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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Agree, however without venting/addition, that lung volume will not be constant over time when depth changes. Buoyancy control is more difficult in the shallows because volume changes more rapidly for a given change in depth.
Isn't the point that the lung volume increase from a breath (of the same size) is not depth dependent? So you get a buoyancy change from that volume increase, and that buoyancy changes has more effect when shallow because (Boyle's Law) the associated pressure change is a larger fraction of the absolute pressure?
True, but consider a constant rate ascent for illustration. The exhale *rate* must be higher when shallower to avoid accelerating. In order to halt an ascent, the exhale rate must obviously increase even further. You mentioned the weight of water displaced in conjunction with a given lung volume, but the same applies to the BCD volume (and drysuit for some).
When shallow, the feedback loop of a diver's awareness and control of their airspace volumes must operate faster or more frequently or with larger magnitude control actions or some combination thereof.
The buoyancy is a Force, upwards, thus an acceleration (F=ma), upwards.. Twice integrated will give a displacement, upwards. It takes time for the buoyancy to result in a displacement. It is that displacement that invokes Boyles Law, and it is the consequent pressure change that has more effect near the surface (at small ambient pressures) than at depth (large ambient pressures).
Reading through these excellent explanations, the simple and the more complicated, highlights something for me. Boyle's law (which is at the heart of the OP's question) is indeed covered in PADI OW (and I imagine in the equivalent in other agencies as well). But in the case of the PADI course at least, it is done at a level that a 10-year-old can understand. It's very basic. And for me it wasn't until I got into the water (and particularly only over my last 20 dives or so) that I started to understand what it really meant in practical terms.
My buoyancy control has improved tremendously since my initial dives. Especially at depth - I rarely touch my BCD throughout the dive; not much need for it. I attribute part of that to a dive computer that is extremely sensitive to ascent rates - I have a nickname for it due to its sensitivity (and wear it on my right wrist so that it doesn't start complaining just because I lifted my left arm to adjust the amount of gas in the BCD). So I've learned (with the help of "Screamin' Leo"
) how to ascend slowly during the deeper stages of my dives. I have gotten to the point where I rarely have an "ascent alarm" during the dive - until I get to the safety stop.
With the exception of exactly one dive in my log, I always have an ascent alarm at the end of the dive, because I still need a lot of practice to properly control my ascent rate in those last 5 metres. Boyle's law is the reason - and of course the last 5 metres is the absolute worst time to be ascending quickly (again, because of Boyle's law), so it is my top priority in "skill development" at the moment.
So much of scuba training is "theory". It isn't until we get into the water that newer divers like us start to understand what it really "means".
My buoyancy control has improved tremendously since my initial dives. Especially at depth - I rarely touch my BCD throughout the dive; not much need for it. I attribute part of that to a dive computer that is extremely sensitive to ascent rates - I have a nickname for it due to its sensitivity (and wear it on my right wrist so that it doesn't start complaining just because I lifted my left arm to adjust the amount of gas in the BCD). So I've learned (with the help of "Screamin' Leo"
) how to ascend slowly during the deeper stages of my dives. I have gotten to the point where I rarely have an "ascent alarm" during the dive - until I get to the safety stop.With the exception of exactly one dive in my log, I always have an ascent alarm at the end of the dive, because I still need a lot of practice to properly control my ascent rate in those last 5 metres. Boyle's law is the reason - and of course the last 5 metres is the absolute worst time to be ascending quickly (again, because of Boyle's law), so it is my top priority in "skill development" at the moment.
So much of scuba training is "theory". It isn't until we get into the water that newer divers like us start to understand what it really "means".
As DDM notes, lung volume is a minor contributor to this phenomena (unless you are doing a CESA) and is mostly masked by the regular changes in lung volume due to the breathing cycle. Especially for experienced open circuit divers who subconsciously initiate descents with an exhalation and ascents with an inhalation. However, it's not zero even if you've exhaled due to the residual gas that's always in your lungs.
How much a wetsuit changes in buoyancy obviously depends on the material and thickness. Neoprene does get a good bit of its insulation value from tiny nitrogen bubbles trapped in the material and these bubbles are governed by Boyle's law. The matrix material is not compressible, so the question is how much gas is in there? I can't find a direct answer, but looking at some photos of neoprene crush at depth my guess is maybe 2/3 of the volume is gas at the surface. So yes, a neoprene wetsuit is going to have a minimum of a liter of trapped gas and a maximum approaching 10 liters for full cold weather 7mm Farmer Johns and hood.
It's not a coincidence that the BCD was invented shortly after the neoprene wetsuit.
You can sidestep this entirely by not using neoprene. If the water is warm enough, you can go without or you can use one of the alternative materials that don't use trapped gas bubbles for insulation such as found in Lavacore and Sharkskin suits. Or use a fabric drysuit.
But you will still have bouyancy changes due to the gas bubble in your BCD (and drysuit if you are wearing one). This will be at least 2.5 liters at the beginning of the dive because you need that much to compensate for the weight of the gas you expect to consume over the course of a dive.
The larger the bubble, the greater the change no matter what depth you are at. Which is why we try to avoid being overweighted.
How much a wetsuit changes in buoyancy obviously depends on the material and thickness. Neoprene does get a good bit of its insulation value from tiny nitrogen bubbles trapped in the material and these bubbles are governed by Boyle's law. The matrix material is not compressible, so the question is how much gas is in there? I can't find a direct answer, but looking at some photos of neoprene crush at depth my guess is maybe 2/3 of the volume is gas at the surface. So yes, a neoprene wetsuit is going to have a minimum of a liter of trapped gas and a maximum approaching 10 liters for full cold weather 7mm Farmer Johns and hood.
It's not a coincidence that the BCD was invented shortly after the neoprene wetsuit.
You can sidestep this entirely by not using neoprene. If the water is warm enough, you can go without or you can use one of the alternative materials that don't use trapped gas bubbles for insulation such as found in Lavacore and Sharkskin suits. Or use a fabric drysuit.
But you will still have bouyancy changes due to the gas bubble in your BCD (and drysuit if you are wearing one). This will be at least 2.5 liters at the beginning of the dive because you need that much to compensate for the weight of the gas you expect to consume over the course of a dive.
The larger the bubble, the greater the change no matter what depth you are at. Which is why we try to avoid being overweighted.
Exactly. Tidal volumes remain pretty much constant which (for general readership) is why SAC and gas consumption calculations work.
Buoyancy is not determined by pressure, it's determined by the volume and thus the weight of the water displaced. A liter of water displaced by the lungs will weigh the same at 30 feet as it does at 120 feet.
Best regards,
DDM
But the movement upwards of a liter of gas by one foot has more effect when shallow; the liter of gas become more than a liter of gas, so the weight of the water displaced is also greater. If you are neutral at some depth, you cannot remain neutral if you breathe in....because that breath makes you bigger so you are displacing more water, so you move up, and become even more buoyant. We know, as experienced divers, how to breath in, then out, at a rate that causes us to stop going up and start going down....we actually yo-you around our neutral buoyancy depth.
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