Basic Buoyancy Question

[KnowledgeIsPower]

Excuse me for how basic this question is, but as a new diver that learned how to dive in cold water, I am a bit confused about dive theory in terms of buoyancy for warm water. What we learned was that we compensate for loss of buoyancy with our BC at depth due to compression of our wetsuits. The other compensation is for loss buoyancy due to loss of air in the tank.

However we recently came back from a tropical dive location where we did 11 dives. We still had to add air to our BC to maintain neutral buoyancy at depth even though we were wearing no wetsuit! Why is this? Is it because we were overweighted? My understanding is that once we achiever Neutral Buoyancy we should have neutral buoyancy at any depth if we are not wearing a wetsuit. If someone could please correct my misunderstanding I would appreciate it!

 

[MB NZ]

We still had to add air to our BC to maintain neutral buoyancy at depth even though we were wearing no BC!

?????

 

[bradells]

Excuse me for how basic this question is, but as a new diver that learned how to dive in cold water, I am a bit confused about dive theory in terms of buoyancy for warm water. What we learned was that we compensate for loss of buoyancy with our BC at depth due to compression of our wetsuits. The other compensation is for loss buoyancy due to loss of air in the tank. However we recently came back from a tropical dive location where we did 11 dives. We still had to add air to our BC to maintain neutral buoyancy at depth even though we were wearing no BC! Why is this? Is it because we were overweighted? My understanding is that once we achiever Neutral Buoyancy we should have neutral buoyancy at any depth if we are not wearing a wetsuit. If someone could please correct my misunderstanding I would appreciate it!

Boyle's law.

Yes, you would have less loss of buoyancy due to less neoprene, but the fact still remains, air spaces change volume as pressure is increased/decreased (depth).

BRad

 

[Scubatude]

Great question! Your BC will usually have some trapped gas that will compress with depth and reduce buoyancy. Also the material the BC is made of often times will be compressible. Lastly there are air spaces in the body that can compress to some degree I am told, that accounts for some small amount of loss of buoyancy. Of course you already know about the weight of the gas in your take that becomes lighter with each breath.

Keep asking question, and keep learning!

 

[Imla]

When you dive without the wetsuit your initial need to add air into your BC comes from the negative buoyancy of the gas in your tank. Because you need to be correctly weighted at the end of your dive, you will nessesarily be slightly overweight in the start.

If you add a certain amount of air to your bc to offset this weight at a certain depth you will be neutral. When you descend this air will be compressed by the pressure of the surrounding water, and you need to add more to stay neutral. When you ascend, the air will expand and you will need to let out air to remain neutral.

 

[KnowledgeIsPower]

?????

Sorry typo, I meant we were wearing no wetsuit I hope we were wearing a BC hehe

 

[fbk]

This is why you want to be weighted correctly and not overweighted... because you need air in your BC to compensate. That's why it's called a Buoyancy Compensator That air compresses as you descend, takes up less volume, so you have to add a bit more. The more weight you have to compensate for, the more air you need, the greater the change in buoyancy. The reverse happens as you ascend.

It's possible to dive with no wetsuit and no BC and just use your lung volume to compensate, if you aren't overweighted. The rest of you is uncompressible, or at least not enough to make any significant difference. Gear with lots of padding, foam, air spaces etc also contributes, so the less of that you wear/carry, the less change in buoyancy that you have to deal with also.

 

[tbone1004]

you don't have to hope you were wearing a BC.... For open water shallow diving with an Al80 and no wetsuit there is really no need for a BC.

That being said, you need to go back to your instructor because he clearly didn't teach you properly. In diving your Buoyancy Compensation Device needs to account for the following
Change in buoyancy of the tank due to air breathed. For an al80 this is only about 5lbs, but on big tanks this can easily become up to 15lbs for a single tank.
Change in buoyancy for wetsuit, not relevant for drysuit diving or diving with no exposure protection.

Your lungs should be able to compensate for the 5lb shift in the AL80 but you have to make sure you're weighted properly.

Full AL80 with a double hose regulator, no BC, just a backpack harness for it. So liberating

 

[TMHeimer]

Good explanations about being properly weighted, compressable material, use of lungs, extra weight (at start, meaning you are purposely well overweight at start) for full tanks from 5 to even 15 lbs., etc. All correct of course. Don't forget the point that except for a possible millisecond you are never exactly properly weighted since you are always breathing down your tank.

 

[spectrum]

Agreed!

If you had the need for any air in your BC that air cell would be subject to compression (loss of volume/displacement) as you descended. You would be adding air just to maintain the status-quo buoyancy.

Within reason this does not mean you were over weighted. Your emply BC buoyancy neutral state wants to be in shallow water with a near empty cylinder. At the onset of the dive you have about 5 pounds of expendable weight (air you will breath) making you that much heavier than at the ideal time. Those 5 pounds equate to a little over 1/2 gallon in air volume. for that reason most divers will have some air in the air cell especially early in a dive.

Now with ideal weighting and practice you can usually work around this on typical sized cylinders. By breathing deep and keping yur lungs "large" most of the time you have in effect created a human BC. You can exhale but then inhale quickly to manage your average buoyancy. You may porpoise or scull a little (watch Sea Hunt) but you can make it all work. As the dive nears midpoint a natural respiration will be perfect. Divers diving in this style may actually be weighted a little less than ideal, they will keep their lungs small in the last third of the dive so they don't cork. Some will even find a rock to carry so they don't cork to the top at the end.