----- Original Message -----
Sent: Wednesday, August 08, 2001 1:43
PM
Subject: Re: [PSUBS-MAILIST] question
about general design?
Ken, your basic reasoning is sound, but as often
happens in submarines, there's more to it than meets the eye at first
glance.
First of all, when it comes to building subs, it
makes sense to stay within guv'mint regulations a.m.a.p.. (Although I
didn't always do this, I know better than to buck the system now.)
Seriously, there's a lot of reasons why one should, and some of them have to
do with staying alive, and in this case, getting back to the surface (which is
the same thing).
Uncle Sam says your sub must be designed with a
certain amount of expendable commodities on board: I'm not looking at the book
right now (and if I'm off by a pound or two I'm sure someone will take
advantage of the opportunity to whack me in the head with volume, chapter, and
verse) but off the top of my head I think a sub is required to carry a minimum
of 72 hours of life support (Oxygen) for every soul on board; and enough
compressed gas (usually high pressure air) to blow the ballast tanks eight
times at design operating depth.
OK. Now, let's use my little NAUTILUS as an
example. She's got about four cubic feet of ballast tank volume; on the
surface those tanks hold four cubic feet of air. Now, let's say
120 psi is about 8 atmospheres over ambient surface pressure; that means
to equal a depth pressure of 120 psi (about 264 FSW) I'm going to need eight
times normal volume or 32 cubic feet worth of air at 120 psi to even get the
mud moving; and if I really want to BLOW, I want more pressure than
that.
Now, I haven't seen the tank you're talking
about, but let's use one of those jobs from Grand Auto that are about 2 cubic
feet in size. At 120 psi, that tank only holds 16 cubic feet of air, and
if I need 32 that won't even blow my tanks once. And as soon as you
crack the valve, the pressure will drop below 120 psi, and then it aint going
to move any water at all; in fact, when tank pressure falls below depth
pressure, you get water in your tank. (Which is why I never burn my
tanks down below 300 psi when scuba diving).
"But hey!." you say. "I'm not going that
deep!" OK, let's look at that. Let's say I'm only going to
half that depth pressure. OK, I start out with barely enough air and pressure
to blow my tanks once; but again, that falls off as tank pressure drops;
so I'm only going to get my tanks part-way empty before my pressure falls
below depth pressure and the system stops working And that's at
only 132 FSW.
Let's go a little shallower, say 66
FSW. It looks like I've got enough to blow the tanks once; but
remember, when tank pressure falls below depth pressure, the system
fails; so I'd say there's still going to be a little water in the ballast
tanks, and I've used up my air.
33 FSW? OK, without running the math
(whattaya want for free?!) it looks like it will blow once; maybe a tad
more. But that's it. Not enough to meet government
requirements, and not enough to give me a practical safety
margin.
Also, keep in mind I'll probably be using some of
that ballast air for minor trim adjustments to the tanks, and maybe other
things (like emergency breathing); so that sparse supply I started with
is looking like less and less with every passing moment.
Nope. And considering the physical size of
the tank in comparison to the amount of air it can carry, yours is a rip off
of your precious space. Why carry a tank that only holds 16 cubic feet
of air at 120 psi when, for the same expenditure of floorspace in your sub,
you can carry 80 cubic feet at 3000 psi? Like I said to David of
Osage: "It's better to have air and not need it, than
vice-versa.")
(One thing just occurred to me, partly based on
your statement that you don't know how deep 120 psi is. Is the reason
you want to use one of the cheapo tanks because you aren't a certified diver,
and can't buy a scuba tank or get it filled? If so, no offense but in my
opinion you really ought to go out and get your training and C-card, for your
own safety I'm a hard-liner about people being certified divers if
they're going to be messing around in experimental subs. A knowledge of
SCUBA will improve your sub design, enable you to operate more safely,
and save your life if the bubble busts. Get your C-card and buy a
scuba tank, brother.)
Anyway, right now I can't make it much clearer
than that; my spare ribs are getting cold. It's a semi-free country; and
you are at liberty to do as you please; but if it were me, I wouldn't use one
of those parts-store tanks when I can get the real thing. One was made
for use at depth underwater; the other wasn't. The choice is
yours, but......
Old saying: "Cheap sub not good; good
sub not cheap". Spend a few more dollars and get 25 times more air;
that's the "akamai" way to go, for sure.
OK, I'm outtahere! ;-)
Pat
----- Original Message -----
Sent: Thursday, August 09, 2001 1:48
AM
Subject: Re: [PSUBS-MAILIST] question
about general design?
Is my reasoning flawed? If you have a off the
shelf compressor bottle that is rated to 125psi that is outside your
pressure hull, it would limit your diving to depths greater than 125psi (I
don't know how many feet that is) Otherwise, you aren't gonna
blow ballast at 150psi with 125psi air. So, if the tank always has
pressure inside of it greater than the pressure outside, it ought to be
happy as a clam. They way I figure it, if the outside presure is 50psi
and the tank is holding 75 psi, that's like having a tank at 25 psi with no
pressure on the outside. Being a newbie, I may be all wet, if so
someone please straighten me out. Ken Harris
----- Original Message -----
Sent: Wednesday, August 08, 2001 4:46
PM
Subject: Re: [PSUBS-MAILIST] question
about general design?
At 19:43 +0000 08/07/01, Captain Nemo wrote:
"Second
question. Are off the shelf air tanks that you would get at Home
Depot that are rated up to 125PSI sufficient for storing
pressurized air for blowing your ballast?"
Nope. Stick with high
pressure air tanks. Lots of reasons why. We'll talk
later...
Please do; I'm interested in understanding these reasons. The main
one I can think of is that at that pressure, those things just don't hold
all that much air. I used up almost all of mine yesterday, pre-charging a
20 gal. household water pressure tank to 28 psi. That wouldn't translate
into much ballast-blowing, at all. Besides, those always make me uneasy by
the time they get up to
120.