Scuba diving
The first diving suits used a heavy copper diving helmet with windows and a hose from an air pump. The helmet was clamped to a waterproof diving suit. Divers would wear heavy weights and walk on the seafloor as it was not safe to swim.  The air hose would send air to the helmet from a pump on land or on a boat above the diver. The air would pass through the helmet and out into the water through a valve, and the diver would breathe from the air in the helmet. This wasted a lot of air, but there was no way to only supply air when the diver needed it. This is called a free-flow system.