Re: [PSUBS-MAILIST] pressure compensation

["Captain Nemo" <vulcania@hawaii.rr.com>]

Sean,

Yes, the concept makes sense.

Dale (correct me if I'm wrong) uses a scuba regulator to keep his external
motor housings balanced against ambient pressure.  Fine for a Minn Kota,
because they are good for one additional atmosphere (and probably three)
right out of the box.  With SNOOPY, the compensated motors "feel like" they
are always running right at the surface, or thereabouts, so the possibility
of leakage is not increased with depth beyond what the manufacturer
intended.

I'm trying to think how Dale's system could be pumped-up a bit over ambient
pressure, since all the overpressure just goes back out the regulator
exhaust.  (It does go back out the exhaust, right, Dale?)  Even if you had a
constant flow regulator like I've got on one of my full-faced dive masks
(one that creeps just a little by design), the excess would just go out the
exhaust.  Which is fine, because as-is, Dale's system does the job of
neutralizing depth pressure effects on the seals, and that's all he needs.

On an early compensation system, I had the capability to hand-valve air
every so often into the motor housing, at intervals determined by my
observations of the Depth and System Pressure gauges.  I'd give it a shot of
air to keep the needles in synch.  There wasn't much danger of blowing out
the seals, because the case was vented with a one-way flow valve, like an
anti-siphon valve.  When the air pressure inside the system became enough to
open the valve against depth pressure, the excess air escaped.  This worked
when I valved it during descent, and automatically on ascent.  It didn't
take much beyond ambient pressure to open this valve, so I doubt I had any
appreciably greater pressure inside the case than was outside.  But if I'd
used an adjustable valve (one that could be set to stay closed until
internal pressures were, say, 5 PSI over ambient external pressures), then
it would have provided the kind of protection we're talking about here.  Of
course, the danger would be if I forgot to add air on the way down.  (D'OH!)

One might want to think about the seal shape and function, though.  If it's
some kind of flexible "cone seal" that advantageously uses external pressure
to squeeze it tighter on the shaft, excess internal pressure just might have
the opposite effect, forcing it away from the shaft, and causing a leak.
Well, like you said: at least we'd see the bubbles and know it was leaking!
;-)

So, I guess the best of both Worlds would be to automatically supply the air
to the motor (or housing, whatever) from a SCUBA regulator, and vent the
housing with a one-way valve that takes 5 PSI to open it, maybe?  That would
do it, but for my own purposes I'm not convinced it's necessary.

In the instance of motors like Dale's and a few other guys I can think of
(including myself, when I put the NAUTILUS back into the water) a simple
SCUBA regulator keeping the housing balanced at ambient pressure works
really well, because the MK seals are good for much more than that.

Pat


----- Original Message -----
From: "Sean T. Stevenson" <sts@telus.net>
To: <personal_submersibles@psubs.org>
Sent: Thursday, June 20, 2002 1:25 PM
Subject: Re: [PSUBS-MAILIST] pressure compensation


> >>I am pressure compensating at ambient..
> >
> >dale.
>
> Would it not make sense to compensate these spaces at higher than ambient
pressure?
>  I can see this having a couple of advantages:
>
> 1)  This prevents water (bad) or sea water (very bad) from entering
whatever
> it is you're trying to protect.  If you compensate just to ambient, water
can
> pass freely through a leak in the system.
>
> 2)  By compensating to higher than ambient, you are alerted to the
presence
> of any leaks in the system because the compensating oil volume will
decrease,
> giving you time to surface before the pressure reaches ambient and water
can
> enter.
>
> -Sean
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