Why is buoyancy more sensitive in shallow waters?

[butwhytho]

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.

 

[lowwall]

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.

The key to controlling those last 5 meters is to minimize gas expansion in your gear. If you are correctly weighted your BCD will be nearly empty at the safety stop, so you only have to deal with wetsuit expansion during the final ascent. It says you are in the Honduras, so the suit shouldn't be that thick.

The following may not be technically correct (I am not an instructor), but it's what I do since I like a really slow final ascent, especially for the last 2 meters.

Fully empty your BCD at the end of the safety stop. If you are bouyant (or neutral but wearing neoprene), go slightly head down and use your breath and if necessary an occasional kick down to control the ascent. You might want to add a pound or two before the next dive.

If you are neutral without neoprene or a pound or two heavy, get vertical and slowly swim it up. If you are still negative at the surface, shed some weight before the next dive.

 

[Duke Dive Medicine]

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.

I totally see what you and @inquis are saying, and you could set up artificial conditions during a dive (i.e. holding peak tidal volume) to prove it if you knew enough not to hurt yourself doing it , but if a diver is breathing normally with dynamic lung volumes, I can't see depth being a huge factor in how much lung volume influences buoyancy.

Best regards,
DDM

 

[therookie]

The key to controlling those last 5 meters is to minimize gas expansion in your gear. If you are correctly weighted your BCD will be nearly empty at the safety stop, so you only have to deal with wetsuit expansion during the final ascent. It says you are in the Honduras, so the suit shouldn't be that thick.

The following may not be technically correct (I am not an instructor), but it's what I do since I like a really slow final ascent, especially for the last 2 meters.

Fully empty your BCD at the end of the safety stop. If you are bouyant (or neutral but wearing neoprene), go slightly head down and use your breath and if necessary an occasional kick down to control the ascent. You might want to add a pound or two before the next dive.

If you are neutral without neoprene or a pound or two heavy, get vertical and slowly swim it up. If you are still negative at the surface, shed some weight before the next dive.

Yes, this is exactly what I'm working on - but thank you those are some excellent suggestions to think about. I'm actually new to wearing even wearing a wetsuit at all (3mm shorty) - corresponds to "my last 20 dives or so" - and that is when I particularly noticed the challenge with the ascent rate alarm at the end (it was always there, just more noticeable with the added neoprene). Planning on sticking with the wetsuit, so weighting with one is still a work in progress, as is getting my BCD fully empty (and tons of threads on SB about both topics...).

And apologies for a deviation from the OPs original question... I need to get into starting my own threads for things like this instead of jumping in on others'

 

[tursiops]

I totally see what you and @inquis are saying, and you could set up artificial conditions during a dive (i.e. holding peak tidal volume) to prove it if you knew enough not to hurt yourself doing it , but if a diver is breathing normally with dynamic lung volumes, I can't see depth being a huge factor in how much lung volume influences buoyancy.

Best regards,
DDM

Isn't the tidal volume about half a liter? That corresponds to about a pound of weight change. That's enough to make you move up as you breath in. Depth only enters into the discussion once you've moved up and Boyle's Law versus ambient pressure takes over.

 

[The Chairman]

Popcorn is too esoteric for me. Kindly hit me with a ball bat if my physics is wrong

Your physics is fine. Things are at best a tad more sensitive when shallow. The real problem is that you just don't have any room for error. A 5-foot swing at 100FSW is no biggie. A 5-foot swing when you're in 5FSW is far more obvious. Of course, I'm predicating that on you being neutral at either depth. A shallow breath will cause less of a swing. At depth, it just doesn't matter.

Fun exercise. Flip over (supine) and instead of using the ocean floor, use the ocean surface as your reference. Now, try to keep 3 feet from the surface. Good? Decrease that by a foot and continue. Still good? Reduce another foot and keep going. Can you get within 6 inches? This is best done with a frog kick. I have done this in caves as I'm exiting. I often see fossils I never knew existed.

 

[Duke Dive Medicine]

Isn't the tidal volume about half a liter? That corresponds to about a pound of weight change. That's enough to make you move up as you breath in. Depth only enters into the discussion once you've moved up and Boyle's Law versus ambient pressure takes over.

In a static lung, yes. In a dynamic lung where the diver is breathing in and out normally, probably not. Again you could probably create a situation where what you're describing happens, but does it actually happen if the diver is just breathing normally?

Best regards,
DDM

 

[tursiops]

In a static lung, yes. In a dynamic lung where the diver is breathing in and out normally, probably not. Again you could probably create a situation where what you're describing happens, but does it actually happen if the diver is just breathing normally?

Best regards,
DDM

I suggest that is what the OP is talking about. So, Yes, to your question.

 

[tursiops]

The perceived difference in buoyancy is more likely due to wetsuit compression.

By the way, at depth there is more air in my lungs (plus maybe in my BCD) than in my wetsuit, so if you are looking for some air to undergo a volume change because of depth changes, the wetsuit has minimal effect compared to the lungs. This assumes you have compensated for your wetsuit buoyancy loss with depth by adding air to your BCD so that you are neutral at depth. If you are not neutral ad depth, then that is an entirely different issue.

 

[inquis]

if a diver is breathing normally with dynamic lung volumes, I can't see depth being a huge factor in how much lung volume influences buoyancy

Again, it's all the airspaces. You've simply learned how to adjust long ago. A 1 ft ascent from 10 ft imparts the same volume expansion -- and therefore buoyancy increase -- as a 2.2 ft ascent from 60 ft. If a diver's breathing pattern, awareness, and skill yields a ±2 ft control at 60 ft, that maps to a range of nearly 9 ft unless they change something.

That's why new OW students often blow up from 10 ft to the surface, but they do not "rocket" from 60 ft to 50 ft. During class they figure out what to change so they don't blow up in that final ascent.

For the OP, you are desiring an even better level control. Physics makes than harder during the final ascent compared to at depth. I'd suggest more frequent blips of the wing vent at a minimum. (This is similar to tapping the brake when coming to a red light rather than waiting, waiting, and then braking hard at the last moment.) You'll also want slightly longer (or slightly faster) exhales because the expanded volume must also be vented. Lastly, you should be actively looking to not outpace the smallest bubbles. (The bubble trick made the largest improvement by far in my awareness and therefore my control.) Best of luck as you progress!

 

[Duke Dive Medicine]

Again, it's all the airspaces. You've simply learned how to adjust long ago. A 1 ft ascent from 10 ft imparts the same volume expansion -- and therefore buoyancy increase -- as a 2.2 ft ascent from 60 ft. If a diver's breathing pattern, awareness, and skill yields a ±2 ft control at 60 ft, that maps to a range of nearly 9 ft unless they change something.

That's why new OW students often blow up from 10 ft to the surface, but they do not "rocket" from 60 ft to 50 ft. During class they figure out what to change so they don't blow up in that final ascent.

For the OP, you are desiring an even better level control. Physics makes than harder during the final ascent compared to at depth. I'd suggest more frequent blips of the wing vent at a minimum. (This is similar to tapping the brake when coming to a red light rather than waiting, waiting, and then braking hard at the last moment.) You'll also want slightly longer (or slightly faster) exhales because the expanded volume must also be vented. Lastly, you should be actively looking to not outpace the smallest bubbles. (The bubble trick made the largest improvement by far in my awareness and therefore my control.) Best of luck as you progress!

I think "unless they change something" is the operative phrase, the change being exhaling. A runaway 9 foot ascent that's driven by lung volume expansion is a setup for pulmonary barotrauma. I could see an inexperienced diver holding an inhalation with an open glottis (a la skip breathing) for long enough for a slight depth decrease that would result in expansion of air in the BC which could then snowball into a blowup, but we're pretty far into the weeds there. A 500 cc volume of air (normal tidal volume as @tursiops mentioned) is going to displace the same amount of water and thus result in the same buoyant force at 30 feet as it will at 120 feet, and that force is very transient as the diver isn't (or shouldn't be) holding the breath, especially after inhalation. Maybe the OP is unconsciously changing breathing patterns when shallow?

Best regards
DDM