Neutral Buoyancy

[Krenath]

You must be talking about a body of water max depth 10 feet..

What makes you think so? While we're at it, who are you even talking to?

Please take the following for the information it contains.

It does not contain any information. Just the ranting of some troll who wants to sound like a know-it-all by insulting someone rather than providing any information to back it up.

Your concepts and ideas are all wrong.

Two can play that game. "No, you're wrong!" See what I did there?

Study diving literature and speak with your instructor before progressing any further.

Right back at ya.

 

[Crush]

I actually did not want to race down to the bottom. I would pay attention to that next time. So it would be better if I leave some air in my BCD while descending ?

Personally, I weight myself so that, with no air in the BC, if I exhale I sink, and if I inhale I rise (only applies in the first several feet before suit compression kicks in).

If I am not in a rush (and I rarely am), I add a shot of air into my BC to stop my downwards acceleration every, say, 10 feet or so for the first 30-45 feet. My ears equalize poorly, so a slow descent is good for me.

If you pinch your nose and blow to equalize your ears, think about adding some air to the BC.

 

[fnfalman]

So I feel comfortable controlling my buoyancy using my lungs now.

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.

However, I am not sure how can I identify neutral buoyancy ? (supposing that I have the appropriate amount of weights)

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.

When I descend, I deflate my BCD completely. Once I get to a desired depth, should I inflate my BCD a little bit ?

When you are properly weighted with single tank diving, you should need to deflate your wing completely or very close to completely before you'd start to sink. On the way down to your desired depth, you should already start putting air back into your BC so that you can descend slowly and in control. Descending slowly allows you to equalize your ears better and give you time to react to anything that were to happen, and to plan ahead for establishing neutral buoyancy at your desired depth. That means if you were to descend to 60-ft, you don't wait until you hit 50-ft or 55-ft then hit the inflator button like mad. That means you'd be putting a bit of air in as you slowly descend. Don't put in too much or else you won't sink to your desired depth.

I feel that it is much easier to exhale and descend than inhale and ascend. Is it normal ?

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.

 

[Superlyte27]

What makes you think so?

Two can play that game. "No, you're wrong!" See what I did there?

Right back at ya.

LOL LOL LOL Perfect.

 

[sansky]

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).

I jump in the water with BC fully deflated. When I get to bottom or whatever my max depth will be, I put little bursts of air in to maintain perfect bouyancy, then use my lungs to fine tune for photography.

As I descend I fully deflate BC, so at the surface I am slightly negative.

Perfect bouyancy takes practice. It does not happen overnight.

 

[Crush]

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.

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.

 

[D_B]

Yep, that's what I do .. all air out to descend, then as I get to about 15 feet add a bit to slow my decent, and keep adding as necessary to maintain a more or less constant rate ... end of dive, neautrial at 10 feet, 500 or so psi , wiggle to get last remaining air out of BC

 

[nns91]

@fnfalman: I did not mean to say fully control my buoyancy. I meant to minutely change the buoyancy. Sorry for the misunderstanding

 

[fnfalman]

I actually did not want to race down to the bottom. I would pay attention to that next time. So it would be better if I leave some air in my BCD while descending ?

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.

I also can ascend/descend using my breath pretty comfortably now. The thing is when I exhale, I descend almost immediately. However, when I inhale, I do not ascend as quickly. I usually have to hold my breath for a couple seconds to be able to ascend. Is it normal ?

Sounds like a) you're slightly overweighted but probably not by much, and b) you haven't established neutral buoyancy before ascending. If you were in neutral buoyancy, just by breathing a full lung (as opposed to normal breathing most of the time) should start to float you up.

As for me, let's say that I'm ascending to safety stop depth, establish neutral buoyancy and stay there for 3-5 minutes. Then for the final ascension to the surface, I'd go from horizontal trim (you're on your belly) to vertical trim (head up, butt down) so that I can see where I'm going and hopefully not going to run into boat traffic, or smack into the dive boat's swim platform or whatever I don't want to smack into. Then I'd breath in deeper than usual to start the rising process. I don't even need to kick up at this point. Then I'd breath out slowly, then breath in deeply again to rise even more and breath out slowly. Repeat until I breach the surface.

You may notice that I didn't say anything about venting my BC. That's because I've already dumped all of the air out of my BC and have held a safety stop with just my regular breathing pattern. Some people prefer to be slightly heavy at this point so that they can have some air in their BC. Some people prefer to weight themselves so that they'd dump all air out of their BC. Whatever works for you. Whatever configuration you use that allows you to hold a safety stop without struggling and allows you to do a controlled ascend.

By the time I reach the surface, I can elect to inflate my BC just a bit to fully float my head out of the water, or I can just kick slightly to keep my head out of the water.

 

[*dave*]

At the start of the dive, you will have greater buoyancy as a result of trapped air and fully expanded neoprene in your wetsuit. You'll also be carrying more ballast in the form of compressed gas in your cylinder. If you are properly weighted, there is a good chance you will have to completely purge your BCD of all gas to descend easily.

Once you descend, water forces out gas spaces in padding and pockets where your suit is loose fitting. Additionally, your suit will compress, resulting in a loss of buoyancy which will need to be compensated for or you will find yourself significantly negative. You can compensate for this to a point by altering your breathing pattern, but that's not ideal because it will force you into a pattern which does not effectively exchange O2 for CO2.

There really is no such thing as neutral buoyancy for divers, as there is always a change as you breathe. Even without the change breathing causes, you can never be perfectly neutral. So, neutral buoyancy is more a matter of balancing positive and negative. Half your breath cycle you will be positive, the other half will be negative.

If you are finding it easier to descend than to ascend using just breath control, you are likely shifted more towards the negative and should compensate by adding some gas to the BCD.

The key to buoyancy control is anticipation and action. Rather than waiting until you are at the desired depth to compensate, you should be adding or subtracting gas to the BCD as you descend or ascend to maintain a controlled rate of ascent or descent. If your descent requires you to clear your ears multiple time, that same pressure is compressing other air spaces and needs to be compensated for.

Every ascent will require you to dump gas from your BCD as the ambient pressure decreases and the volume of gas in your BCD and other spaces increases. Every descent will require you to add gas to your BCD as those same spaces are compressed.

The amount of compensation required will depend primarily on the thickness of your exposure protection, but being over weighted will also have an impact. If you are 8 pound negative at the start of the dive, you will need 8 pounds of buoyancy in your BCD to be 'neutral' at the surface. If you descend to 33fsw without adding gas to the BCD, the 8 pounds of buoyancy you had in your BCD at the surface will be reduced to 4 pounds and you will now be 4 pounds negative. This is ignoring all other dynamic sources of buoyancy and ballast, which would cause an even greater shift.

You can use your lung volume to compensate, but that should be a very short term fix as you are tweaking the gas in your BCD.