This thread has suffered because we keep bouncing back and forth between
talking about a fixed amount of gas and a fixed volume cylinder.  Of course,
more gas weighs more.  So, Jack, your point is obvious but that wasn't what
I've been arguing.

David was stating that sucking 104 cuft of gas out of a PST 104 would change
his buoyancy less than sucking 80 (or 77) cuft of the same gas out of a
Luxfer 80.  We've established (I hope) that the buoyancy characteristics of
a cylinder aren't changing, so all we're talking about is the weight of the
gas.  There are more molecules in 104 cuft of gas than in 80.  So tell me
how it can possibly weigh less.

--Rob

-----Original Message-----
From: Jack [mailto:omegasac@ei*.ne*]
Sent: 24.Feb.2001 12:18 PM
To: Rob Fuller; David M. Thiesfeld
Cc: techdiver
Subject: Re: DIHUL, was: Fins question


If a cu ft of gas  weighs .0001 @ 1 atm it will weigh .0100 @ 100atms.
Simple maths.
Jack
----- Original Message -----
From: "Rob Fuller" <rfuller@te*.ne*>
To: "David M. Thiesfeld" <dthiesfeld@su*.ne*>
Cc: "techdiver" <techdiver@aquanaut.com>
Sent: Friday, February 23, 2001 10:14 PM
Subject: RE: DIHUL, was: Fins question


> David,
>
> The physical size of the containers, for all intents and purposes, is
> constant.  I.e. they don't become significantly smaller as the pressure
> decreases.
>
> Therefore, all we're really talking about is the change in the weight of
the
> gas.  So, if you're telling me that a cubic foot of gas is significantly
> heavier at 3000 psi than it is at 2600, perhaps you can explain the
physics
> of this.
>
>
> --Rob
>
> -----Original Message-----
> From: David M. Thiesfeld [mailto:dthiesfeld@su*.ne*]
> Sent: 23-Feb-2001 4:42 PM
> To: Rob Fuller
> Cc: techdiver
> Subject: Re: DIHUL, was: Fins question
>
>
> Rob,
> No, and neither does the tank charts.
>
>  The tank charts are not telling you the physical weight of the air inside
> them, only what the buoyancy characteristics are. When you start
> pressurizing air in different sized fixed containers at different
pressures
> things do not remain linear.
>
> Remember the 104 contains it air in a different sized container and at a
> lower pressure than the 80 does. You have to look at Archimedes's
Principal:
> " Any object wholly or partly immersed in a fluid is buoyed up by a force
> equal to the weight of the fluid displaced by the object."  The key is how
> much water the tank physically displaces because of its physical size and
> weight, with and without the air.
>
> A cubic ft. of air may weigh .08 lbs, but that's at atmospheric pressure
> (sea level). You change the pressure (as in putting the air in a tank) and
> the weight of that cubic ft. of air is going to change. The change is
> governed by the physical size of the container and what the pressure is.
>
> DT
>
>
> ----- Original Message -----
> From: Rob Fuller <rfuller@te*.ne*>
> To: David M. Thiesfeld <dthiesfeld@su*.ne*>
> Cc: techdiver <techdiver@aquanaut.com>
> Sent: Friday, February 23, 2001 3:16 PM
> Subject: RE: DIHUL, was: Fins question
>
>
> > David,
> >
> > ????  Are you saying that 104 cu. ft. air in a PST weighs LESS than 80
(or
> > 77) cu. ft. in a Luxfer?  Some new laws of physics must be out that I
> don't
> > know about.  A cu.ft. of air weighs approx. 0.0736 lbs. so I would say
the
> > Luxfer spec looks about right but the PST one can't be.
> >
> > --Rob
> >
> > -----Original Message-----
> > From: David M. Thiesfeld [mailto:dthiesfeld@su*.ne*]
> > Sent: 22-Feb-2001 8:58 PM
> > To: Marv; Don Burke
> > Cc: techdiver
> > Subject: Re: DIHUL, was: Fins question
> >
> >
> > Guys,
> > One of the things I see missing from your discussion is how much the
tanks
> > buoyancy changes from full to empty. Look at the abyss tank chart and
you
> > will see the PST 104 has a change of 4.6 lbs vs. 5.9 lbs. for the Luxfer
> 80
> > (77), now double those numbers and look at the weight difference needed.
> >
> > http://www.abysmal.com/pages/features-cylinders.html
> >
> > Also from looking at this chart I'd say that the Luxfer 80's are going
to
> > start swinging positively buoyant closer to 2000 psi than 1100 psi. A
> > doubles manifold may push it down some but not that much.
> >
> > DT
> >
> >
> >
> > >
> > >
> > > Don Burke wrote:
> > >
> > > > From: Marv <ajmarve@ba*.ne*>
> > > >
> > > >> ok
> > > >> first, if you read Pauls post you should have been able to infer
that
> > he
> > > >> was discussing the weight of the tanks on dry land.
> > > >
> > > >
> > > > The _only_ place that is true is on dry land.
> > >
> > > um, ok?
> > >
> > >
> > > >   For a given gas capacity, a
> > > > diver wearing aluminum tanks is a bit heavier on the boat since the
> > > > displacement of the aluminum requires wearing more weight.
> > >
> > >
> > > only if you breathe the tanks below approx 1100psi, the point at which
> > > they swing positive.
> > >
> > > >
> > > >
> > > >> If you ever tried to lift a set of 104s and then right away move a
> set
> > > >> of 80's, like i do when im filling in my garage, you would see the
> > > >> difference between the two,
> > > >
> > > >
> > > > That's comparing apples and watermelons.
> > >
> > > Thats what i was saying, that the tanks were different. So much so
that
> > > if dry land weight was a consideration, as Paul indicated in his
> > > original post, then the 80s "beat" the 104's. i never mentioned steel
> > > 80's and esp not hp 80s like the pst. There is no reason to bring them
> > > into the discussion in fact because they swim horribly. you guys can
> > > talk numbers all you want, try diving the tanks and then tell me about
> it.
> > >
> > > > I made up a set of AL 80s and I'm wearing 11# of lead (with a steel
> > plate)
> > > > with a wetsuit in fresh water.  I haven't hit the ocean with these
> yet,
> > so
> > > > my calculation of 17# is approximate at best.  As soon as I go dry,
> > those
> > > > tanks will become deco/stage/rec bottles.  If I didn't already own
the
>
> > AL
> > > > 80s, I certainly wouldn't have bought them.
> > > >
> > > > Those 104s are a pretty heavy item as steel tanks go.  They weigh
> about
> > the
> > > > same as an OMS 125.
> > > >
> > > > Capt JT wears a set of 104s and more power to him.  I haven't asked
> him
> > what
> > > > he like about them.
> > > > I have a welder and a generator I can lift when I feel the need for
a
> > > > hernia. :)
> > > > I think I'll go for 112s when I get to that.
> > >
> > > the 104 is a  good choice for cave, where extra buoyancy is a minus.
one
> > > can manage to dive them in the ocean,but i dont see them as ideal in
the
> > > ocean.
> > >
> > > > The wall thickness on an aluminum tank is so much greater than that
of
> a
> > > > steel tank, the weight of the tank is more and the displacement is
> more.
> > >
> > > and in the water you have the wt of the valve, the gas, the reg to
> > > consider, along w/ all you other gear.
> > >
> > > Marv
> > >
> > > --
> > > Send mail for the `techdiver' mailing list to
`techdiver@aquanaut.com'.
> > > Send subscribe/unsubscribe requests to
`techdiver-request@aquanaut.com'.
> >
> >
> > --
> > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
> > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
> >
> >
>
> --
> Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
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>

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