Buoyancy control

MY BUDDY AND I prepared to dive with rented twin 12-litre aluminium tanks in the Red Sea. I asked for 10kg of lead. He was amazed.
He thought he would need only 6kg.
I’m a lot taller than he is, so I wasn’t surprised.
I displace a lot more water than he does, so I knew that I would need more weight than he
did. I dived very comfortably.
Later, he was pleased to drop the pockets full of rocks he had collected during the time we spent in shallow water, coming up the reef from depth. He added a couple of kilos to
his weightbelt before the next dive.
We’re all different. We each displace a different amount of water, so our requirements for added ballast will vary. However, to achieve perfect weightlessness is the very essence
of the art of diving. To have control of your buoyancy, to be neither floating up nor sinking down, and to be able to go in any direction at will, with the least effort, is the trick.
So how do we achieve this state of nirvana? First, get your weights right for the equipment and the suit you’re wearing and the water you’re diving in.
The overall weight that you need in a particular body of water, when all your equipment is in place, should be enough to allow you to float vertically with your eyes just above the surface with a full tank and a full lung of air.
When you exhale, you should begin to sink, but if you quickly inhale again from your regulator you should rise back to where you were, rather than dropping like a stone.
This simple test will account for the weight of air you will consume during the dive. At the end of a dive, with your tank empty and the weight of the gas that was in it shed, you will find that a full lungful of air will raise your chin and shoulders well clear of the surface of the water.
I do this on every dive trip. It takes only a minute, and is a minute worth the investment.
You should get into the habit of exhaling when you leave the surface too. Of course, this is contrary to a normal swimmer’s reflex, which is to take a deep breath before dipping under.

WITH THE MINIMUM AMOUNT of lead weight as ballast, you will use the minimum amount of air in your BC or suit. Some say that you should pile on plenty of weight, because you can always add air to your BC or suit to compensate. This is true to a degree, but it makes achieving true neutral buoyancy far more difficult.
This is because the air is subject to ever-changing compression and expansion as you go deeper and come up shallower, so the amount of water it displaces is forever changing.
If you put a lot of air into your drysuit, it will move around inside it, causing you to pitch and yaw. You will always be making corrections.
It never ceases to amaze me that so many divers haven’t got a handle on this. Advice on Internet forums often doesn’t help, either. Statements such as “a drysuit is for keeping you dry and a BC is for controlling buoyancy” or “I put air in my suit to take off the squeeze and air in my BC to neutralise my buoyancy” can be very misleading too.
Let’s look at why we need to use a BC. You are incompressible, as is most of the equipment you are wearing. The amount of water you and it displaces is fixed. The suit makes the difference.
If you had no need for a suit, you’d probably have no need for the BC.
If you use a wetsuit or a semi-dry suit, the material from which it is made contains millions of tiny integral bubbles that get compressed as you go deeper. In this way you displace less water, although your weight remains the same. You become less buoyant.
You add air to the BC to compensate.
If you’re using a drysuit, the air within the undersuit that you use for insulation gets compressed (as does the material of the suit, if it is Neoprene).
You add gas to the suit as you go deeper to both stop the unpleasant squeeze that would happen otherwise and, more importantly, to maintain the volume of the suit.
If you maintain the volume in this way, your buoyancy will remain the same because your displacement stays constant. Job done!
Naturally, you will need to release gas from either drysuit or BC during an ascent for the same reason. Otherwise it expands on ascent and you displace more water, becoming exponentially more buoyant.
Many inexperienced divers can be seen swimming in a semi-upright position through the water. This is usually because they have either too much lead or too little air in their BCs.
They are swimming upright because a lot of their effort in finning is used to maintain their depth, rather than to progress in a horizontal direction.
If your buoyancy is near-correct and you try to swim in this way, you will tend to head slowly upwards, and as you go up the air in your BC will expand exponentially, with the ever-present risk that you will find yourself embarking on an unplanned and unpleasant buoyant ascent to the surface.
A rule for good buoyancy control is to use the minimum amount of lead combined with the minimum of buoyancy-compensating air.
Another reason for this upright stance may be that there is a lot of weight on the belt, and a lot of gas had been added to the BC or drysuit to compensate.
The fulcrum is between the weightbelt and the gas that migrates automatically to the top of the BC or drysuit. So the body is forced to pivot into an upright stance.
The gas in a typical full tank for open-circuit diving weighs around 2.5kg. If you use multiple tanks, you may need to start a dive with gas in BC (jacket or wing) rather than to put all the gas needed for neutral buoyancy into a drysuit. This gas will be dumped as the dive progresses.
Remember, constant displacement equals constant buoyancy. You’d need to be wearing a very thick Neoprene drysuit (with a lot of material that compresses) to require putting gas into a BC as well as the suit.
Neutral buoyancy means that all the effort you put into finning will go towards moving you in a horizontal direction. A side-effect is that your air consumption will be dramatically reduced.

DON’T BE ASHAMED to do a buoyancy check before diving. If you have changed any of your equipment or the circumstances in which you are diving since your last dive, your ideal weighting requirement will be different.
The specific gravity of the water you dive in will make a difference too. Fresh water is less dense than sea water, so you’ll need less weight.
If you use a borrowed or rented tank, you’ll need to do a weight check.
The buoyancy of a tank varies according to its all-up weight and the water it displaces. Aluminium tanks are bigger and lighter than the equivalent steel ones. A 15-litre aluminium tank will be less negatively buoyant than a 12-litre steel tank, particularly one that has a lot of excess metal in its base.
Once your buoyancy control is perfected, minor adjustments can be made instantly by varying inhalations.
Perfect control frees you to pursue other interests under water. For example, you can take close-ups of the reef without accidentally trashing the delicate parts while doing it.
It’s the nearest you’ll ever come to flying without a plane!