EVERY DIVING INSTRUCTOR will be aware that it’s a big step to ask someone to kneel at the bottom of a swimming pool, when all they’ve done up to that point is try to stay swimming at the surface.
There’s an understandable anxiety that induces an increased heart-rate and necessarily large amounts of air inhaled.
For my own part, I’ve had trainees anxiously signalling that they have a problem when the surface is rippling only inches above their head. I’ve responded by telling them simply to stand up if they’re worried.
The fact is that we were all taught to take a deep breath before ducking under while swimming, and that’s a tricky habit to break.
The side-effect of this is that trainees tend to have much greater lung volumes than they need to have, and this precipitates an increase in buoyancy. So what does the instructor do He loads more lead onto his charges, so that they can kneel conveniently 2m deep while getting to grips with basic scuba skills.
How many of you were taught to exhale fully in order to leave the surface, and then take your first breath at around 2m deep or more
It’s something that experienced divers pick up later, but sadly few of us do enough diving to say that we are really experienced.
Twenty descents in a year from 20 dives does not make a diver “experienced”, and buoyancy control is usually taught at entry level only in a very rudimentary way.
This means that many divers get into the habit of being over-weighted. I see them at inland dive sites, taking deep breaths from their regulators before leaving the surface.
If you have, say, 6 litres of lung capacity fighting to keep you afloat, you’ll already need an extra 6kg of lead to counteract it. Then there’s the air in the suit.
Back in the old days, when men were men and Jacques Cousteau blew up coral reefs “for reasons of conservation”, there were no scuba buoyancy aids.
The bodies of divers without suits are incompressible, so there was no problem.
Once they started wearing those famous skinny silver suits, Cousteau’s divers had to get their weight just right so that they could leave the surface by exhaling fully and maintain some semblance of buoyancy control at depth, which they did by taking bigger breaths from their twin-hose regulators.
Then the adjustable buoyancy life-jacket (ABLJ) was invented, and it became easy. Anyone could do it. It meant that suits could be made of thicker material, too.

WHY WAS THAT Neoprene is composed of thousands of tiny bubbles of inert gas that compress as it goes deeper, causing it to lose its volume. Pioneer aquanauts couldn’t use thick suits, because it meant that if they wore enough lead to be able to leave the surface, by the time they got to 40m they were so over-weighted that they would find themselves trudging around on the seabed.
The ABLJ allowed them to compensate for this loss of buoyancy with a thicker suit. It was the first BC or “buoyancy compensator”, and divers could stay down for longer because they didn’t risk freezing their butts off.
So the BC was developed to compensate for the compression of the suit, but why does this compression appear to make a diver less buoyant
Ships weigh thousands of tonnes, but they are defined in size by the amount of water they displace. A loaded ship weighs less than the amount of water it displaces. If it didn’t, it would be an Italian cruise liner.
Divers want to do more than simply float on the surface. They want to have neutral buoyancy – neither to float nor to sink. Neutral buoyancy is the very essence of diving. If you are positively buoyant you’re a floater. If you are negatively buoyant, you’re a mooring block.
Let’s revisit that British inland site. It’s so cold that nobody in their right mind would be in it
in anything other than a drysuit (OK, I’ll concede that a few of you are used to suffering
in semi-drys).
If you contrive it so that you are neutrally buoyant near the surface with just enough lead worn so that you can descend on exhaling, all you will need to do is maintain the volume of the suit to match your depth.
You will need to add air, because the small amount in your suit is compressed as you go deeper, but you will not need to inflate your suit so that it displaces any more water than it did at, say, 3m. It simply means that the air in the suit is denser at depth, rather than having
more volume.
Now some of you will say that you need to add so much air that it would swill around inside your suit, causing problems of trim. You’re worried that it will go to your feet and send you into an uncontrolled inverted ascent.
This may be true – but only if you start off with too much air in your suit in the first place.
We often hear people say that they like to add air to the suit to take off the squeeze and then put air in the BC to compensate for buoyancy. Why would you need to do this if you are keeping the volume of your suit and hence your displacement the same
Here’s the answer – it’s because you’re starting the dive with too much lead. You are still stuck in the days when you were learning to dive, and your instructor piled on the lead to stop those big lungfuls of air from sending you irrevocably upwards.
Get your weight right for your displacement near the surface and you won’t ever need to put air in your BC.
What do you look like at the surface in your drysuit Does it look as if it’s clinging to the contours of your undersuit, or does it have a profile of its own
Do you look like a wet spaniel or a plump turkey If it’s the latter, you probably have too much air in your suit before you even get into the water.

CROUCH DOWN and break open the neck-seal before you put on the rest of your kit, expelling as much air as possible. Once in the water, open that shoulder dump fully to allow any spare air out, before you leave the surface.
Start your dive with only the air contained within the fibres of your undersuit. Start off with minimum volume of the suit and keep it that way throughout the dive, and you’ll use your BC only for surface support.
How do we respond to those who say: “Drysuit to keep you dry and BC for buoyancy control” Shed some weight!
Keep your drysuit at constant volume and the displacement will stay the same, whatever
the depth.
If you find that there’s air sloshing around in your suit when you do this, you’ve started off with too much air in it, which is why you needed too much lead on your belt.
You need a BC for buoyancy control under water only when you wear a wetsuit or semi-dry suit that compresses at depth, with nothing else you can do to compensate.
Open-circuit divers using multiple tanks will start a dive over-weighted thanks to the weight of the gas that they will eventually exhale into the water in the course of a dive.
They might need to add air to a BC as well as their drysuit to compensate for this at the beginning of a dive, but let’s face it, if you are using multiple tanks, you ought to know what you’re doing!