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I'm not a physicist, but I play one at work, so to explain cavitation (go
ahead and skip down a paragraph if you understand it) if you put a liquid in a
container with a vacuum, the liquid will fill the vacuum with gas. If you have
tanks of liquid N2, there will be gaseous N2 in there above the liquid. If
that container happens to be a piston, like a syringe, you can increase the
volume of the chamber all the way out until only gas is present. Another place
you see cavitation is in the low pressure zone right next to a boat prop,
creating a trail of bubbles (it's not all exhaust). Sub props take a lot of
design time to minimize cavitation and noise.

That said, my image of what happens in a knuckle is a little different than
the syringe Rat mentions. Yes, the bubbles in the syringe won't go away, but
that's because volume stays high. Perhaps what happens in a knuckle is that
the tissue in the joint gets "fluffed up" by the sudden decrease in pressure,
and expands to allow the bubbles to go back into solution. I'm really reaching
here, and I'm sure the docs are going to kill me if they even bother trying to
correct the mess I'm making, but this would be why your joints feel like they
need cracking: that bearing in the joint is getting compressed. When there is
a bubble present, you can't crack them. You can crack them again after it goes
back into solution, but you don't feel the need to crack them again until the
bearing gets compressed from use.

Now, so what has this got to do with depth? It seems unlikely to me, from my
reknowned work on turkeys at Thanksgiving and buffalo wings every friday,
that a bubble here is not likely to block bloodflow. Could be wrong on that,
but if bubbles could get into your bloodstream from joints, cracking joints
would be really bad for surface dwellers, too, and we'd know about it. If the
fluid is well loaded with nitrogen, cracking a joint could lead to an
abnormally large bubble, which could cause problems. Cracking a joint a depth
could result in a bubble growing unacceptably large on ascent. Don't want to
crack my neck at depth.

Just got back from a 3 day trip to La Paz (low vis, cold water, great time),
and while diving with a speargun (that new Russian SeaBear pneumatic) my hand
got tired and I cracked my knuckle at 50 feet. Watched myself do it the way
you watch yourself slam the car door with the keys in the ignition. Couldn't
wait to tell you all about it. Stayed down another half hour or so and came up
with no effects. Kind of disappointed. ;-)


Topic: narcing and noticing the symptoms.

I used to agree with people who said that all divers got narced earlier than
they thought, and didn't notice it or just found a way to operate impaired. I
often dive in the 100-150' range, and every time, like clockwork, at about
110-120' the air tastes sweet. Some other divers, including my buddy, have
noticed it. All who notice it have been anesthitized with nitrous oxide and
agree: the taste is the same. When I taste that, I know I'm getting narced,
but don't really feel any other symptoms. My head still SEEMS clear. That's
why I threw up the post about games to play while narced; I don't trust
myself. In the last week, I've gotten three new data points:

1. the diver I've mentioned a couple of times who got paranoid at 120' and
went up ballistically told me he was diving under a couple of Contacs and a
slug of Vick's. Idiot. Without drugs, he's since made it to 135.

2. took a tour of a hyperbaric chamber. They mentioned that for $20 a head,
they'll take people down to 165'. They said everyone gets really really narced
and has a great time. People get much more narced there, they say, then when
on scuba. No need to fight the feeling and try to operate.

3. got amazingly narced at 70'. Astoundingly, profoundly drunk. The air
changed flavor on me and I looked at my depth gauge and could not believe how
shallow I was. Factors: I had gotten a lot of exercise the day before, much
more than usual, and only about 4 hours of sleep. Felt fine on the surface,
not at all like I should call off diving. When it came on, I knew we were
going no deeper than 80', I could see the boat, was on the bottom, and felt
safe enough to purposely not fight the feeling. In retrospect, I may not have
been clear headed enough to have made the right decision there. It was a
fantastically warm narc. The beauty of the wall was overwhelming. I could hear
Jaque Cousteu narrating my dive. My air consumption dropped to 6 psi/min. Went
back up above 70 and it faded completely within a few minutes. I know this
much: I was so narced so suddenly that I COULD NOT have missed the symptoms.
It would have taken a very conscious effort to fight it, but with tasks laid
out simply (perhaps written down), I could have fought it. Wish I had tried
to, now.
	I guess my conclusion is that most of the time, I get a little narced,
and only notice it because I've been sensitized by the taste of Nitrous. I
can see where others might not notice it. Throw the dice often enough, and
anything can happen.

Roger.

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