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No change in weight, but weight is not
displacement. You can spread out a lead sheet and make a toy boat that floats
from it or crumple it into a ball that sinks
and its weight is the same, but its displacement
was different when spread out and EXPANDED. Same with air.
Bill.
----- Original Message -----
Sent: Thursday, November 10, 2005 12:14
PM
Subject: Re: [PSUBS-MAILIST] Forces of
Nature
That sounds right to me Dan, I bet you'd have a small buoyancy
change as the cabin bled air over time. That compressed air volume in
the cabin weighs a little bit but I'm not sure if it's important compared to
you and the sub. Here are some numbers:
Take a dry ambient
compartment that's 60 cubic feet (about 3 ft diameter by 8 ft long). The
standard weight of air is 0.0766 lb/cf at sea level which gives a cabin air
weight of 4.6lbs on the surface and 18.4lbs at 100ft down. So you'd gain
about 14lb of cabin air weight while descending to 100ft.
But like you
say, the air came from onboard flasks so there's no overall change in the
weight of the boat!
Paul
On 11/10/05, Daniel
Edwards <dan_f_edwards@hotmail.com>
wrote:
True
If the tank/ cabin is open to the water and no further is added to
keep
the same displacement and keep the air at ambient pressure.
A
point ive never really considered though is that say you have a
dry
ambient design obviously air is being added to keep the inside of the
cabin
slightly above or at ambient pressure and since its dry also at
the same
displacement. however since you are increasing the pressure
therefore
density of the gas inside the cabin this would surely affect
the overall
weight slightly, much in the way a full steel scuba tank
starts as
negatively bouyant but as it is emptied throughout a
dive progressively
becomes more nuetrally bouyant as the gas inside
becomes less dense.
however since the pressure inside an ambient
design is very unlikely be very
great ( unless your a loon building a
saturation bell) do you think that any
weight/bouancy change would be
significant considering also that whilst you
are increasing the density
oa gas inside the cabin you are also decreasing
the density of gas inside
the supply tanks.
whoops i think ive just answered my own question
and in fact overall i guess
the whole system will just become slightly
more bouyant due to the loss of
exhaust gasses.
Let me know if
you think that sounds about right,
Cheers,
Dan
>From:
"Akins" <lakins1@tampabay.rr.com>
>Reply-To:
personal_submersibles@psubs.org
>To:
<personal_submersibles@psubs.org>
>Subject:
Re: [PSUBS-MAILIST] Forces of Nature
>Date: Wed, 9 Nov 2005 19:57:46
-0500
>
>Water, thru ambient water pressure, compressing the air
inside the cabin or
>tank, changes the air displacement if you do
not
>
>add any air to replace the compressed air.
>
>Bill.
>
>
>
>
>
----- Original Message -----
> From: Daniel
Edwards
> To: personal_submersibles@psubs.org
> Sent: Wednesday, November 09, 2005 6:33
AM
> Subject: Re: [PSUBS-MAILIST] Forces of
Nature
>
>
> the bouancy should not change if
the exact shape of the vessel does not
> change because it
still displaces the same amount of water which does
>not
> become denser due to compression. therefore
bounancy is about
>displacement
> not wether the air
inside is compressed if you see what
imean
>
>
> >From: "Akins" < lakins1@tampabay.rr.com>
>
>Reply-To: personal_submersibles@psubs.org
>
>To: <
personal_submersibles@psubs.org>
> >Subject: Re:
[PSUBS-MAILIST] Forces of Nature
> >Date: Wed, 9 Nov
2005 00:59:57 -0500
> >
> >Hi
Joe.
> >
> >Not sure if I am
reading your questions correctly, but I will try to
>help
> >and hopefully be corect for what I
think you are asking.
> >
>
>Remember that 1 atm is the air pressure at the surface and is the
14.7
>psi
> >we have pressing against our bodies
(GENERALLY speaking) ALL the time.
>
>
> >If you want to submerge something to say 33
feet, that would be 2 atm.
> >Remember to count
the
> >
> >single atmosphere we
already have pressing on us all the time. So 33 ft
>is
> >2 atm, 66 ft is 3 atm,
etc.
> >
> >You asked.....#1 One
submerges a bubble of air (enclosed in
any
>material)
> >and open to ambient pressure to
1atm . Equal forces of pressure are
>applied
>
>to either side of the material. I assume that the lifting force of
the
>air
> >in pounds of buoyancy is not
cancelled out by the equalization of that
> >pressure.
That the material is subjected to a stress in psi equal to
>that
> >force. Then if one were to apply an
opposing force ,as in ballast, that
>the
>
>material is then subjected to both forces. Is this a
correct
>assumption?
> >
>
>The first part of your question I assume you mean't submerging a
bubble
>of
> >air at ambient pressure at greater
than 1 atm.
> >
> >The second part
of your question states about applying equal force to
>
>either side of the material. I assume you mean the water pressure
force
> >
> >going into the main
soft ballast tanks or even part of the subs ambient
>
>hull (depending on design) and then having equal force applied by
> >
> >releasing air into the
same space and therefore the material (hull or
>
>tanks) has equal force applied onto it from both sides which
cancels
>each
> >other's force
> >
> >out, remembering of course
that there has to be a slight pressure
> >difference
inside, in that the inside air pressure force holding
out
>the
> >water has
> >
> >to be slightly greater than the water's pressure
trying to get in. That
> >sounds correct if that is
what you mean't. I wasn't quite sure what
>your
>
>third part of the question mean't
>
>
> >when you asked whether the lifting force of the
air was not cancelled
>out
> >by the equalization
of that pressure. Are you asking if by
pressurizing
>the
> >vessel, (material, bubble,
whatever) if
> >
> >the lifting
force of the air is decreased by being compressed
against
>the
> >outside water pressure? If that
is what you were asking, I am not the
>best
> >to
answer this, but I do know that the deeper
>
>
> >you go and the more you compress air, it does
have an effect on its
> >buoyancy I believe. I'll leave
that one for someone a bit more
> >knowledgeable in
math to compute buoyancy loss due to air molecule
>
>
> >compression. But I hope I helped you some, and
remember we are always
>at
> >approximately 1 atm
all the time.
> >
>
>Bill.
> >
> >
> >
> > -----
Original Message -----
> > From: Joseph
Perkel
> > To: personal_submersibles@psubs.org
>
> Sent: Tuesday, November 08, 2005 12:15 PM
> > Subject: [PSUBS-MAILIST] Forces of
Nature
> >
> >
>
>
> > My learning curve is flattened
again, too many distractions, Please
>help
> >me
with some basics here.
> >
>
>
> > #1 One submerges a bubble of air
(enclosed in any material) and open
>to
>
>ambient pressure to 1atm . Equal forces of pressure are applied
to
>either
> >side of the material. I assume
that the lifting force of the air in
>pounds
>
>of buoyancy is not cancelled out by the equalization of that
pressure.
>That
> >the material is subjected to a
stress in psi equal to that force. Then
>if
>
>one were to apply an opposing force ,as in ballast, that the
material
>is
> >then subjected to both forces. Is
this a correct assumption?
> >
>
> #2 One submerges a piece of marine ply to one atm. Is the
cellular
> >structure of the wood irreversibly
compressed creating a permanent
>change
> >in
density and therefore buoyancy?
> >
>
> Thanks
> >
>
> Joe
> >
>
>
>
>
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