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post #49 of Old 07-26-2013
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Re: Need answer to something I've always Wondered About

I don't have any desire to make anyone look bad, I just don't want to see anyone get hurt.

Air at the surface is under pressure, which is the reason that air is moved into solution (blood) in your lungs. That pressure is approx 15 lbs per square inch. It's actually the weight of a 1 inch column of air that extends from sea level to the top of the atmosphere that is pressing down on your head. We call that pressure, 1 Atmosphere.

Since water is significantly more dense than air, it only takes approx 33 ft of water to weigh the same 15 lbs, therefore, pressure will double at 33ft beneath the surface. It becomes 2 atmospheres.

When pressure doubles, gas volume halves and when pressure halves, gas volume doubles. Boyles Law.

Therefore, air that is compressed by the surrounding pressure at 33 ft (not the scuba tank) will double as you ascend to the surface. Gas compressed at 6 ft will expand 18%, which can be plenty to cause damage if you start with a full lung and don't exhale as you ascend. Although, not so much that you will likely feel it along the way.

This is not a problem with snorkeling, as the air you inhale at the surface is first compressed as you dive down and then re-expands to it's original volume, as you ascend. However, even breathing through a hose that extends to the surface will draw compressed air into your lungs at depth. This is both why it would require more exertion to inhale and how one could get hurt if they don't properly exhale as they ascend.

There a lot more to it, but I'll stop there for the sake of this discussion.

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Jeanneau 54DS

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