Buoyancy and weighting, worked out

One sum, and only two of its terms move

Everything you take into the water either weighs more than the water it displaces or less. Add it all up — you, the suit, the cylinder, the lead, the fins nobody thinks about — and the total is your buoyancy. Get it to zero and you hover.

The useful part is that almost none of that total changes during a dive. Your body does not. The lead does not. The regulator, the fins and the mask do not. Two things change, and only two: the gas trapped in the foam of your suit, which is squeezed smaller as you go down, and the gas in your cylinder, which leaves through the regulator and does not come back.

That is the whole problem. Everything divers argue about — steel against aluminium, how much lead, why the safety stop is the hard part — is one of those two terms, and both of them are computable to the nearest tenth of a kilo.

Water first, because it sets the scale. A litre of seawater weighs about 1.025 kg and a litre of fresh water weighs 1.000. That two and a half per cent is small until you notice it applies to your whole displaced volume: a diver and kit displacing 90 litres is 2.25 kg more buoyant in the sea than in a lake. It is why the belt you set at Hemmoor is wrong in Bonaire, and wrong in the direction that lets you sink.

What a wetsuit is actually worth

A wetsuit floats because most of it is not rubber. Foamed neoprene is solid polychloroprene — 1.23 g/cm³, denser than water, a sinker on its own — blown full of nitrogen until the block comes out at around 0.25 g/cm³. Work backwards from those two numbers and four fifths of your suit, by volume, is gas.

The volume is easy to estimate. A one-piece suit covers about 1.75 m² of a person, so its neoprene volume in litres is that area in square metres times the thickness in millimetres. A 5 mm full suit is 8.75 litres of foam weighing 2.19 kg, displacing 8.97 kg of seawater, for 6.8 kg of lift at the surface. A 3 mm is worth 4.1 kg and a 7 mm is worth 9.5 kg.

Those numbers are worth holding up against the rule of thumb every instructor teaches, which is to start with lead equal to ten per cent of your body weight in a 6 mm suit and five per cent in a 3 mm. For an 80 kg diver that is 8 kg and 4 kg. The model says a 6 mm suit is worth 8.14 kg of lift and a 3 mm suit 4.07. The rule of thumb is not a rule about your body at all — it is the suit, and your body weight is standing in for how much suit it takes to cover you.

Now take the suit down.

The dashed line is what every dive course implies: Boyle’s law applied to the bubbles, which would leave a 5 mm suit with 1.4 kg at 30 m (100 ft). Real neoprene does not do that. When Bardy, Mollendorf and Pendergast put wetsuit foam under hydrostatic pressure and measured it, they found about 30 per cent of the volume gone in the first 10 m, 60 per cent by 60 m, and the loss levelling off near 65 per cent by 100 m. The rubber matrix carries part of the load, so the bubbles are stiffer than free bubbles.

The gap matters. At 30 m the measured curve leaves you 3.0 kg of lift where Boyle predicts 1.4 — more than twice as much. A diver who assumes the suit disappears at depth will be badly overweighted, which is the commonest weighting error there is.

The aluminium argument is about the offset, not the swing

Ask around a dive boat why anyone prefers steel and you will be told that aluminium cylinders “go positive at the end”, as though steel somehow avoided the problem. It does not, and the arithmetic is unusually clean.

A cylinder gets lighter by exactly the weight of the gas you take out of it, and nothing else. A litre of air at the surface weighs 1.225 g, so an aluminium 80 — 11.1 litres at 207 bar, which is 2,298 litres of free gas — loses 2.81 kg over a dive. A European steel 12 at 232 bar holds 2,784 litres and loses 3.41 kg. A steel 15 loses 4.26.

Steel does not swing less. It swings more, because it holds more gas.

Read the chart as six bars of nearly the same length sitting in completely different places. Every steel bar stays left of the neutral line: a steel 12 is 4.2 kg negative full and still 1.2 kg negative empty. All three aluminium bars cross it: an aluminium 80 starts 1.1 kg negative and finishes 1.7 kg positive. The difference between the two cylinders at the end of the dive is 2.9 kg, and the difference in how far they moved is 0.2.

So the real statement is this. Aluminium and steel change your buoyancy by about the same amount during a dive; steel starts about three kilos lower on the scale, so you carry about three kilos less lead to compensate. That is a comfort and trim argument, not a safety one — and it is exactly reversed if you ever have to hand the cylinder off, because a positively buoyant empty aluminium stage is far easier to manage than a steel one trying to sink.

The published buoyancies confirm the mechanism rather than just asserting it: for every cylinder in that chart, the gap between full and empty matches the weight of its air to within an eighth of a kilo.

The dive, drawn

Put the suit and the cylinder on the same axes across one ordinary dive and the whole problem becomes visible at once.

This is 18 m for thirty minutes on a rental aluminium 80, in a 5 mm suit, breathing 15 litres a minute — a dive most people have done a hundred times. Measured against the surface at the start, the diver is 3.1 kg heavier as soon as the suit has compressed on the bottom, drifts back to 1.5 kg heavy as the cylinder empties, and arrives at the safety stop 0.4 kg light.

Follow the two dashed lines rather than the total. The suit gives its lift back on the way up, and the cylinder has already given away everything you breathed. On the ascent those two work in the same direction, which is the answer to the question in the standfirst: the belt that felt right on the bottom was set when the suit was flat and the cylinder was full, and by 5 m neither of those is true. A diver who trims their weighting to sit still on the bottom with a full cylinder is 3.4 kg light at the safety stop.

Three or four kilos does not sound like much on land. At 5 m, with an empty jacket, it is the difference between holding a stop and being pulled off it — and a safety stop you cannot hold is the part of the dive where being underweighted actually costs you something.

Getting the number right, then keeping it

The weight check is one minute of work and it is specified the way it is for a reason. In water too deep to stand up in, empty your buoyancy compensator completely, hold a normal breath, and float: the water should sit at eye level. Breathe out and you should sink. That is the whole test.

The part that gets skipped is the cylinder pressure. Do that check with a full cylinder and you will be right at the start of the dive and light at the end, because the 2.8 kg of air you have not breathed yet is still on your back. Do it with about 50 bar left, at the end of a dive, and the number you get is the one that works when it matters. If you can only check on a full cylinder, add the swing yourself and take it off at the end.

Then there is the tool nobody counts. Three litres of air between a comfortably full breath and a comfortably empty one is 3.08 kg of lift — more than an aluminium 80 loses over an entire dive. Your lungs are a bigger and vastly faster buoyancy device than your jacket, which is why competent divers barely touch the inflator once they are down, and why the first fix for a diver bobbing at a stop is to breathe shallower rather than to dump more air.

Two adjustments follow from the sum rather than from experience. Fresh to salt costs you about 2.5 per cent of everything you displace — call it two to two and a half kilos for a diver and kit — and it goes the other way coming home. And at altitude the surface itself is at lower pressure, so the same descent compresses a suit proportionally more: at Rubicon Wall, 1,897 m above the sea, ten metres of water is a larger relative pressure change than it is at sea level, and everything on the chart above happens slightly faster.

Where buoyancy is the skill

Some dives forgive a rough weight check. These do not: entries you have to walk down and climb back up, cold water in a drysuit where the same arithmetic runs on a suit you can inflate, and walls where the only thing under you is more water.

The gas half of this is in how long a tank of air actually lasts, and what the same compression does to your ears is in why your ears hurt.