When I was newer to diving, I got the advice "breathe shallow when shallow, breathe deep when deep", and I noticed on my own dives that I change depth more easily with breathing in shallow waters vs deep. Why is that? The best answer I could find was something regarding pressure gradients, but I still don't know why pressure gradients in deep and shallow water causes the difference.
Why is buoyancy more sensitive in shallow waters?
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Your physics is right, but if taking in that breath moves you up slightly in the water that breath, your wetsuit and any air in your BCD expands much more in the shallows than it does when you are deeper. That expansion tends to move you up more, and the process repeats to bad effect.Popcorn is too esoteric for me. Kindly hit me with a ball bat if my physics is wrong
I'd agree the tidal volume alone is small/short enough not to be an issue on its own.
My position is based on there being more air than the 0.5L tidal volume in play. The functional reserve volume is 2-2.5L (probably larger for the anxious new diver, but let's ignore that). The inhaled volume is what it is. (I believe it's also significantly more than normal resting tidal volume, but let's ignore that, too.) The wing will probably have a liter to compensate for gas weight (cuz Rock Bottom) or more for those who are overweighted. (Ignore the drysuit and wetsuit expansion cases for simplicity.) That's at least 4 liters that will expand IAW Boyle's law, contributing to an additional upward acceleration. Importantly, this is over and above that due to the inhalation, as presumably that's handled by the imminent exhalation.
Rising by a couple feet during a 6-second inhale/exhale cycle is quite easy. The cause (inhalation/propulsion/combo) isn't terribly important. Expansion of that 4L adds +0.2 lb (60 ft) or 0.4 lb (10 ft) of lift. Longer or deeper breathing cycles will obviously deviate further. I agree it's not a huge issue at 60 ft, but at 10 ft it's almost the same lift as if you just inhaled again. This additional buoyant force needs to be countered by exhalation (which is a problem because you just finished your normal exhalation!), BC venting, or propulsion. Take another breath before getting back to 10 ft (starting another inhale-based ascent) or a misaligned kick and it can quickly get out of hand.
My position is based on there being more air than the 0.5L tidal volume in play. The functional reserve volume is 2-2.5L (probably larger for the anxious new diver, but let's ignore that). The inhaled volume is what it is. (I believe it's also significantly more than normal resting tidal volume, but let's ignore that, too.) The wing will probably have a liter to compensate for gas weight (cuz Rock Bottom) or more for those who are overweighted. (Ignore the drysuit and wetsuit expansion cases for simplicity.) That's at least 4 liters that will expand IAW Boyle's law, contributing to an additional upward acceleration. Importantly, this is over and above that due to the inhalation, as presumably that's handled by the imminent exhalation.
Rising by a couple feet during a 6-second inhale/exhale cycle is quite easy. The cause (inhalation/propulsion/combo) isn't terribly important. Expansion of that 4L adds +0.2 lb (60 ft) or 0.4 lb (10 ft) of lift. Longer or deeper breathing cycles will obviously deviate further. I agree it's not a huge issue at 60 ft, but at 10 ft it's almost the same lift as if you just inhaled again. This additional buoyant force needs to be countered by exhalation (which is a problem because you just finished your normal exhalation!), BC venting, or propulsion. Take another breath before getting back to 10 ft (starting another inhale-based ascent) or a misaligned kick and it can quickly get out of hand.
Specifically on this point... It is easy to continue upward in spite of empty lungs. In fact, I'd say this is probably the typical case when someone "loses it". They give a hard exhale upon realizing a problem, inducing a downward acceleration (courtesy of gravity). It all comes down to the fact that before they can stop, they have to slow down. During that slowdown time, they're still ascending (though at a decreasing rate). Unfortunately, every foot of ascent increases the lift in the BC & exposure suit, which gradually reduces the effectiveness of gravity.
It's much like putting on the brakes in a car after riding them coming down the mountain. They're hot and don't work very well, but they're sort of working. Unfortunately, applying them heats them even more, reducing effectiveness. It's an open question whether you'll stop in time. Tip: don't ride your brakes going downhill!
I'm certain I'm not the only one to have ever surfaced in spite of laying on the deflator and exhaling for all I'm worth. Again, I expect this is far more common than failing to exhale at all.
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