Neutral Buoyancy

[Krenath]

Then you're doing it wrong. And if you were to get a massive headache while diving while everybody else is fine, then you'd know that you had not been breathing properly and had CO2 build-up. One uses breathing to minutely change buoyancy, not to control it. That's why you have a Buoyancy Control Device.

When you can float effortlessly without moving much other than your fins to keep yourself stabilized, when you hve the feeling of weightlessness, when your breathing doesn't appreciably change your depth.

If you're at neutral buoyancy, you can inhale deeply to start the ascension process because the air in your lungs will cause your body to be positively buoyant. Once you start to ascend, breath normally again and start deflating your BC until you get to new desired, shallower depth.

Yeah, I think this is what the OP meant by controlling buoyancy by breathing.

I know that's what I have to do, as no matter how close I get to neutral with my BCD, I'm so skinny that breathing in or out will significantly alter my buoyancy.

I consider myself close enough when my depth doesn't change a lot when I'm breathing normally (and I may counter the small changes with swimming), but either a deep breath in or deep breath out will cause me to change depth rather rapidly.

 

[Krenath]

It's all in the weighting process. I weight myself so that I have to empty my BC AND fully exhale AND kicking down past 10-ft.

If I were to do this, I would be entirely unable to perform a safety stop at the end of a dive.

 

[fnfalman]

If I were to do this, I would be entirely unable to perform a safety stop at the end of a dive.

That's why you have to figure out what works for you. What works for me doesn't work for others and what work for others don't work for me.

 

[likestodive]

My goal is to be neutral at a 10-15 foot safety stop at the end of the dive. With a skin (ie. the suit doesn't affect buoyancy much), I am slightly negative at the beginning of the dive and 'squeek' a little air into my BC. With the air out of my BC, I still have to swim slightly to get down - otherwise it would just take too long. Anyone that 'rockets' to the bottom at the beginning of the dive without air in their BC is NOT (repeat NOT) neutrally buoyant. You should barely have to put any air in your BC at any point in the dive if you are truly weighted correctly - unless you are wearing lots of neoprene.

Just my observations, but my guess is roughly 90% of the divers I see in the Carribean are overweighted. I am a 230lb 5'10" guy (ie. not "Mr Trim") and I need 5-6 lbs max without a suit. I dove for years with 9-10lbs before someone did me the favor of telling me to take off 4 lbs and they would carry extra with them in case I needed it. I resisted and said "I've been diving for years and know I need 9-10lbs". I was wrong. That was 10 years ago and haven't looked back.

If you can, get the lead out. Diving is way more fun without having to push around a bunch of air and always have to adjust your BC.

 

[likestodive]

Breathing shouldn't change your depth? If you are correctly weighted, breathing absolutely WILL change your depth. That's how buoyancy control works.

I once saw a guy who was at least 10lbs overweighted say that the idea that breathing could be used to control buoyancy/depth was ridiculous (same guy that was always 'fins down' because he had so much air in his BC). Maybe to him. To someone that actually was weighted properly and understood buoyancy control - not so.

 

[Thalassamania]

Short answer - if exhaling at the surface causes you to sink, and inhaling causes you to float, you are doing very well with your buoyancy.

Adding air to your BC as you descend is a good idea as it will keep you from accelerating downwards as your suit (and BC air) compress.

If exhaling to descend is easier than inhaling to ascend, you might be slightly overweighted. Frankly, it sounds like you are doing well. You can always swim up to ascend.

However, beware:

• Some will tell you that you should be neutral at the surface at the start of your dive, and suit compression at your safety stop will offset the 5# loss of air by the end of your dive. This assumes that you are wearing a suit, and makes no distinction between, say, 3 mil and 7 mil.. Trust me - there is a big difference between diving 3 and 7.
• Some will telly you that you should be neutral at the surface at the end of your dive.
• Some will tell you that you should be neutral at your safety stop at the end of your dive.
• I have not yet heard anyone say that you should be neutral at your safety stop at the start of your dive, but I am sure someone will state this.

Several weeks ago TSandM posted (correct me if I am wrong) that she preferred to err on the side of being slightly overweighted as she once had to swim downwards for the duration of her safety stop in order to not pop to the surface. Her advice seems sound to me.

Finally, some food for thought - in the bad old days divers did not use BCs - they weighted themselves for their operating depth and kicked downwards to get there. Once at the chosen depth, they were neutral (owing to suit compression). At the end of the dive they were even more buoyant that at the beginning (air consumption), and their ascent was straight to the surface (no safety stop).

Keep in mind that most divers with a single 80 are carrying four to five lbs of gas at the start of their dive. The ideal configuration is to be able to hit your stop (real or safety) at the end of your dive, with whatever reserve you desire (usually 500 lbs, but maybe 300 lbs in a LP steel), an empty BC, and stay right there. This translates into being negative at the start of you dive by the weight of your gas plus whatever compression your wet suit experiences down to 15 (or so) feet. If you are diving dry you want to negative only by the weight of your gas.

In the past you used to weight yourself for the depth of the dive. Most competent divers had a graph that they'd put together over time and use that to determine how much weight they'd wear, based on the planned depth of thier dive. You can see an example of such a graph in the University of Michigan Reseach Diver's Manual (Figure 3-5).

Ascent were easy, you, exhaled, kicked up and then flared. It was rather easy to hold a rate at or below 60 fpm (5 feet on your gauge, 5 secs on your watch, then 5 seconds on your watch, 5 feet on your gauge, etc.). There were no "safety stops." If you needed to deco you did that on a down line or a pipe under the boat (sometimes with a seat belt to hold you to the pipe.

Then you're doing it wrong. And if you were to get a massive headache while diving while everybody else is fine, then you'd know that you had not been breathing properly and had CO2 build-up. One uses breathing to minutely change buoyancy, not to control it. That's why you have a Buoyancy Control Device.

Moving the mid point of your breathing cycle to shirt your buoyancy and skip breathing are entirely different things. There is nothing wrong with moving the midpoint as a fine tuning of your buoyancy.

Personally, I like to have 1-2 pounds of weight over what is perfect, so I can get down fast when I need to (Galapagos is a good example).

When I need to get down fast I do a surface dive and actively fin down.

I don't think that the OP was suggesting that he controlled his buoyancy entirely with his lungs. I'd love to see someone try to do that in a 7mm wetsuit.

It depends on the depth and the size of your lungs. You are right, most people would have trouble with that, but I know a few individuals who, like Bob Croft, have lungs that can shift fourteen or more pints (lbs.) of air.