Buoyancy Question

[nereas]

You should be neutrally buoyant with a nearly empty tank. At that point you won't want to descend...

Exactly right, Walter. I am glad you also pointed that out, as did I.

...With a full tank, you'll be about 6 lbs negative. ...

Also right, and he apparently did not know that compressed air has weight. That is a science thing, that molecules have mass and therefore near planets and moons they therefore have weight, since the gravity of planets and moons attracts molecules and gives them weight. It's too bad that the scuba books don't explain this better.

Almost everybody seems to think that air is thin and has no mass nor weight. Philosophy explains how human perception often leads to false conclusions like this.

... You should be finetuning your buoyancy with your lungs ...

This is over his head, Walter. Besides, his question was about the surface, about surface buoyancy. And you are talking about underwater buoyancy. So you are unfortunately just going to confuse him more, with this.

...Unless I'm wearing a wet suit, I don't put any air in my BC during the dive.

With this you are going to confuse the heck out of him. This has nothing to do with his question, nor with good diving. What you are saying is that you use your drysuit like a B/C, which is old school, and not necessary good procedure anymore.

Plus, if you were diving with a 1/2 mm dive skin, this statement would not even be true, unless, of couse your never go to Florida, or the tropics, or the Red Sea, which are the most beautiful diving meccas in the world!

 

[SkipperJohn]

Actually, my degree is in physics and mathematics. Maybe that's why the instructions confuse me. Yup, air = 14 pounds per square inch (at sea level). Still, I would not have thought that the tank of air had any appreciable extra weight but that was simply because I didn't think about it (until I read it here or in the book).
I think you guys are pointing out my question even more. You said that I should be weighted so that I will be neutral with no air in my BC and my tank "nearly empty". So doesn't this reinforce my thought that a bit of air in the BC when starting out to compensate for the "6 pounds negative"?
I got the whole idea that the weight of the water beyond 10 or 15 feet compresses all air spaces causing them to displace less water. So I assume that once I reach this depth I wont need to use my lungs as a buoyancy compensator but rather I can breath & use them to oxygenate my blood as I have been comfortable since that doctor slapped me almost 55 years ago... And beyond that inflate my BC.
I just thought that a little extra lead and a small pocket of air in the BC to compensate, then release this air to descend and save the lungs for ummm breathing. Then as my tank gets lighter as I consume and exhale the air, less BC and my extra lead comes in the save the day.

something tells me my dive instructor tomorrow is going to remember me. heh...

 

[nereas]

...So doesn't this reinforce my thought that a bit of air in the BC when starting out to compensate for the "6 pounds negative"?
...

Exactly right!

It is also true that you would inflate your B/C quite full, before your dive, so that your head is nice and far out of the water for a beach entry, or so that you do not sink into oblivion on your giant stride entry from a boat.

Then you would begin to dump air, in order to start your descent. And YES there WILL be a little air left in your B/C to compensate for your 80 cu ft of compressed air/nitrox in your tank, which at the beginning of your dive will "weigh" 6 lbs more than it will at the end of the dive.

...I got the whole idea that the weight of the water beyond 10 or 15 feet compresses all air spaces causing them to displace less water. ...

Indeed the crushing weight of Davy Jones' locker (as they say in Pirates of the Caribbean) will not only squeeze all the trapped air in your padding and whatever, it will also dangerously compress your wetsuit as well. This is why you need a really good B/C when you dive with a wetsuit. And this is why you may need to add air to your B/C as you descend, to compensate for the crushed wetsuit as well, at deeper depth.

This problem varies with depth. The deeper you go, the more the problem manifests itself. Thus with a wetsuit and deep diving, you will need a very reliable B/C to get you back up to the surface, particularly if you need to abort the dive early on, and go up with a full, heavy tank of air/nitrox.

...So I assume that once I reach ... depth I wont need to use my lungs as a buoyancy compensator but rather I can breath & use them to oxygenate my blood ...

Remember this rule, by Cousteau back in the 1950s: "Never hold your breath while using it." Just breathe normally, not too deeply, and not shallow either.

What you are doing now in thinking about all this in advance is called "visualization." It is a learning mechanism that really intelligent persons have access to. It is the best form of preparation that you can do before a learning event.

Visualization is also an excellent manner in which to plan all your future dives, even after you are certified and trained.

I think you will become a very excellent diver, based on your enthusiasm and intelligence.

 

[spectrum]

Actually, my degree is in physics and mathematics. Maybe that's why the instructions confuse me. Yup, air = 14 pounds per square inch (at sea level). Still, I would not have thought that the tank of air had any appreciable extra weight but that was simply because I didn't think about it (until I read it here or in the book).

The operative fact here is that air at 1ATM weighs .08 pounds per cubic foot. An 80 CF cylinder contains 6.4 pounds of air. This is why you will experience a 5-6 pound increase in buoyancy as you breathe the tank down. The displacement is a constant but the mass drops increasing buoyancy.

I think you guys are pointing out my question even more. You said that I should be weighted so that I will be neutral with no air in my BC and my tank "nearly empty". So doesn't this reinforce my thought that a bit of air in the BC when starting out to compensate for the "6 pounds negative"?

Neutral buoyancy is a fleeting and dynamic concept as your cylinder contents change and as you change depths. You should expect to be adding some air to your BC before you get very deep especially if you are wearing much of a wetsuit.

It is true that one can dive without a BC but that works in a fairly narrow band including limited neoprene and cylinder sizes that don't have a huge swing. It's way out of the scope of modern training but it's worth looking into once you get competent with the basics. This is a good thread on the topic.

I got the whole idea that the weight of the water beyond 10 or 15 feet compresses all air spaces causing them to displace less water. So I assume that once I reach this depth I wont need to use my lungs as a buoyancy compensator but rather I can breath & use them to oxygenate my blood as I have been comfortable since that doctor slapped me almost 55 years ago... And beyond that inflate my BC.
I just thought that a little extra lead and a small pocket of air in the BC to compensate, then release this air to descend and save the lungs for ummm breathing. Then as my tank gets lighter as I consume and exhale the air, less BC and my extra lead comes in the save the day.

Yes, use your lungs for breathing in a normal/deep fashion. Use your BC to become neutral. This will require adjustment as the cylinder gets more buoyant and as you change depths. It called a buoyancy compensator for a reason.

There is a secondary concept of using your lungs to manage slight variations. This skill is usually not pushed in basic training since the overriding message is to never hold your breath. Should you start to ascend while holding your breath the expanding air stands to create a possibly fatal embolism. Bad, bad, bad.

However since you are now aware of it in a nutshell it's like this.... You can take extra air into your lungs to rise over a ledge for instance however you MUST keep your airway open, this can be done by exhaling very slowly, just like the bubble stream you are being taught to blow while doing a regulator R&R. You need to remember that if you push this too far the air in your BC as well as your neoprene suit will expand, gaining displacement, making you buoyant and up you go. Like wise you can exhale deeply to drop down into a space and you will rise when you refill your lungs. Drop too far too long and that bubble will shrink, your suit will crush and you will hit the bottom unless you give your BC a shot of air. It's a skill you will grow into. The important thing is that you NEVER close your airway! For now fly your inflater and try to hold a depth as you swim.

Pete

 

[SkipperJohn]

OK, now that I did my CW dive(s) I have a question on this topic.
I practiced doing the neutral buoyant thing for what seems like forever. I did hovering and fin pivots. Here's the thing. I would make minor adjustments to my BC with these little puffs of air that could not be more than a coffee cup full. A little in or out and it would make a decent difference helping me towards my goal. Then I would breath. Now, at 6'4" 205 pounds and what I've been told are oversized lungs, my breaths would laugh at my BC and really ake quite a difference. This made hovering really difficult. How do you handle it? How do you not float and sink with each breath-kind of like what the fin pivot demonstrates? And... then there's that delay/yo yo thing.

 

[nereas]

...How do you not float and sink with each breath-kind of like what the fin pivot demonstrates? And... then there's that delay/yo yo thing.

Excellent question!

You have 3 choices.

The first choice is to adjust your B/C-wing to be neutral when you have exhaled. Then when you inhale, you will go up a little bit, and then come back down when you exahale again. For those who breathe normally, this is probably the best choice.

The second choice is to adjust your B/C-wing to be neutral in the middle of a breath. Then when you inhale, you won't go up as much, but when you exhale you will go down a little instead. For those who breathe more rapidly, this is probably the best choice. And that may be your problem, right now, since you are just new.

The third choice is to adjust your B/C-wing to be neutral when you have inhaled. This can be very dangerous for a beginner, however. It is akin to skip-breathing. Photographers do something like this, as well as speargun hunters, both of whom need steadiness while they pull the trigger or push the shutter. So you may want to hold off trying this for a few years.

P/S: I would like to torture whomever invented the fin pivot. It messes up the water,and it destroys the bottom, so STAY OFF THE SAND OR REEF!!!

 

[under water]

...Then I would breath. Now, at 6'4" 205 pounds and what I've been told are oversized lungs, my breaths would laugh at my BC and really ake quite a difference. This made hovering really difficult. How do you handle it? How do you not float and sink with each breath-kind of like what the fin pivot demonstrates? And... then there's that delay/yo yo thing.

You will always become more buoyant when you breath in and therefore go up and likewise, you will go down when you breath out. I think there might be something like a 12 pound swing (correct me someone) from a full breath in to a full breath out. The trick is to become familiar with this and use it to your advantage. Because of that slight lag in response discussed earlier, when you feel yourself start to rise it is then time to breath out, when you feel yourself start to sink it is time to breath in. The right timing is probably before you actually start to change directions.. remember that lag.. The effect is that you will eventually be able to hover by timing it right.
Never hold your breath!

 

[Walter]

This is over his head, Walter. Besides, his question was about the surface, about surface buoyancy. And you are talking about underwater buoyancy. So you are unfortunately just going to confuse him more, with this.

Not over anyone's head. You can't strive toward a goal unless you know to set the goal.

With this you are going to confuse the heck out of him. This has nothing to do with his question,

Actually, it does. It let's him know that he's mistaken when he says:

one would be able to descend without messing with your breathing or air in your lungs

nor with good diving.

Of course it's good diving.

What you are saying is that you use your drysuit like a B/C

I've never said that. What I'm saying is I use my lungs for buoyancy control. That is old school, but it's excellent technique.

Plus, if you were diving with a 1/2 mm dive skin, this statement would not even be true, unless, of couse your never go to Florida, or the tropics, or the Red Sea, which are the most beautiful diving meccas in the world!

I never "go to Florida." I live here. Most of my diving is here. I do go to the tropics. I've never even worn a sissy suit. In Florida, I dive 6 months of the year with a swim suit (not a dry suit), I weight myself properly and I don't add air to my BC, because I don't need to add air to my BC. I control my buoyancy with my lungs. The other 6 months of the year, I do wear a wet suit and I do add air to my BC as I descend. In those rare times I travel to really cold water (40s or colder), I wear a dry suit and I do not use it as a BC. I try to keep just enough air in the dry suit to eliminate suit squeeze.

 

[TSandM]

I would make minor adjustments to my BC with these little puffs of air that could not be more than a coffee cup full. A little in or out and it would make a decent difference helping me towards my goal. Then I would breath. Now, at 6'4" 205 pounds and what I've been told are oversized lungs, my breaths would laugh at my BC and really ake quite a difference. This made hovering really difficult.

Excellent observation. HERE is a good piece written by Rick Murchison, on the unachievable ideal which is neutral buoyancy. If you're hovering, immobile, and breathing, you will gently rise and fall a few inches with each breathing cycle. The key is to make sure that you're cycling around a steady central point. You get your buoyancy very close to idea with your BC/drysuit, and then you will learn to make very small adjustments in your breathing to stay exactly where you want to be. The goal is to do as much as you can with your buoyancy compensator, so that your breathing can be natural. This is for two reasons -- One, too much use of the breath leads to CO2 buildup, which at best causes headaches, and at worst precipitates anxiety and panic. Second, when you're distracted or stressed, you'll forget to alter your breathing, and the next thing you know, you're rising or sinking.

It's actually a pretty cool feeling, sitting at a safety stop, and feeling yourself gently rise and fall with your breath. It brings home the utter weightlessness that is part of diving.

 

[Bob Vincent]

SkipperJohn there are so many variations in getting neutral. At first everyone was trying to tell you what to do to get there without any protection. Then came wet suits, dry suits etc. At least they kept with 80 AL tanks and didn't bring steel's into the equation. Which would keep you negative during the whole dive. Every BC (99%) has inherited buoyancy. Different thickness of wet suits, different compression rates at different depths. There are nitrogen compressed wetsuits that don't compress as nearly like regular wet suits. Dry suits ... I read someone talking about putting air in the dry suit instead of the BC. (in the Arctic maybe above water to stay warmer)

You've been told a number of ways to accomplish this. Pick one and get it done. Once your really close you can fine tune. But remember practice and having time spent underwater will get you where you want to be. About using your lung volume to move up or down in the water column after awhile you'll see how easy it is.