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Re: [PSUBS-MAILIST] question about general design?
Title: 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.