> I want to make a sensor that I
> can screw in to a hig pressure port on my 1st stage regulator that
> will output an analog or digital output that is proportional to the
> pressure inside the tank. It would be even better to convert that to
> volume, but that's not the issue here.

The obvious answer is a pressure sensor designed for the job. They
are available with the right threads and everything. But that's $$$.

Cheapest solution would be to disembowel an analog SPG and
attach the pointer to a pot or glue on a card with a gray scale
printed on.

If you really want to do it the way you propose, get an old balanced
piston first stage. The housing is going to be the right thickness
and material. The pressure on the needle is going to be the cross
sectional area (in inches) * 3000lb full-scale. Go to a spring maker
and tell him you want a coil spring x inches in diameter and the
two heights you want (corresponding to 300psi & 3000psi).

> I
> have read that mechanical failure of such a vessel is analogous to a
> small hand-grenade accident (if there is such a thing!)

I don't know about small. I was just reading about a tank going
bang while it was being filled last year. It was in a water tank with
8" concrete walls. It killed the operator, blew the roof of the dive
shop and made a 2 foot hole in a concrete block wall.
Not exactly the same thing but then your first stage sits about an
inch behind your head.

Getting way off topic - A couple of weeks ago at an automotive
swap meet, I spotted an old two hose, USD, single stage regulator.
It was being sold by an ex-commercial diver who had bought it
new. He even had the authentic "old diver" limp. All the hoses,
valves, etc are in excellent shape and apart from tasting very
rubbery it seems to work OK on a tank out of the water. Can't wait
to blow some bubbles with it.

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: steveb@tla.co.nz      fax +64 9 820-1929
======================================================

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