How does a small divers tank supply a diver with air for long periods of time underwater?

Answer 1

Scuba cylinders can hold large volumes of air (or other, more exotic, breathing gas), because the gas is compressed to very high pressures. If you're reading this in your living room on Earth, the air you're breathing is at "one atmosphere" of pressure. This pressure is often termed "1 bar" or 14.7 PSIg*. A typical scuba cylinder, an "80 Cubic Foot" aluminum tank, is usually compressed to about 3000 PSIg, or about 200 bar. This is about 200 times higher pressure than the air you're breathing there in your living room.

The scuba cylinder is fitted with a pressure regulator system which delivers the breathing gas at the much lower pressures needed by the diver. Modern regulators are built in two "stages". The first stage, mounted to the tank, reduces the thousands of pounds of pressure in the take to an intermediate pressure of about 150 PSIg. That's three to five times more pressure than a typical car tire. At this point the second stage, mounted at the diver's mouthpiece, reduces the pressure to match the ambient pressure of the water at whatever depth our diver is at. This allows the diver to breathe more-or-less normally with minimal effort.

It is important to note that the pressure needed by a diver to allow breathing increases as the diver descends. The air in your living room, as we've discussed, has a pressure of about 14.7 pounds per square inch. This is essentially the weight of the tens of thousands of feet of air stretching between your living room and the outer reaches of Earth's atmosphere. That's why 14.7 PSIg or "1 bar" is also considered "1 Atmosphere" of pressure, or "1 atm". Water weighs significantly more than air. It only takes 33 feet of sea water to exert as much pressure as an entire atmosphere of air. This means that a diver descending to 33 feet is under "2 Atmospheres" of total pressure; the one of air she started with at the surface and another of water. That's 2 bar or about 29.4 pounds per square inch. What does this mean to our diver's breathing? It means her air is delivered at about twice the pressure at 33 feet as it was on the surface. This also means that the air in the tank will last half as long at 33 feet as it would on the surface.

Let's go a little deeper. Unlike air, water also has the property of not being compressible, so each additional 33 feet of sea water "weighs" the same 14.7 pounds per square inch. If our diver descends to 66 feet, she is under a total pressure of 3 atmospheres -- one of air and two of water. That's about 44.1 PSIg (about 3 Bar). Her air is being delivered at 3 times higher pressure, hence three times greater air volume per breath, than it was at the surface. The scuba tank will support her for 1/3 as long at 66 feet as it would at the surface.

This pattern continues as our diver descends. A depth of 99 feet of sea water adds yet another atmosphere of pressure, resulting in the scuba regulator delivering our diver's air at 4 Bar (59 PSIg) and the tank lasting 1/4 as long.

It's worth noting that recreational divers are generally trained to dive to a maximum depth of between 20 and 40 meters (100-130 feet). Due to a number of factors such as CNS Oxygen Toxicity, Nitrogen Narcosis and Decompression Illness, diving deeper is considered unsafe for recreational divers. Specifically, decompression stops become mandatory on deeper dives and recreational divers are not taught decompression diving techniques. Another consideration for deeper dives is that, due to the increased ambient pressure, the diver will probably not be able to carry an adequate amount of breathing gas in a single recreational scuba cylinder.

*Note that there are some conventions used in Imperial vs Metric measurement units that cause the numbers used in this discussion to be inexact. 1 "bar" is actually defined as 100 kilopascals which equates to 14.5038 PSIg, rather that the 14.7 used in the article. By contrast, one atmosphere, or 1 "atm" is defined as 101,325 kilopascals or 14.696 PSIg. We have overlooked these small differences in measurement conventions for the sake of simplicity and readability.