properly embarrassed to have gotten it wrong…

One reason bubbles can exist in a fluid is that the pressure of the liquid is the same as the pressure of the air inside the bubbles. If the bubbles are small enough that they weigh the same as the fluid around them, the bubbles have neutral buoyancy and stay put. When you draw a (partial) vacuum at the surface of the fluid, you no longer have as much air pressure on the liquid, and so the liquid pressure is reduced by a like amount. The bubbles, now having a higher pressure than the liquid around them, expand. As they expand they displace enough fluid to gain positive buoyancy and rise to the surface. If the bubbles can move all the way to the surface of the fluid, they will pop and thus be removed.

This explanation does not address issues like viscosity or surface tension of the fluid because you didn't identify what fluid we're talking about. But it's a lot easier to get bubbles out of brake fluid than it is to get it out of fluids like marine spar varnish because of the viscosity and surface tension issues.

By the way, this is similar to the mechanism that causes divers to get the bends if they rise too quickly to the surface; gases in the blood expand faster than the lungs can get rid of them.
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Old 03-12-2015, 08:59 AM   #3
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Protagonist,

It in fact is brake fluid. I have previously set up a visual of the phenomenon and seen the bubbles move up, IIRC the vacuum made them move noticeably quicker but also looking for the name of the beast. If I'm gonna explain what it is to others I need to understand more than "do this and it does this"

Any further info?

Thanks,
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Old 03-12-2015, 09:11 AM   #4
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This is a pretty complex topic, and I doubt that I could explain more than Protag. However, what I think you're looking for is "Henry's Law" which addresses the solubility of gasses in a liquid. Using a vacuum will change the "P" (pressure) variable in the formula. You'll have to know the viscosity of brake fluid to make it work. A Google search of Henry's law will give you all you need to know. I hope your chemistry education is fresh in your memory.
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Old 03-12-2015, 09:29 AM   #5
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You probably know this, but there are brake vacuum pumps and many other vacuum pumps based on this principle.



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Old 03-12-2015, 09:43 AM   #6
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Hey Pro, clearly once an Engineer, always an engineer. Good job. Jim


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Old 03-12-2015, 10:55 AM   #7
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Gang,

This "procedure" for bleeding has been presented by GM and Ford as TSB's for certain clutch hydraulic release systems and many techs that I talk to seem to use it to burp power steering systems. I'm preparing a YouTube video and I have used this Dry Vacuum Bleeding and it works, (only in certain very specific situations) but I'd like to back it up with a reference to something other than I don't know how it works but it works like a air magnet or for example Henry's Law (which I stumbled on prior to this thread) but not likely to explain at that level but sure would like a minor understanding of this Air Magnet.

Not the usual campfire tire kicking.

Thanks,

Gary
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Old 03-12-2015, 11:07 AM   #8
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Quote:
Originally Posted by GCinSC2 View Post
I'd like to back it up with a reference to something other than I don't know how it works but it works like a air magnet or for example Henry's Law (which I stumbled on prior to this thread) but not likely to explain at that level but sure would like a minor understanding of this Air Magnet.
I can't explain it any better than I already have. Someone else will have to take a stab at it. "Physics for Dummies" is a whole lot more difficult to explain than "Physics for Physicists."

I'm not calling you a dummy— just that you want an explanation that the average high-school dropout will understand, since your video has to appeal to the lowest common denominator.
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Old 03-12-2015, 11:13 AM   #9
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Quote:
Originally Posted by Protagonist View Post
just that you want an explanation that the average high-school dropout will understand, since your video has to appeal to the lowest common denominator.
Ding Ding Ding!
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Old 03-12-2015, 11:53 AM   #10
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My mountain bike hydrolic brake system is a closed system. In changing the fluid, topping off and pressurizing the fluid, gas will be released. Keep repeating these steps until no more gassing out. If you didn't do this, the brakes would quickly become spongy.
Same idea as your blood 'boiling' in space or getting the 'bends' from deep diving.
Do I even know what the hell I'm talking about?
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Old 03-25-2015, 12:21 PM   #11
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Quote:
Originally Posted by Protagonist View Post
I can't explain it any better than I already have. Someone else will have to take a stab at it. "Physics for Dummies" is a whole lot more difficult to explain than "Physics for Physicists."

I'm not calling you a dummy— just that you want an explanation that the average high-school dropout will understand, since your video has to appeal to the lowest common denominator.
Well, Protag hit it pretty darn spot on with the LCD comment. I decided if I was not ready to teach the why it works, I'd have to settle for watch this, it works.

I setup a visual demonstration of the vacuum applied to a fluid idea with clear vinyl tubing showing the behind the curtain view.

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Old 03-25-2015, 02:18 PM   #12
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Vacuum will expand the bubble that is already there and decrease the viscosity of the fluid allowing the bubble to move more freely. If there is any gas dissolved in the fluid it will also come out of solution, link up with a bigger bubble and not go back into solution.
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