Divers today are fortunate to have the latest technology available in breathing equipment. It, however, wasn’t always so. A hundred years ago, breathing underwater was accomplished by long tubes, sacks of air carried with a diver and many other ingenious methods. But there were problems associated with those methods that modern demand valve regulators and pressurized tanks have solved.
As you dive deeper the pressure increases about 1 atm (one atmosphere) for every 33 feet (10 m). Since air is easily compressed, your lungs and chest muscles have to exert a force to overcome the pressure. But that’s more difficult if you’re breathing, say, free air delivered from the surface. That air remains at 1 atm. In order to help counteract the increasing pressure, pressurized air is needed. In addition to that, as you dive deeper the gases inside your body get compressed a small amount. Air near sea level is about 78% nitrogen, 21% oxygen with trace amounts of other gases, mostly argon. But that mixture isn’t ideal underwater as the pressure changes. It has to be regulated to deliver the proper concentration of oxygen. Modern air delivery systems compensate for those changing circumstances, providing the right mixture and pressure as divers descend.
Since most recreational dives are done with a mask that encloses the eyes and nose, breathing is done mostly through the mouth. That introduces another problem, since the mouth is wrapped around the regulators’s mouthpiece.As you inhale, you take in air. Your body uses it up, then exhales the product, part of which is carbon dioxide. Inhale 21% oxygen, exhale 18% oxygen and about 3% carbon dioxide. Carbon dioxide itself isn’t toxic as carbon monoxide is, but breathing too high a concentration decreases the relative amount of oxygen you intake. Ultimately, that would result in oxygen starvation. Oxygen starvation leads to light-headedness, disorientation and ultimately death. The regulator/tank system solves that problem too by the simple expedient of venting the exhaled gas to the surroundings.
Since you don’t typically exhale through the nose underwater (that would fog the glass and open the mask to the water), that solution is more clever than it appears. You are breathing out the same mouthpiece and hose you use to breath in. Venting the carbon dioxide to the water requires some ingenious engineering.
Today’s designs are predominantly open circuit 2-stage diving regulators. The original Aqua-lung design by Jacques Cousteau and Emile Gagnan, developed during WWII was only one stage. In either case, air is delivered at a pressure that matches the ambient pressure of the diver.
Inside the tank, air is pressurized to about 200 atm. In the 2-stage design, the first stage regulator reduces the air pressure to around 10 atm. In the second stage, the demand regulator lowers the pressure again to match that of the surroundings. This delivers air at the best pressure to the diver’s lungs, in order to counteract the ambient pressure.
Modern air delivery systems provide safe, healthy air to breath with reliable equipment. That’s a tribute to the years of dedicated effort and inventiveness of scuba gear designers. Bravo!