Re: [PSUBS-MAILIST] pressure hull sizing

[Dan h <machine@epix.net>]

Sounds like a great idea but with one note.
An "off the shelf" scuba second stage won't work as a demand valve for this
application as it's designed.  A second stage will provide air, or what ever gas
your dispensing IN RELATION to the pressure that surrounds IT SELF.  To have it
control oxygen levels by air pressure, the second stage has to have a reference
pressure. It would take some modifying of a second stage to do this, and even
then, the pressure difference your talking about may be to slight to work
properly.  The principal is sound, but I'm not sure a scuba second stage is
sensitive enough, and I know it would have to have a reference pressure like the
bellows in a barometer does.

More input please.  I could use such a system myself.  Has anyone tried a
modified second stage in this application?

Dan H.

Alec Smyth wrote:

> This is correct, if you are releasing O2 at a constant flow. An alternative
> is to release it only upon demand. As you metabolize CO2, the compartment
> pressure will drop. Via a simple demand valve (think a scuba second stage)
> you can then release O2 automatically to restore the original pressure. This
> sounds complicated but is straight forward and entirely automatic.
>
> Another idea is to do a combination of constant flow AND demand flow. The
> constant flow can be set at a level compatible with consumption by a crew
> that is unconscious, and the demand valve can provide whatever you need
> beyond that. The advantage is redundancy without having a heavy constant
> flow.
>
> The above are Phil's ideas, I don't want to steal his thunder.
>
> Of course one could also put in all sorts of gas analyzers, a
> microcontroller, electrically actuated valves, etc. But it seems to me a
> simple mechanical approach has fewer failure points.
>
> Alec
>
>
> -----Original Message-----
> From: Al Secor [mailto:wreckdiver@usadatanet.net]
> Sent: Sunday, May 05, 2002 9:11 PM
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] pressure hull sizing
>
> This has been covered before in the list archives.  You input O2 at the
> rate your body metabolizes it...somewhere between .8 and 1.0 liters per
> min at rest.  You circulate the cabin air through a CO2 scrubber which
> extracts the CO2 from the air.  As a result there should be no net
> increase or decrease in cabin pressure and it remains at 1 atm.  A
> simple baramoter can be used to keep track of the cabin air creeping up
> or down.  A valve to vent the cabin environment to the outside at the
> surface will equalize and small pressure differential to avoid any
> difficulties in opening the main hatch.
>
> Al
>
> DBACKIDS@aol.com wrote:
> >
> > It seems that whether you put the excess air into a tank, or vent it
> > (which seems a better idea), you will still need a pump, because as
> > Adam said, the surrounding pressure also increases. A valve would only
> > equalize the pressure, would it not?
>
> --
> Al Secor  ARS: WA3PWX  Scuba Instructor SSI DCSI #5528 TDI #1413
> http://www.geocities.com/SubDiverI