Re: [PSUBS-MAILIST] Dif. Between Ambient & 1ATM

["Sean T. Stevenson" <cast55@telus.net>]

First off, Joe, I think you might be confusing terminology, or maybe I'm just
getting confused - a lock is not necessary in an "ambient" design, since the
interior of the sub is already at ambient pressure.  Is that what you meant, or
is it a 1ATM design which requires blowing the lockout down to depth?

Dealing with the exit of an occupant is easier in designs with larger internal
volumes, since the proportional volume change is so much less.  In my design, a
diver exiting the lockout chamber will cause the water level in the exit trunk
(between double hatches) to rise a bit.  With a single diver, this is not a
problem as the level restores itself (more or less) when he returns.  With more
divers exiting, the water level may rise so much that it overflows the trunk
into the lockout compartment bilge - not a problem, but not something I want as
as SOP.  In this case, pressure is increased in the chamber to compensate for
the lost personnel volume, pushing the water level down.  The water level iself
is not measured, but rather, the pressure of the compartment is automatically
maintained at the pressure of the sea at a level just above the bottom of the
exit trunk. (Just above, so that the system does not continuously pump gas out
the bottom of the trunk).  When the divers return, their volume pushes the level
right to the bottom of the exit trunk, so once the lower hatch is closed there
is only a little bit of water remaining that needs to be pumped out.  That takes
care of volume change.  The weight loss is compensated for automatically by
pumping seawater into the auxiliary ballast tank(s).

With a small sub, this is not so easy to do since the effect of the change is so
great.  As you mentioned, you need to compensate for weight by either being
negative (only safe if bottomed), or by adding ballast as the diver exits.  You
also need to compensate for volume, if the design of the boat doesn't lend
itself to easily accomodating the water level rise.  One simple way of
accomplishing both tasks is with a varable volume "soft" tank within the
interior, to which you can admit seawater when a diver exits - simultaneously
compensating for volume and weight.  This tank would then be pumped out when the
diver returns.  This might be getting complicated for a small psub, though.

Interesting photos of that sportsub - maybe I'm looking at the wrong pictures,
but I don't exactly understand the under tail entry.

-Sean


Quoting Joseph Perkel <joeperkel@hotmail.com>:

> I have been thinking about this problem for my (ambient)design and came up
> with the "wet trunk" idea as well but, ruled it out because of the buoyancy
> problems you mention and access. 
> I found that to egress, you would have to be bottomed, well "negatively"
> flooded and stable to accommodate the loss of the occupant'(s).
> Where to put access was problematic for me as well so I ruled it out. But I
> have to tell you, Sportsub has this problem licked nicely with their
> particular ambient design. http://www.ivccorp.com/
> 
> Joe





************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003.  Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.

If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
	removeme-personal_submersibles@psubs.org

Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of
our server receiving your request.

PSUBS.ORG
PO Box 311
Weare, NH  03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************