DCI video clip help

[Woz]

I am doing the BSAC OT6 'what happens if' lecture in a few weeks time and would like to plop up onto the magic lantern display thingy a nice little video about nitrogen absorbtion, release and what happens when the bubbles get bigger when in there (if you don't now what I am talking about then you are on completely the wrong forum and should go to knitting.net).

I also have to do a bit on burst lung and narcosis.

Anyone got any funky video clips knocking about (and no, I don't mean ones titled "Weapons of Ass Distruction" or "Shaving Ryans Privates") that I could squirt up on the projector so the Ocean Divers go "aaaaaah I see now".

[NotDeadYet]

There is a particularly good BBC documentary on the effects of deep diving in the commercial industry from the late 70's or early 80's. It's full of footage of convulsing USN divers on huge ppO2s, people writhing from bends, an unconcious diver that had been smacked on the head with a lump of steel and a rescue of some divers trapped for 24hrs in a lost diving bell. Guaranteed to put off anyone ever going near a SCUBA set ever again.

[Woz]

I want to educate them not scare the BeJesus out of them

[Chardy]

Like it says on the tin !

[Andy the Coastie]

Bottle of FIZZED up lemonade, balloon attached to a cylinder containing 100 bar direct ?


More hands on for the students , rather than sat back watching some boring slide / film show

[Woz]

Originally Posted by Andy the Commie 2

Bottle of FIZZED up lemonade, balloon attached to a cylinder containing 100 bar direct ?


More hands on for the students , rather than sat back watching some boring slide / film show

Yep all that planned in too plus a can of Boddingtons (the widget would be offgassing nicely thank you).

[NotDeadYet]

In a similar vein, get a balloon with just slightly inflated so it is small, get another which is well (but not fully) inflated and a short piece of plastic tube then join the two together. What happens goes against what you'd expect (well, what I expected at any rate). I would have thought the big balloon would have blown air into the little balloon but it doesn't, it sucks gas from it making it smaller and the big one bigger.

This I am told is what happens when you get a bend, the big bubble feeds off any little ones, something to do with small bubbles having more energy than big ones so the driving force goes the opposite way from what you'd think.

[MattS]

Originally Posted by NotDeadYet

I would have thought the big balloon would have blown air into the little balloon but it doesn't, it sucks gas from it making it smaller and the big one bigger.

Ahha. Surface tension. Blow into a balloon and it gets easier to inflate the more you it inflate it.

The pressure in the balloon is somewhat dependant on the force exherted by the elastic skin. As the skin stretches the elastic resistance is spread over a wider surface area. So the gas inside has less resitance to expansion causing pressure to drop. Which means you need less pressure to drive additional gas into the balloon. A smaller balloon has an equal elastic force spread over a smaller surface area so the pressure inside is higher. Connect the two and gas moves from the high pressure zone to the low pressure zone.

Substitute bubble for balloon and you have a scary DCI mechanism where the bigger a bubble grows the more likely it is to cause a bend.

It is much easier to show the effect than describe it. Be nice to know if anyone understands my explanation though ;-)

Originally Posted by Woz

I want to educate them not scare the BeJesus out of them

Oh how things have changed. I distinctly remember my BSAC novice lectures as 8 weeks of learning new and more horrible ways to die underwater.