Subject:
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Re: Home-made One-way valve
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 20 Dec 2001 05:34:56 GMT
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Original-From:
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Steve Baker <sjbaker1@airmail.SAYNOTOSPAMnet>
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Reply-To:
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sjbaker1@AVOIDSPAMairmail.net
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Viewed:
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1474 times
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Duckie Dave wrote:
> > Somehow I'm a bit queasy about this business of "storing vacuum" in a
> > tank.
>
> How about a vacuum flask (thermos), domestic lightbulb, or even CRT (cathode
> ray tube such) as in your TV or monitor. These all rely on a stored vacuum.
Argh!
<rant>
It's been a long time since domestic lightbulbs were even partially evacuated - these
days they are filled with some kind of relatively inert gas. Back in the 1940's
and 50's they were partially evacuated - and on some old movies you see the
hero whose gun has been emptied fool his enemies into thinking he reloaded
by unscrewing a lightbulb in the room and dropping it - thereby making a loud
**BANG** as the air rushed back into the space it occupied.
If you drop a modern lightbulb, all you get is a rather sad little 'tinkle' -
certainly nothing that sounds remotely like a gunshot!
What all these devices (except the lightbulb) have is reduced atmospheric
pressure. They all store some small amount of gas - just less than in
the case of a container at normal atmospheric pressure. As such, there is
absolutely no "vacuum" inside any of them.
But still, I don't like the concept of storing nothing. Answer me this:
* When you take more air out, do you get more vacuum?
* If you double the size of a tank containing a fixed amount of
air, do you have more vacuum or the same amount?
* If you maintain that there is *some* vacuum stored in your Lego tank
when it is at half normal atmospheric pressure, how much vacuum is
there in the air we are breathing?
* At what pressure can we say there is catagorically no vacuum present?
* If there is no vacuum stored in a sealed vessel at one atmosphere here
on Earth - will it somehow gain some vacuum if we take it to someplace
on Jupiter where the ambient pressure is 10 atmospheres?
* Does the vacuum leak out of the sealed tank if we take it up to the top of
Mount Everest?
The whole concept of vacuum as a tangiable 'thing' that you can "store"
is just silly. Like Phlogistron was to the ancient alchemists or 'The Ether'
was to pre-Einsteinian physicists.
You may argue that it's only a matter of terminology - akin to the mythical
"centrifugal force" that upsets physicists so much...but fuzzy thinking does
not belong in a scientific pursuit such as robotics - and we should endeavor
to stick with the correct way of viewing the world.
Just because we talk about 'Vacuum Pumps' and 'Vacuum Cleaners' doesn't
mean that the terms have physical meaning. Scientists also use the term
'Centrifuge' even though they know that these machines work by momentum
rather than by some peculiar force that operates on objects that move
in a circle.
</rant>
Lowering the air pressure in the Lego tank allows the system to do work
by having the higher pressure outside the tank refill it. There is no
"vacuum" anywhere in the system.
> > So I checked with the local supplier of industrial gases. If you need a
> > tank of compressed air, that's no problem. But a tankful of vacuum, er
> > that's not listed in the catalogue.
I'm sure they have a price for "Empty Tanks" in that catalog. If they
delivered the empty tank with air at one atmosphere in it, that would be
just a bonus! You'd have a tank full of vacuum *AND* some free air.
:-)
> But then, they are a supplier of gases rather than a supplier of empty tanks
> ;)
> Besides, almost everyone owns a vacuum cleaner, and so can create their own
> vacuum easilly enough.
A vacuum cleaner is poorly named. If it actually did create a vacuum,
it would have to be bunged up because it relies on the outside air rushing
into it's lower pressure chamber to push the dust and dirt inside.
Creating an actual vacuum (a total lack of particles) is essentially
impossible - even deep interstellar space has hydrogen gas at densities
of millions of atoms per cubic meter.
----------------------------- Steve Baker -------------------------------
Mail : <sjbaker1@airmail.net> WorkMail: <sjbaker@link.com>
URLs : http://www.sjbaker.org
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Message has 3 Replies:
 | | Re: Home-made One-way valve
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| In lugnet.robotics, sjbaker1@airmail.net writes: [snipped rant about light-bulbs] (...) Well, since a perfect vacuum has never (AFAIK) been shown to exist anyware (even in space), I guess you're right. However a partial vacuum is easy to "store" and (...) (23 years ago, 20-Dec-01, to lugnet.robotics)
| | | Re: Home-made One-way valve
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| (...) a (...) tanks (...) own (...) So, define vacuum as an area of lower pressure than the surrounding environment. I think that satisfies most if not all of your requirements, and is what most people seem to think of when they mention vacuum. Andy (23 years ago, 20-Dec-01, to lugnet.robotics)
| | | Re: Home-made One-way valve
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| (...) If you had a 1 atmosphere vacuum in a blue LEGO tank and you took it to the bottom of the Mariana Trench you would have about 13,000 atmospheres of vacuum in it. And if you took it out into space you would have *no* atmospheres in it. All this (...) (23 years ago, 20-Dec-01, to lugnet.robotics)
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Message is in Reply To:
| | Re: Home-made One-way valve
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| c s soh <cssoh@singnet.com.sg> wrote in message news:3C213F4F.CC899E....com.sg... (...) and (...) How about a vacuum flask (thermos), domestic lightbulb, or even CRT (cathode ray tube such) as in your TV or monitor. These all rely on a stored (...) (23 years ago, 20-Dec-01, to lugnet.robotics, lugnet.technic, lugnet.build)
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