Buoyancy Question

[SkipperJohn]

In 2 more days I start my CW dive. I studied and learned my PADI book and have a question on buoyancy. The book says you are properly weighted when you can float eye level with a normal amount of air in your lungs and no air in your BCD.
Reading into this, one then must exhale to descend? I would have thought that a small amount of air in your BCD would be normal to be properly buoyant and therefore one would be able to descend without messing with your breathing or air in your lungs.
I understand that as you descend you become less buoyant due to the volume of any air pockets getting compressed. Is this the reason?

 

[Walter]

You should be neutrally buoyant with a nearly empty tank. At that point you won't want to descend. With a full tank, you'll be about 6 lbs negative. You should be finetuning your buoyancy with your lungs. Unless I'm wearing a wet suit, I don't put any air in my BC during the dive.

 

[Teamcasa]

Skipper, You are essentially correct. On the surface, at the beginning of the dive, you will have some air in you BC to account for the negative buoyancy of your tank. In a practical sense, most (boat) dives will require you to stay on the surface for a few minutes to get organized, wait for your buddy and signaling the deck crew you are OK and preparing to descend.

Your descent will begin with dumping any air in your BC and exhaling. As you descend, the pressure will compress any air pockets and your wetsuit will also start to compress. This compression will start to increase your rate of descent and you will need to start compensating by adding puffs of air to your BC to slow you down and to prevent you going too deep. The trick is that you don’t want to add too much and start ascending. Your instructor will help get you and your BC adjusted for proper buoyancy.

Now, as you gain experience, these things will become second nature to you. You will also find that you will be able to reduce your ballast (lead) and improve your overall buoyancy control and start to make fine depth adjustments simply by adjusting your breathing.

 

[ScubaSpicher]

A quick answer to your question: You are "properly weighted" according to the text books when you can float at eye level, while holding a normal breath, with no air in your BCD, with 500psi in your tank.

Air has weight, if you start the dive a little light on lead, by then end of the dive you could actually be positively buoyant by the end of the dive. You want to take the minimum amount of lead to allow you to complete your full dive. Therefore, you plan for the time you are the most buoyant, the end of a dive (hence the near empty tank)

More importantly buoyancy comes with practice and although proper weighting helps, experience is just as important. Good luck and have fun diving!

 

[don Francisco]

Skipper, there's a degree of lattitude for perfect weighting based on preference, but most folks try to optimize their weight for the end of the dive where they'll need to hover at 15' with a nearly empty tank. Some like to have a bit of air in their BC at this point so they can easily go negative in case they float up a bit. Some, like myself prefer to be neutral with the BC empty and would exhale to counter any tendancy to float up.

Either way, there's a catch-22 in that you can't do a weight test at the beginning of the dive on the surface duplicating the conditions at the end of the dive. So this is an aproximation to get a starting point, and with experience you'll fine tune it later on.

If you're floating at eye level on the surface, you'd be about 1# positive submerged (the displacement of the top of your head). So yes, you'd need to empty your lungs to start the descent, and after the first few feet the compression of your wetsuit would help you to continue the descent as you inhale some air.

Remember this is only a guideline for starters and you'll be fine tuning later on.

 

[40Fathoms]

Like Teamcasa and don Fransico says, there are usually two points to a dive where you can check your buoyancy somewhat objectively: At the beginning of a dive where, like your textbook says, you should be able to float at eye level with BCD (and dry suit if wearing one) completely deflated, and again at your safety stop at about 15' where you should be able to hover with no air in your BCD.

However, there are personal aspect as well, as they also say. One could be personal preference. Some like to be a little on the heavy side. Another reason could be the type of type. On dives with a lot of swimming close to the surface (shallower than 15') due to dive site layout I like to be a little heavy so I don't struggle with too much buoyancy as I consume air. Also, beginners tend to fin more at the safety stop, so they might want ot be a little heavy, too.

 

[TSandM]

SkipperJohn, floating at eye level with the BC empty and normal lung volume (neither holding a full breath, nor fully exhaled) is where you are approximately NEUTRAL. Due to wetsuit compression and compression of any residual gas in your BC, you will then be negative at 15 feet, with the same weighting. The estimate is that you will be about five pounds negative, so that when you return to 15 feet with most of the gas used in your tank, you will be neutral at 15, and able to hold a safety stop.

The problem with that, for me, is that as you ascend OUT of that 15 foot depth, you become positive, and it gets difficult to control the ascent rate. Since the proportional pressure changes are greatest in the shallows, that's a time when I want to be able to ascend as slowly as I desire. For that reason, I weight myself to be five pounds NEGATIVE at the surface with full tanks, so that when I return to the surface with an empty (500 psi) tank, I'm neutral when I get there.

It is quite easy to do a weight check with a full tank. Simply add weight until you are neutral at the surface with everything empty, and then add the weight of the gas you intend to use on your dive. Air (or Nitrox) weighs about 1 lb for every 13 cubic feet in the tank, so if you know the tank capacity, it's pretty easy to figure out how many pounds of air you are going to lose during the dive -- That's the amount of lead you need to add at the beginning.

 

[pasley]

You understand correctly, that with 500 PSI in the tank, normal amount of air in lungs, you should float at about eye level.

To descend you need to do the following:
Dump the air in your BC
Exhale, Exhale, Exhale, Exhale (small sip of air) Exhale till you are past 10 feet
STOP MOVING YOUR FEET. Normal reaction in water is to kick your feet back and forth. I caught myself after 50 dives trying to go down and still moving my feet back and forth which of course, since I am vertical in the water propels me back up, the opposite of the direction I am trying to go. Cross your ankles to break this natural habit/tendency.

One puzzle to me when I first started diving was why, when I add air to my BC on decent it takes a few seconds to take effect, but on ascent it is almost instant? The answer of course is momentum. On decent air added to your BC must overcome 3 factors: Momentum, Gravity, Compression with every foot you decent reducing the volume of air or buoyancy/lift of the BC and of course wetsuit compression. On Ascent, your BC has more easily overcomes momentum as gravity is pulling against you and every foot of ascent increases the volume of air and therefore the lift/buoyancy of the BC and the wetsuit decompresses. Remember to add air in very small puffs and wait for it to take effect then add another small puff if required after 10 seconds or so. Failure to wait, or adding large puffs causes the yo-yo game to start.

Yo-Yo game explained: Add air, downward momentum continues to pull you down while the increased volume/lift slows (and will eventualy stop)your decent, add air again, decent slops as first puff of air overcomes momentum (which it would have done just fine without the second puff) and start ascending as the second puff increased your lift/buoyancy more than required and you are now positive. So now you dump air, too much, and start to descend, ad air and repeat, and so the yo-yo game is played.

 

[MauiScubaSteve]

Another problem with the pre-dive surface weight check is that some of the trapped air in your gear may still be in your gear. It is not uncommon for BC's and tight wetsuits to release some bubbles up to 10 minutes into the dive, when you finally roll onto your side or get into a head down position.

Like TSandM says the last 15' of the dive is very important. In the dumbed down gas management of most OW training, the 500psi reserve is for emergency use, to save a buddy who is OOA. That means in an emergency you could very well be under 500psi, so if you are neutral with 500psi (and your buddy is too) you will both be positive when the poo hits the fan.

In an emergency fast breathing is the norm, which is usually not condusive to full exhalation, which further contributes to the positive buoyancy. I personally think a couple pounds more than neutral with 200psi at 15' is much more logical if we are really preparing to handle all emergencies. What most of you call 4lbs over-weighted is what I consider properly weighted!

As an instructor, 4lbs extra would not be enough for all emergencies, because we have to sometimes arrest bolting students. All this talk about perfect weighting being the key to good buoyancy control is a crutch. If you understand the physics and mechanics of diving and buoyancy the weight has very little to do with it. On intro dives, with 4 non-divers on their first dive with only a fast pool session #1, many instructors dive with twice as much weight as perfect, and they still manage to dive.

You can dive well and safely with too much weight, but you can not dive well and safely with too little weight.

 

[nereas]

...Reading into this, one then must exhale to descend?

...I would have thought that a small amount of air in your BCD would be normal to be properly buoyant and therefore one would be able to descend without messing with your breathing or air in your lungs...

It is a confusing statement, unfortunately. It's no wonder that you are confused by it.

At the beginning of your scuba dive, you will have 80 cu ft of air/nitrox in your tank. At 0.08 lbs per cu ft, this will weigh about 6 lbs. Thus at the beginning of your dive, you are slightly negative, not neutral, and thus you need air in your B/C to make you neutral, at the beginning.

By the end of your dive, most of this is gone. So it is at the end of your dive that you would be neutrally buoyant, in the manner described in the statement, with no air in your B/C. And that is the time to go back to the boat or shore. No more descending then.

After your dive, during your 3 min safety stop at 15 ft, you should check this, by seeing if there is any air left in your B/C, while you are suspended as you hold onto the anchor or buoy line.

If there is air in your B/C at the end of your dive, then you are indeed overweighted, and you should reduce your weight by 1 or 2 lbs before the next dive, and then try this test again.

Good question. It shows that you are thinking and properly preparing. Not everyone is this good of a diver, as you obviously are already.