Rowan Hayes, of Kings High School, asks :-
How does an aqualung work?
John Campbell, a physicist who has been diving for nearly half a century, responded.
Ah, your question takes me back to the old days. One great advance in diving equipment was the invention of air-pumps which could keep an open-bottom diving bell full of fresh air. I trained in a modern version of this, the old copper-helmet standard diving dress. If the pump or pipes on the surface failed so that the air pressure in the helmet fell to surface pressure, then the diver would be compressed into the helmet by the surrounding water pressure.
Sports divers use compressed air because oxygen is poisonous at pressures greater than two times atmospheric pressure. The main advance in self contained underwater breathing apparatus (SCUBA) was the 1815 invention of two Frenchmen, Beniot Rouquayol a mining engineer and Auguste Denayrouze a navy officer. They used a membrane subjected to the surrounding water pressure to regulate a valve which allowed air to flow to the lungs only while a diver was breathing in, hence the term demand valve. This invention was forgotten until 1942 when again two Frenchmen, Emile Gagnan an engineer and Jacques-Yves Cousteau a navy officer and keen skin diver, applied it to make true scuba equipment.
Nowdays aqualungs are two-stage devices. The second stage, at the end of the hose by the mouth, uses a modern version of the Rouquayol-Denayrouze valve. That way the diver gets air at the same pressure as the surrounding water. Modern diving cylinders contain air compressed to about 200 times atmospheric pressure. To make control easy the air in the hose is maintained at an intermediate pressure of about eight times atmospheric pressure. A spring loaded valve where the hose is attached to the tank (the first stage) lets air flow from the tank into the hose only when the air pressure in the hose falls, ie when the second stage opens as the diver breathes in.
Thirty years ago I saw film of mice using their own water-filled lungs to breathe under highly oxygenated water. Shades of the Kevin Costner character in the film "Waterworld."