Physical principles of decompression sickness

[keith_henson]

morning all (well just)

I have been doing some reading (got to be careful here) about decompression sickness being described by pouring a pint of worthies or tetleys. I enquired with our resident HSE dive team on campus and my thesis tutor dug out an old exam paper from 4 years ago. One of the questions being "Opening and pouring a tin of beer into a glass reveals many physical properties of decompression sickness. Describe them" - 8 marks

Needless to say this got my brain (sometimes called brian) ticking over and started reading into it. I can understand that the imperfections in the glass surface create cavities for nucleation of micro bubbles - hence bubbles appear in the beer.

Other than that im a bit in the dark here - any ideas?

Cheers

Keith


P.S. the other half of the question was "Draw a typical M-Value graph for a dive, indicating how a diver would use the graph to minimise his decompression time" - 12 marks

[kevFISH]

Don't forget about the action of 'opening' releases the initial compressed gas - ie surfacing.

Feeling too Friday'ish to help any more though - sorry

[Mark Powell]

The initial pouring causes disturbances and cavitation which can cause new bubble nuclei. Analogous to vigourous exercise after diving.

[hopper]

I know very little about this but:

Simply put, I can imagine that if you tilt the glass at an angle and pour slowly (as if you were ascending from depth) and stopped to let the beer settle at various stages of the glass filling you would get less bubbles than if you just poured it into the glass in one go when it would overflow!! As I said simply put but I see the analogy.

[turbanator]

By slow pouring, you're reducing the distubance, not the pressure drop.

A slow ascent is more analagous to opening a screwtop slowly.
In the case a beer tin, you have remove all of the pressue at once, when you pop the lid.

So the slow pouring is more like a rapid ascent followed by laying down, whereas fast pouring is like a rapid ascent, then climbing a boat ladder followed by a 100m sprint.
r
P

[Janos]

Clouds in a glass of beer

This is a great book. You could well find it very useful and it explains the mechanisms of bubble (of gas or water) formation very well. It's not too mathematical. I read it for an option (in Atmospheric physics) in my fourth year.

Laters,
Janos

[Ainsleyberryman]

When explaining deco and slow ascents after your safety stop I used to shake up a bottle of coke and get the guests/students to open it. They all of course opened it slowly, seeing the bubbles and controlling the opening. This was a very good way of slowing their ascents at the end of their dives and generally helped people visualise what was actually going on very well.

Sometimes real life analogies like this are better than lectures and books. Especailly when your guiding on a livaboard and everyones just on Holiday and not thinking about their diving techique.

Impurities in the liquid also allow bubble formation - so perhaps lending credibility to modifying your diet and avoiding certain drinks prior to deco dives. Free Radicals and orange juice comes to mind (whatever they are)

[turbanator]

Originally Posted by Ainsleyberryman

When explaining deco and slow ascents after your safety stop I used to shake up a bottle of coke and get the guests/students to open it. They all of course opened it slowly, seeing the bubbles and controlling the opening. This was a very good way of slowing their ascents at the end of their dives and generally helped people visualise what was actually going on very well.

I went so far as to take a soda stream machine to a lecture once.
Shake water - not many bubbles, add CO2, release pressure - bubbles, shake carbonated water - more bubbles

[wreckweasel]

hmmm having caused BQI (beer quaffing illness), hyperbeeric therapy allieviates the symptoms, but requires a longer/slower ascent from the depths of your glass

[chrisch]

Originally Posted by wreckweasel

hmmm having caused BQI (beer quaffing illness), hyperbeeric therapic allieviates the symptoms, but requires a longer/slower ascent from the depths of your glass

That's true but if you stop at the 6 pint mark and switch to a beer-free mix (whisky is good but the BIR chaps use a more complex volatile mixe) you can ascend to the required state that much faster.

Chris
BTW I believe that the so-called "techies" use another thing called "travel mix" for long sessions, but I am not at that level - Zak, you will need to talk to Frank about that I guess.