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Hi Ken,
You raised a couple of great points in your recent letter. Trouble
is that I've gone and deleted it before I could answer them, and I
can't get my web browser to play, so I can't even look up the
archive.
From memory (shaky ground) we were discussing ind/man in OOA situations.
I contended that an OOA (OOGas?) situation was less likely in an indie
than mani, and you said that OOA's were due to human error (in most cases)
and illustrated your point with a diver who has a gauge stick, and then
trusts it. So he uses 1/3, then switches to the other cylinder and
trusting his gauge, drains the whole lot. 1/3+3/3 in, 2/3 left, BANG,
he is dead.
I thought about this all day today at work. (if some sailor gets
a breathing set that don't go, we know who to blame: Ken!)
I admit the indie diver is dead as a door nail, in a cave. But then
so is the manifold diver who follows his gauge. He breathes 3/3+3/3
and then has *none* remaining to get out with. Not much difference.
*BUT* in OW there is a difference! I don't know about others, but I
retain not thirds, or exact halves, or halves + 200, but the amount
of gas needed to reach the first stop from the wreck. Hence if my
second cylinder should "just stop" be it from failure, or from
breathing it dry, there "should" be enough gas in the first cylinder
to get me back to the anchor, and up to my first stop. I think
that this is a major advantage for indies, over manis.
The other point that I can remember concerned burst disks. I haven't
seen many go. A couple of Spare Deaths, while I was filling them. They
seemed more to just go "bang" than empty. A 150 cuft 200 bar (3000 psi)
and a 120 cuft 300 bar (4350 psi), both full. From memory, they seemed
to empty a *LOT* faster than 10 minutes. Just a few tens of seconds.
One minute at most. Of course I didn't time them.
So today I measured the hole in a burst disk plug. It was 2.5 mm in
diameter. (sorry I have no idea what this is in imperial) This gives
an area of about 5 square mm. (1.25*1.25*3.141=4.90....) Now the gas
would flow through the hole at the speed of sound, which in TMX is about
600 m/sec, or about 60 000 mm/sec. 600 000*5=3 000 000 cubic mm/sec.
Now there are 1 millon cubic mm/litre, so this is 3 litres/sec. Now
the maths gets hard, and I can't cope with any exactness. The actual
amount of gas is 3 litres, times the current cylinder pressure.
Because the cylinder pressure is going down, this varies. It's
been a long time since I did intergral math, and I'm not up to this,
so I considered each second on individually. Not accurate, but close
enough for back of the envelope calculations.
Now if we have manifolded twins, we should have big ones. The largest
I've seen were Rodney's 18x2 litre 232 bar set. 8350 litres of free
gas (about 300 cuft @ 3350 psi). I considered it as though the internal
volume had increased by 3 litres each second (or if you like, as though
we had decant filled a 3L cylinder once per second)
232 (start pressure) x 36 (cylinder volume) / 39 (volume with 3 litres
racked off elsewhere) = 214 (end of first second)
Then with the same workings:
Second Pressure
2 197
3 182
4 168
5 155 (this is where the manifold divers reckon they get
6 143 it shut down)
7 132
8 122
9 112
10 104
11 96
12 88
13 81 This is where I think they would get it shut down under
normal circumstances.
[snip]
22 40
23 37 This is where the guys who loosen the crotch strap
first, and use two hands,get the isolation valve shut.
(200 useable psi or about 3 breathes)
81 bar is about 1100 psi. He has about 1450 litres left out of
8350 (about 50 cuft left out of 300) Except of course he can't
draw out the last 20 bar (due to the depth) leaving him with
38 cuft out of 300. Which at 300 ft would last about 2 minutes.
Hey! that's about the same as a spare death lasts at recreational
depths! And that's with bloody great big *full* cylinders. Oh, and
a single hole burst disk. Lots of them have two holes these days,
(so the cylinder doesnt' fly around) and they, of course, would flow
gas twice as fast.
If I've missed anything, or if you can think of anything else, I'll
have a go at that. It seems that Carl is going to *make* me use
a manifold if anyone can find any advantage to them for OW TMX.
Cheers Jason =:)
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