A question for divers

[SteveD]

Here we see people snorkelling down to a "Bubble Room" which is maybe at 20 meters' depth. What will be the air pressure inside the bubble? Is there any risk of the bends when they come (quickly) back to the surface after having spent time inside?.

http://www.youtube.com/watch?v=Phj3lLEtWlk

Bubble Room

[ChuckFickens]

Pressure = density * gravity * depth

sea water is approx 1025KG per meter cubed
Gravity is close as makes no real difference to 10m/s2
Depth is 20meters

so at 20m P = (1025 * 10 * 20)
P = 20500 Pascals (which is just above 3 times atmospheric pressure once you've added the 1 atmosphere of pressure that is always present even on the surface)

The easiest way to figure out if there is a risk from the bends is to use a dive table. which shows that you can stay at that depth for about an hour and then come straight up without problems, anything over that you would need to make decompression stops on your way to the surface. for example if you stayed down for 200minutes you would need to stop at 20feet for 1 minute and then 10feet for 70 minutes!!

[ChuckFickens]

I used the US navy dive table for those figures BTW, there are other tables that may give slightly different times (for example the PADI table says about 40min is the maximum safe time without decompression stops)

[joggerjayne]

Well done, Chuck.

The only practical risk on the way to the surface is having a lungful of air at 20m ambient pressure, which will expand as you ascend, and could cause a simple pressure trauma if you held your breath. You'd need to breathe out constantly as you went up, to let the expanding air vent off. Breathing out underwater feels unnatural.

[joggerjayne]

I thinkneither of the tables would apply accurately, Chuck, because the tables assume a normal ascent rate at the end of the bottom time. A snorkeller can't ascend that slowly.

[ChuckFickens]

That's quite true for a snorkel dive, I don't think there are too many people that can hold their breath for 70+ minutes at 10feet under water to decompress :) (never mind the staying under for 200 min :))

I never found it a problem breathing out when surfacing, I don't really see why people have a problem with it, to me it always seemed to be the only thing you can do!

I really should get the gear out again, I used to love going on diving holidays down off the Cornish coast.

[joggerjayne]

I've dived out of Falmouth. It's lovely there when the viz is good.

[ChuckFickens]

You shouldn't be reading those childish comics while diving :-p

[joggerjayne]

LOL

"get yer t!ts oot fer the lads"

"look out ... it's the Fat Slags" (oh no, sorry ... that's Croydon, LOL)

[jake-the-peg]

Interesting

Doesn't look much like 20 meters to me - more like 10