Buoyancy for beginners
Demonstrating
Demonstrating how to rest easily with just the fin-tips touching the side of the pool

Good buoyancy control is the essence of pleasurable diving. Making yourself perfectly neutral so that you neither float up nor sink renders your diving effortless. So its a pity that so many divers never seem to master this basic technique, says John Bantin

WHEN YOU LEARN TO DIVE IN CONFINED WATER, the major preoccupation of your instructor is to keep you down comfortably on the bottom while you learn the various skills that make you a diver. It would be very distracting for you both if you were to continually float away.
     The simple solution embraced by most instructors is to strap on sufficient lead to keep you where you are - firmly on the bottom. That may be good for teaching, but its not diving.
     But the die is cast. Everyone starts off with the idea that you need enough lead to be able to get down. You dont. What you need is sufficient lead so that you neither sink nor float up.
     Try this experiment in a swimming pool. Wearing nothing but your bathing costume, float vertically in the water. The human body is more or less neutrally buoyant, depending on the fat-to-muscle ratio. People drown only because their head is the densest part of their body so, given a natural state of affairs, they would float vertically but head-down.
     By breathing in a large lungful of air you become positively buoyant, and your head will stay out of the water, supported by the buoyancy of your body. Exhale and empty your lungs as much as possible, and you sink below the surface. You need no lead. This changing state of buoyancy depending on lung volume is crucial to understanding buoyancy control. So whats that lead all about
     It was a long time ago that Archimedes discovered that when an object was submerged in water, its loss of weight was equal to the weight of the volume of water it displaced.
     There is an upforce applied, and if the item is less dense overall than the water, this upforce will make it float. The builders of massive steel ships know this only too well.

src=http://divernet.trafika.co.uk/data/images/10604buoyancy2.jpg
Perfectly weighted, these divers dont have to waste any gas
or energy to maintain their chosen position in the water

losing weight
Not all items of diving equipment sink. However, they all lose weight when submerged. Heavy diving tanks displace water and become less heavy. Aluminium ones almost become neutrally buoyant when empty, but remember, even the air contained within them has weight, so you must allow for that when calculating the lead you need. More of this later.
     Some other items of diving equipment will tend to make you float. Your mask and the air within it add to your buoyancy. When you put on a diving suit you increase your overall volume and decrease your overall average density. That adds buoyancy. The same goes for a hood and gloves.
     Averaged out, your diving equipment tends to increase your total buoyancy, so to become neutral again we put on extra weight in the form of lead to counteract its effect.
     But never add more than you need. Never lose sight of the idea that you should rise when you breathe in and sink when you breathe out. That remains true whether youre wearing your diving kit or not, provided you are correctly weighted.
     Do this buoyancy check again, fully kitted, at the surface in the water in which you are about to dive. Take a normal breath from your regulator and empty your BC. You should float at eye level and start to sink only when you start to exhale.
     Inhale and rise, exhale and sink. If it doesnt work like that, adjust your weight until it is correct. Then add lead to counteract the weight of the air you are likely to use.
     The amount of air in a cylinder, and therefore its weight, is calculated by multiplying the cylinder capacity in litres by its pressure in bar. If you use 180 bar from a 12 litre cylinder, you will have used 2160 litres of free air, which weighs around 2kg, so you will have to allow for that amount of weight lost during the dive by adding it at the start.

living animals
It would stay that simple if not for the effects of water pressure. Unfortunately, as you go deeper so pressure increases, and that part of your equipment which is compressible, mainly your suit, maintains its weight but loses volume and therefore its buoyancy. We wear a buoyancy compensator to counteract this, but it must be used properly.
     Air is added to a BC by way of its direct-feed control, and with it buoyancy, as we go deeper. That air is later released as we ascend back towards the surface.
     Things are slightly complicated by the fact that the air too is compressed at depth, and therefore has less effect, but then expands and has more effect as we ascend.
     Its not much different for a drysuit diver. If you are correctly weighted at the surface, inhaling to keep your mouth above the surface and exhaling to sink below it, you need only put enough air in the suit to make up for its compression and loss of buoyancy, and to keep its volume constant.
     A correctly weighted drysuit diver need never put air in the BC unless the drysuit fails for some reason.
     However, being the living animals that we are, our buoyancy is dynamic rather than static. The way in which we breathe makes a lot of difference to our buoyancy. If we breathe in a relaxed and shallow way, our buoyancy remains stable. If we work harder, we use larger lung volumes.
     Our buoyancy tends to vary much more from moment to moment as our lungs change their dimensions when we breathe heavily. A big man breathing heavily may find that his buoyancy varies from inhalation to exhalation by as much as 8kg of lift. Thats a lot. The same happens if we start to breathe heavily for any other reason.
     Theres reason to assume that smaller people and females have better natural buoyancy skills, because their buoyancy changes less from breath to breath. That said, however big your lungs, good buoyancy control is about relaxed diving.
     Our finning action also has the ability to fool us into thinking that we are neutrally buoyant when we are not. Many divers are over-weighted and would naturally sink if they came to rest, but they continue to swim onwards and a lot of their effort goes towards maintaining their chosen depth rather than progressing forwards.
     In fact they would be swimming onwards and upwards if they were perfectly weighted. How do you avoid that happening Start off with just the right amount of lead and add only the right amount of air to the BC or suit to be neutral at a particular depth. Make a point of swimming horizontally, rather than trying to walk through the water.
     Do this and youll discover its amazing how little air in a BC or suit a correctly weighted diver needs.

dont fiddle
New divers often fiddle with the direct-feed controls to their BCs. They think its like using an elevator. I remember one recently certified pair firmly believing that you let air out to go down and put air in to go up. It seems reasonable enough until you realise how frighteningly dangerous that would be!
     You need to add air only as you go down, and dump air as you come up. You aim to maintain that neutral buoyancy you started with at the surface. Keep fiddling and youll never get it right. Buoyancy is controlled within a given range through your breathing and you will find that far easier to achieve spontaneously than trying to manipulate a mechanical control.
     Assuming that you plan to swim at one depth, simply take a big breath to get over some obstruction in your path and exhale fully to duck under something. Its as simple as that. After all, divers were doing this long before BCs were invented.
     Putting a lot of air back in your BC or suit once at the surface is done only to raise your head comfortably clear of the water, as you would do with a life-jacket.
     A diver with neutral buoyancy uses his fins to go forwards or back, up or down. At the end of the dive, a neutrally buoyant diver can make his way up an anchor or shotline with little effort. He runs his hand over the line as a guide. He doesnt need to pull on it. He fins upwards, dumping a small amount of air from BC or drysuit from time to time so that he stays neutrally buoyant all the way to the surface.

TEST YOUR Buoyancy control
  • Do you find that you cannot stay at the surface by simply holding a lungful of air without putting any air in your suit or BC
  • Do you plunge downwards as soon as you let air out of your suit or BC, instead of descending gently only after you exhale fully
  • Do you find that you are finning all the time you are under water, unless resting on something. Are you unable to relax and stay where you are in the water column
  • Are you panting from exertion all the time during a dive in still water, instead of feeling relaxed
  • Are you working too hard
  • Do you find it a long and tiring climb up a shotline to the surface
If the answer to any of these questions is yes, youre not using your buoyancy-control skills effectively. Get them right and you will feel a lot less tired after dives. The big bonus is that, as you work less, air consumption drops dramatically.

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