On 21 Dec 2003 at 22:41, Peter L. Peres wrote:

> >> I have heard of, and have recommended to others, using a scuba air
> >> tank for this, with a very small window.
> >
> >Why do you say that?  most scuba tanks are very close to
> >neutrally boyant; they sink SLIGHTLY when full, and float
> >SLIGHTLY when empty.  That's actually pretty ideal for your
> >RV, but not so easy to dump off the side of a boat for
> >testing.
>
> That is with the regulator and stuff on. Without, they float, full or
> not, afaik. (aluminium tanks anyway)

The tanks themselves vary quite a bit, depending on type,
manufacturer, model, etc. For example (Aluminium) Luxfer AL80 are
considered the ideal tank for deco gas as they are 6lb negative when
full and 6lb positive when empty. That means the diver doesn't need to
carry excessive weight or bouyancy to cater for the extra tank. I can't
remember the exact numbers, but a PST steel tank is bloody heavy full
and still bloody heavy when empty.

That's all a bit academic anyway. If a tank is being used as a pressure
hull, it would generally have the neck end machined off and other things
stuffed inside. The whole bouyancy thing goes out the window. The
aluminium 80cu ft tanks are ideal for this type of thing. The walls are
abou 0.5" thick so there's enough room to get a face sealing O-ring and
plenty of bolt holes for a good seal. You can also machine flat surfaces
on the outside to get a good sealing surface for connectors.
You should be able to pick these things up for next to nothing from a
dive shop that does testing. A failed tank is fine for this application and if
the owner wants peace of mind, have him remove the valve and run a
file across the tank threads or drill a hole in the neck.

Someone mentioned using the scuba tanks for the ballast tanks. Ballast
tanks are quite different from the pressure hull and don't need to be very
strong in comparison as there is very little pressure difference between
the inside and the ambient pressure.

Steve.

==========================================
Steve Baldwin                          Electronic Product Design
TLA Microsystems Ltd             Microcontroller Specialists
PO Box 15-680, New Lynn                http://www.tla.co.nz
Auckland, New Zealand                     ph  +64 9 820-2221
email: steve@tla.co.nz                      fax +64 9 820-1929
=========================================

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