Re: Air Use Calculations

[protek@shreve.net]

    I find it unusual that so many people who plan to build small subs want
to design the air rebreather system first.  It seems like this is their
first priority.  The thought of diving without O2 is unthinkable to so
many.  But, depending on the volume of the air inside the hull, there is a
time interval where one can breath the inside air only with no problems.
Many subbers do this continuously with no use of 02.  The SAR (surface and
replenish) method has worked for me in my sub, but I don't go for long
duration dives where more is needed.  The cost of the materials for
scrubbing for me would not warrant their use on short dives.  In other
words, for me personally, I would not even turn it on till I had been down
long enough to need it.
    A no-scrubber time limit needs to be established though and adhered to.
 Also, one must realize that emergency situations may extend down time to
greater duration than the SAR method can handle.  In my case I have
compressed air on board.  This does raise the inside pressure if used and
that is another problem I will admit.
    Scrubber systems do not take up much space in most cases.  The main
weight is the O2 bottle itself.  If I were designing a sub again, I would
leave space for the system and allow for the estimated weight but not
concentrate on this systems fine details till I had a workable sub.  Then
if you choose to use a scrubber it can be installed.
    Once again, if you don't have a scrubber there is less safety built
into your system.  So I am not saying to not use one.  I am saying that I
don't in my sub.  And IF (and that can be a big IF), I never have to sit on
the bottom without the ability to surface, I will probably never need one
because of the duration of my dives.  Scrubber or no scrubber, if you do
use the SAR method for a particular dive you should have an efficient
method of purging the old air from the subs hull.  Opening the hatch for a
couple of minutes is not enough as the air does not have time to exchange.

Gary Boucher