THE BASICS OF DIVING BUOYANCY CONTROL - Abyss Ocean World Liveaboard

[Liddell Marilyn]

If you are a scuba diver, you appreciate the importance of understanding buoyancy not only for a better diving experience but also for your safety. This is one of the trickiest parts for learners but if you understand the basics, it becomes much easier to control your dive. Diving Buoyancy Control in the simplest terms means the tendency to float, sink or remain neutral. Here are some of the basics of buoyancy to help you become a better diver.
Main Diving Buoyancy Control Terms

• Positive buoyancy: Where a diver floats upwards or is able to remain floating on the surface.
• Negative buoyancy: Inability to float upwards or where a person sinks downwards
• Neutral buoyancy: Where a diver remains suspended in the water at a single depth

How Buoyancy Works

Any object or person submerged in water displaces an amount of water equal to their volume. The water around such an object will try to fill the space, which exerts an upward pressure or buoyant force. The Archimedes principle helps understand whether an object will sink or float; if weight of the object is higher than the buoyant force, it will sink and vice versa. In diving, the idea is to sink at the beginning of the dive and remain neutrally buoyant before ascending. Achieving this naturally is impossible hence the use of buoyancy control device (BCD) or inflatable jacket.

What Affects Buoyancy in a Dive?

• Your BCD helps control BCD underwater. When inflated, the amount of water displaced increases thus increasing buoyancy.
• Lead weights: These are used to overcome positive buoyancy during a dive and allow you to stay down.
• Exposure protection: The thicker your wetsuit the more the buoyancy hence the need for more lead weights.
• Dive gear: This includes scuba tank, fins, regulator among others. The more the weight of your dive gear the more your negative buoyancy hence the need to measure buoyancy in order to use the correct weights.
• Tank pressure: As you breathe more air from the tank, it becomes lighter thus increasing positive buoyancy. You have to weigh yourself to ensure you remain negatively or neutrally buoyant even at the end of the dive.
• Air in the lungs: To become negative buoyant, exhale before the dive to decrease the amount of water displaced in order to sink.
• Nature of water: A diver is more buoyant in salt water than in fresh water
• Body weight: The fatter the diver the more buoyant they will be.

Applying Diving Buoyancy Control

To apply these buoyancy concepts for your dive, inflate the BCD before diving so as to float. This ensures you confirm everything is in place before descent. Second, deflate the BCD to start descent and continue adding small bursts of air as you descend. At your desired depth, add air to achieve neutral buoyancy. You might need to deflate the BCD to compensate for the depleted air in the scuba tank. Finally to ascend, deflate the BCD and try to swim as opposed to floating. On the surface, inflate the BCD in order to float as you remove the regulator.

 

[rongoodman]

Finally to ascend, deflate the BCD and try to swim as opposed to floating.

Why you do think that's a good idea? Do you anticipate the diver being vertical and having to fin to remain in place during the entire ascent?

 

[Altamira]

I agree with @rongoodman. I want to be neutrally buoyant throughout the dive except when trying to descend, and then just slightly negative under most circumstances. On ascent, I do not want to find myself sinking rapidly if I stop finning to deal with some issue, nor to I want to cork to the surface. Buoyancy is a constant balancing act, and a regimented thought process of dumping all bcd air before ascending will likely bite you at some point.

 

[scrane]

Fin up with an empty bladder to see if you're over weighted. Theoretically you should be neutral or a little heavy to compensate for a situation where you have trapped air.

 

[rongoodman]

Fin up with an empty bladder to see if you're over weighted. Theoretically you should be neutral or a little heavy to compensate for a situation where you have trapped air.

Even if you're weighted perfectly, you'll still be heavy by the amount of gas you're carrying. Loss of bouyancy due to the compression of a thicker wet suit would only make things worse.

 

[TMHeimer]

Disagree with one thing. A "fatter" person USUALLY is more buoyant. There are other factors.
Not sure of the thread's purpose, other than to review OW course material.

 

[boulderjohn]

Finally to ascend, deflate the BCD and try to swim as opposed to floating.

This is a very bad thing to be teaching people. It works when they are not too much overweighted and and when they are using a thin wetsuit.. Put them in a heavy wetsuit and they will start to plummet the moment they deflate the BCD.

 

[ScottFL78]

Deflating as you ascend is a good idea but you should remain in trim. But most divers have a console mounted computer and they ascend vertically with their head down watching their depth. It looks like someone texting and walking. I find it amusing. I would have missed a few cool schools of fish today if I would have had my head down during ascent.

 

[TMHeimer]

This is a very bad thing to be teaching people. It works when they are not too much overweighted and and when they are using a thin wetsuit.. Put them in a heavy wetsuit and they will start to plummet the moment they deflate the BCD.

Yes, sorry, I missed that. No matter what thickness wetsuit (or dry) and weight you use, you should be neutrally buoyant when you are (probably) near the bottom (or at least at depth) when it's time to ascend. You start up with your BC the way it is, and gradually let air out as it expands when during your ascent. Seems to me this should work no matter what.
Deflating the BC as you ascend is not an exact science of course, since as you go up the pressure change gradually increases. With practice, you can get pretty sharp at it. Once in a while I still find myself kicking mid water and not making upward progress--oops, let just a little too much air out...
Now if you're fooling around with that idea of trying to see how much weight you can swim up with a completely deflated BC and know what you're doing with that, it's a different thing.

 

[nolatom]

I'm not sure all the "why" you need X amount of weight is super useful. Have an instructor or experienced buddy help you figure out whether, say, 8 pounds is enough. "Enough" means you can exhale with an empty BC and full tank at the surface, and start sinking/descending. I would then add maybe two or three pounds so I'm not too light after I burned about what, four pounds of air from my 80cf tank but maybe "gained" a pound or two from my suit--and me--getting "un-bubbled"?

And remember a breath of air gives you a swing of around 3-4 pounds? So you can "breathe" your way up and down as a fine-tune thing.

Now when you begin ascending, you're starting out at neutral buoyancy, so I wouldn't empty the bc, just start with what you have, and bleed out a little air every 15 feet or so so you won't get too buoyant. It's okay to "swim against negative buoyancy" if it's only slightly negative, but not way negative, you'll wear yourself out. Me, I try to just be neutral-ish.