Posted on 18 January 2012 at 12:42h
Last week my son went diving with one of the teachers from the college, I went to watch which wasn't very interesting since all I could see were bubbles coming to the surface. Occasional excitement was induced when the bubbles stopped for a few seconds but that sort of excitement I could do without. As I watched the bubbles moving around the fjord I noticed that sometimes they were complete spheres but at other times in the form of many small bubbles. have no idea why this is and to find out I'd have to get under water with them and I'm not doing that. I think I've got reverse fear of heights. I get scared when I see the surface of the water getting higher and higher. After the dive I asked them about all the bubbles and was interested to learn a bit about the role of the BCD (buoyancy control device). This looks like a life jacket and has a similar function. Because of the weight of the air tank the diver would sink so to compensate for this they put air into the BCD. To go down they release air. As they dive deeper the air in the BCD becomes compressed so has less volume therefore the up thrust is less so to keep the forces balanced they have to add air. On the way up the air expands etc. so they have to release air to rise at a constant rate. The same thing happens to the air in your lungs. If you breath in air at depth it will be at high pressure and will expand as you rise to the surface. If you hold your breath whilst doing this your lungs would explode. Charming.
A bit more about bouyancy from Jelena (the diving biology teacher).
The diver has to be warm enough - must have a suit - best is made of light & insulating material, the one filled with air bubbles = neoprene. The suit because of those air bubbles has + buoyancy so you are absolutely not able to sink in only a neoprene suit. Thus you must have weights (lead) to neutralize the suit's buoyancy. Bottles have negative buoyancy only when full (200 bars of air - weights a couple of kg, depending of bottle volume). If a bottle is empty it may have almost neutral buoyancy (depending on its volume and material) - that means that during a dive your total buoyancy at same depth grows as you are spending air. That is not a problem at depth because your suit (the air bubbles in it) and your lungs are compressed. But when you go up they expand and creat positive buoyancy and it may become increasingly difficult not to be propelled to surface (after a deeper and/or longer dive that may not be a great idea because the dissolved nitrogen in blood may start champagne-ing little bubbles which the diver's brain will not like, for sure)
We didn't have a BCDs in my first decade of diving, so proper weighting was essential - and eventually you learned to use your lungs as a buoyancy device (breathing normally at surface before dive, at top full lungs at depth or at half empty lungs in shallows at the end of the dive. BCD makes it much more comfortable: you put slightly more weight, so you don't have to hassle about empty bottle buoyancy, and you can just keep it neutral by deflating and inflating your BCD as you go down and up. Much less tiresome and/or air-costing (once when you spent enough hours under water to be near adrenaline-free).
But with a dry suit it is yet another story. With it we keep BCD only for safety and comfort reasons - especially on surface. Because you deflate and inflate your suit instead, while under surface. At the descent you deflate the BCD completely and you will inflate it again only when back on surface to keep yourself afloat with no effort. If you tried to do that with the dry suit it would make your swimming veeeery clumsy and difficult.
Now, physically interesting is why I prefer neoprene dry suits to the "trilaminate ones": Because of the squeeze effect. In latter, when your position in water is vertical your legs will be squeezed - the air in your legs is noticeably more compressed then at your shoulders - 1.5m depth difference. The amount of air you have to inflate into that kind of suit (if you need vertical position - for photographing for e.g.) in order to neutralize the leg squeeze effect would make you positively buoyant, because most of air goes up where there is more space for it - those suits in order to enable movement are usually less snug since not elastic. The squeeze effect is less noticeable in neoprene dry suit - due to its elasticity and compressibility and also because it is usually more snugly cut and thus has less ineffective volume.
and on and on I could go.......
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