Re: Here's the fallacy.


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Posted by Kendall Raine on August 27, 2002 at 09:44:18:

In Reply to: Fallacy in your argument. posted by Chris on August 26, 2002 at 20:11:41:

As I read it, you don't dispute the physics, just the degree to which wetsuits are buoyant because of the blown nitrogen in the material, versus inherent buoyancy in the rubber. As such, you argue that using Boyle's Law is inappropriate due to the inherent residual buoyancy in the rubber. Right? If that were true, how is it that compressed neoprene drysuits, where the nitrogen bubbles have been crushed, have virtually no inherent buoyancy?

I think your post contradicts itself. What you are saying is that your wetsuit has roughly the same buoyancy at the surface as it has at depth. Were that true, by the time you had used half your air, you would be about 5 lbs buoyant at depth and need to carry a bag full of rocks around just to hover. Is this what happens?

The only way you are able to stay neutral thoughout your dive is by adjusting the depth (degree of wetsuit compression) and gas in your tank and compensate with lung volume. As you use gas you will become less negative. As you work your way down a reef, it is possible through a combination of depth, gas consumption, forward propulsion and lung volume to appear to be neutral. There is no way for you, with a steel 120, a wetsuit, neutral at the surface, to descend directly to 130 and be neutral. That is physically impossible unless you were neutral at the surface with no air in your lungs and had a very impressive vital capacity. Even then, you would need to exhale at some point.

Much of the buoyancy of a neoprene wetsuit comes from the imbedded blown nitrogen bubbles in the material. The purpose of the bubbles is to create the insulating quality of the suit-nitrogen being a much lower conductor of heat than water, and the nitrogen thickens the suit without adding much stiffness and weight. Cyclical compression of these bubbles reduces their size over dives. The pressure experienced at depth increases the pressure gradient between the nitrogen bubbles in the suit and the surrounding material. A tiny amount of nitrogen diffuses out of the bubble. When the bubbles expands again after the dive, it has lost a little of it's original nitrogen volume-nitrogen does not diffuse back into the bubbles because there is still a positive internal gradient even at the surface. That is why the suit loses some of its buoyancy and some of its insulating quality over compressions. It is also why the suit compresses with depth. The nitrogen bubbles compress with depth thereby reducing the thickness and buoyancy of the suit at depth. This is not an urban legend, it is Archemedes Principle together with Boyle's Law. It also explains why compressed neoprene drysuits have no natural insulating quality and almost no inherent buoyancy. The gas has been compressed out of the material. If the buoyancy of neoprene were due to factors other than the bubbles, a compressed neoprene dry suit would still have substantial residual buoyancy. It has a little, but not much.

At depth, you can offset negative buoyancy with forward propulsion. You are in effect creating lift by planing through the water-the hydrodynamic version of the Bernoulli Effect. That uses energy and requires a higher consumption rate. The higher the consumption rate, the bigger the tank you need. You are trading the additional streamlining gained by removing the wing for the added consumption and CO2 you get from having to plane through the water. You're also giving up a significant safety device. At the other end, the need to hold on to a rock in order to complete a safety stop if you use most of your air works just fine as long as you have the right sized rock handy and don't drop it. I'm not saying you can't dive this way and get away with it. You and many others are evidence you can. The fact that you have survived as long as you have speaks to the fact you know your limitations and dive within the envelope, are resourceful enough to deal with minor contingencies and haven't had a debilitating problem at depth, or had to help a buddy with one. But the tradeoff you make, aside from the reduction in basic safety, is perverse from the standpoint of efficiency based on simple physics.




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