It was like wearing a mini-submarine, I thought when I tried on the Avelo kit at the recent Diving Talks. What looks like just a slightly oversized tank with a fancy little backplate and two small canisters along the flanks, which resemble battery packs, are in fact also a buoyancy device.

The tank is buoyant and can become lighter or heavier by using water from its environment. It works by taking in or expelling water, just like a submarine. When a diver is ready to dive, he or she presses a button and water is pumped into the Hydrotank – that is what they call it – and around an inner bladder, which holds the air, or some breathing mix. Once the diver is neutrally buoyant, the pump is stopped, and the diver can swim about retaining that same buoyancy until enough gas has been consumed to make the diver a little buoyant.

The pump can then be turned on the pump again for 20-30 seconds. The water in the Hydrotank replaces the mass of the gas consumed, and the diver can experience another 20-25 mins with neutral buoyancy. At the end of the dive, a purge valve is opened to drain and all the water from the Hydrotank becomes buoyant and lightweight, making the climb back aboard or ashore easy.