Tools · Buoyancy

Float, sink, or hang? It's a tug-of-war.

Drop something in water and two forces argue over it: gravity pulling it down, and an upward push equal to the weight of the water it shoves aside. Whichever wins decides whether you float, sink, or hover. That second force is buoyancy, and we've understood it for over two thousand years.

One fun fact: the legend says Archimedes worked it out in the bath around 250 BC — water rose as he climbed in, and he supposedly ran out shouting "Eureka!" The bathtub-and-streaking part is almost certainly embellished, but the physics is real and still carries his name: the upward push equals the weight of the fluid displaced. It's the same reason a steel ship floats and a steel weight doesn't — it's all about how much water you push out of the way.

Set your body and suit, and the tool works out your baseline lead. Then take it underwater: the two knobs — depth and the air in your BCD — are what you manage to hold neutral, the same job you do on every real dive.

What sets that balance is mostly you and your suit. A wetsuit floats, and the thicker it is — or the bigger you are — the more neoprene you're wearing and the more lead you need to sink. That's why the tool asks your weight and height: it estimates your skin area (the Du Bois formula), scales the suit's lift to your size, and adds it into a baseline lead, shown beside the plain 10%-of-bodyweight rule of thumb. Salt water shifts it too — dissolved salt packs more mass into every liter without adding volume, so the water you push aside weighs more and pushes back harder. It's not drag or "thickness"; it's weight: about 2.5% extra lift across your body, a few pounds, which is exactly why you carry more lead in the ocean. Your breath works the same way on a smaller scale — full lungs lift, empty lungs sink — a fine-tune of a pound or two, never a fix for bad weighting.

Now take that baseline underwater. Descend and the rising pressure compresses your wetsuit — its gas squeezes down exactly like the bubble on the last tool — so it loses lift and you trend heavier; you feed air into your BCD to make it back. That makes a neutral depth a knife-edge: drift below it and you keep sinking, drift above and you keep rising. Holding a depth is something you do actively, with your BCD and your breath — which is also why two divers can be neutral together while each is badly out of trim.

Going up is where it bites, and it's the whole reason you vent on ascent. Two gas pockets expand at once as the pressure drops: your wetsuit re-inflates and regains lift, but the bigger one is the air you added to your BCD on the way down. That air is a fixed amount of gas, so as it expands its lift grows — and if you don't bleed it off, your lift climbs past your weight, you go positive, you rise, and rising drops the pressure further, expanding the air more and lifting you faster still. It's a runaway, steepest in the last ten metres where a fast, uncontrolled ascent is most dangerous — lung overexpansion and decompression sickness. The fix is constant: keep the deflator — the inflator hose — in your hand the entire way up and vent continuously to stay neutral. Try it above: flip trim to Manual, neutralise yourself at 30 m, then add a little air — and watch the diver climb away on its own, faster the shallower it gets, until it hits the surface. Pull the air back down to stop it. That runaway is exactly what happens to a diver who forgets to dump. (Leave trim on Auto and the tool vents for you — watch the air gauge fall as you rise.)

And that active control comes with a catch beginners feel hard: it answers slowly. Add a squirt to your BCD or take a breath, and for a moment nothing happens — your body has its own inertia to overcome before it starts to move — and then, just as the knife-edge warned, the move feeds itself: rise a little and the BCD air expands and lifts you faster; sink a little and it compresses and drops you faster. So the classic new-diver loop goes: you feel yourself sinking, add air, feel nothing, add more — now there's too much, and you're climbing toward the surface; you dump it all in a panic, overshoot, and plummet back toward the bottom. Up, down, up, down — the yo-yo. The cure is counterintuitive: do less. Make one small adjustment, then stop and let the dust settle — give it several seconds to show its effect before you touch anything again. Anticipate the change a beat early, trim with your breath and save the BCD for the coarse moves, and above all, wait. Nearly every buoyancy problem a beginner has is really an impatience problem.

One the sliders leave out, worth a mention: your cylinder. A steel tank is heavy and stays negative even when empty, so it doubles as ballast; an aluminum tank is lighter and turns slightly positive as it drains. And the air inside has weight — a typical aluminum 80 holds about 5 lb of it — which you breathe away over the dive, so you finish a few pounds lighter and more buoyant than you started. That's the real reason you weight for the end of the dive, not the start: you have to hold a safety stop on a near-empty tank without drifting up. Tank, suit, water, breath, gear — they all collapse into one question: how much lead? One consideration among many.