In the 1950s, when I joined the Royal Navy (RN) as a medical officer, the RN did not use contents gauges for scuba diving. Instead, they used twin tanks yoked together. The tanks were upside down, with the valves within easy reach just above the divers bottom. The diver turned on one cylinder (A) and used it until it ran out; he then knew that he had used half his air. Turning on the second cylinder (B) equalised the pressures in both cylinders and then cylinder B was turned off. At this stage, the diver had used half his available air and was breathing from half the remaining half. When that air ran out, he still had a quarter of his original air supply in cylinder B. At this point, the diver turned off cylinder A and used cylinder B to return to the surface. This procedure was still in use in 1972.
