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 11:32:54 GMT
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Viewed:
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2894 times
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In lugnet.robotics, sjbaker1@airmail.net writes:
[snipped rant about light-bulbs]
> 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.
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 "measure".
> 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?
Yes. As partial vacuum is expressed as pressure difference, reducing the
contents would increase the partial vacuum.
> * If you double the size of a tank containing a fixed amount of
> air, do you have more vacuum or the same amount?
As above.
> * 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?
As the pressure in our lungs is reduced when we breathe in, you could say
there's a partial vacuum in that particular situation. There are also
constantly minute pressure differences in the atmosphere, which cause nasty
stuff like wind & cyclones, so I guess you could say there's little partial
vacuums all around in the air.
> * At what pressure can we say there is catagorically no vacuum present?
A more meaningful question would be "at what pressure *difference* can we
say there is categorically no vacuum present?" to which the answer is zero.
> * 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?
Yes (or more correctly, the partial vacuum will increase).
> * Does the vacuum leak out of the sealed tank if we take it up to the top of
> Mount Everest?
The partial vacuum decreases.
> 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.
However, it can be a useful tool.
> 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.
I have no problem using "fuzzy thinking" if it helps to explain something.
> 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.
There is not a complete vacuum. There is a partial vacuum.
> > > 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.
However creating a partial vacuum is fairly easy, and as most people agree
total vacuum is impossible, I think it's reasonable to assume that most
people mean "partial vacuum" when they talk about "vacuum" (even if they
don't realise it). I used this assumption myself in answering your questions
above - if the assumption was wrong, the answers are meaningless (as are
most of the questions) 8?)
ROSCO
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Message is in Reply To:
 | | Re: Home-made One-way valve
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| (...) 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 (...) (25 years ago, 20-Dec-01, to lugnet.robotics)
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