[arachne] OT... Re: Pressure, etc. ... Was: Best tip of the year

Arachne at FreeLists---The Arachne Fan Club!

Hi Steve,
                   If you take a steel tank with a valve 
on it and hold it at the surface of a lake and open
the valve and let equilibrium occur then the air 
inside the tank is at a pressure of 1atm..
Now close the valve so air can't go in or out of the
tank and assume the volume of the steel tank
can't change, then take the tank to a depth of
32 feet in the lake. the air pressure inside the tank
is still at 1 atm. and the pressure trying to push
water into the tank is 1 atm.
     Now cut the end off the tank and hold it still with
the open end of the tank just slightly submersed
below the surface of the lake. Open the valve which
is above the water surface level and let equilibrium
occur. Now close the valve and take the the tank to
the depth of 32 feet in the lake without letting any
air escape. The pressure of the air inside the tank
will now be at 2 atm. and the volume of the air will
now be 1/2 the volume it was on the water surface,
if the temperature is the same. The pressure exerted
on the steel tank is zero.
     WE are of course assuming that water is not
compressible.
     If you inflate a tire to 20.3 psi the the air pressure
pushing on the tire is only 20.3 psi trying to blow it up.
The air pressure of the air inside the tire is however
 35 psi. absolute pressure.   The pressure printed on 
the side of the tire is the difference between the 
pressure of the air inside the tire minus the pressure 
of then air outside the tire.
If you take the tire to a depth in the lake of 32 feet, then
you can inflate the tire to an absolute pressure of 64.4 psi
absolute pressure since the absolute pressure outside 
of the tire is 29.4 psi.
  ie.   64.4 - 29.4 = 35.0  recommended operating pressure
In other words, if you are going to use a tire at a depth
of 32 feet below the water surface, if you inflate the tire
to a pressure of 35 plus 14.7 = 49.7 psi on your tire gauge,
when you take the tire down to the 32 feet depth in water
then the tire will be at the recommended operating pressure
of 35.0 psi.
Confusing...... isn't it !!  <ggg>

Eric


On Sun, 6 Nov 2005 10:35:27 -0700 (MST) Steve <steve@xxxxxxxxxxxxxxx>
writes:
> Arachne at FreeLists---The Arachne Fan Club!
> 
> 
> > SCUBA is very real world, and not a bit laboratory -- honest!
> 
>    Absolutely.  However it's still a non-sequiter.
> 
> >>> water proof even at only 30' depth (2 atmosphere's pressure at 
> that 
> level)"
> > However, to answer your question:  If a watch is rated at one 
> atmosphere of 
> > pressure, that means you can go under water if you'd like but the 
> watch can't 
> > survive.
> 
>    Ok.  Let's start from the beginning:
> 
>    When internal pressure and external pressure are equal, the 
> amount of 
> pressure experienced by the tank/watch is ZERO.
> 
>    To illustrate this, think of an air compressor tank open to the 
> atmosphere.  The gauge pressure is zero, but we know that there 
> there are 
> 14.7 PSI inside, and 14.7 PSI outside (if we're at sea level on a 
> "normal" 
> day).  It really doesn't matter if the internal pressure is 50 PSI 
> and 
> external is also 50 PSI.  As long as internal and external pressures 
> are 
> equal, the pressure vessel "experiences" zero pressure.
> 
>    Seal the tank.  It's now like a watch.  Internal and external 
> pressures 
> are still at equilibrium, and the pressure exerted on the tank/watch 
> is 
> still zero.  Take the watch 32' under water.  The pressure 
> differential 
> seen by the watch is 14.7PSI or 1 atmosphere.
> 
>    What you've done above is analogous to inflating your tires to 
> 20.3 PSI 
> on your tire gauge because adding in the 1 atm (14.7 PSI) you 
> actually 
> have 35 PSI like it says on the side of the tire.



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