Dive theory on a beer coaster

We’ve all had to learn it: diving. Or maybe you’re just about to start — in which case, an exciting journey awaits you.

Learning to dive means mastering a range of practical skills, but also gaining a basic understanding of theory. In most sports — like skiing, surfing, tennis, or kayaking — you mainly learn by doing. Only in a few activities, such as flying, skydiving, or sailing, does theory play a more prominent role. Diving belongs in that category — perhaps more than any other sport.

In practice, you’ll learn how to clear your regulator and mask, equalize your ears, share air in an out-of-air situation, swim efficiently with fins, and communicate using hand signals. But learning to dive also means understanding what happens when you go underwater: how depth and pressure increase, what that does to the volume and density of the air you breathe, and how your body responds. We call this dive theory.

Dive theory can seem complicated at first. Tables, computers, gas laws, safety rules… But the basics of diving are actually surprisingly simple. Once you understand the key principles, almost everything in diving can be traced back to just a few simple rules. In fact, the essence of dive theory fits on a beer coaster. Literally — take a look.

American manuals therefore usually use a trick. Instead of using depths that make sense to divers (as above), they convert the metric depths of 10, 20, 30, and 40 meters to feet. This results in 32.8, 65.6, 98.4, and 131.2 feet. These are then rounded to 33, 66, 99, and 130 feet. And instead of displaying pressure in the usual unit psi, they introduce ATA (Atmospheres Absolute)—a unit rarely used outside of diving physiology and almost equal to bar, but not quite (1 ATA = 1.01325 bar). You then get the table on the right coaster above. The result suddenly looks surprisingly clear again.

We won't bore you further with more calculations in the imperial system. For practical diving, the metric system simply fits better with the physics underwater. That is also why most divers worldwide eventually use meters, bar, and liters—even if they learned their first dive training in feet and psi. Still, American divers often remain remarkably attached to the imperial system. So much so that a certain dive training organization continues to mention imperial units in all translations of its teaching materials—as if feet and psi are universal units!

Bar is the simplest unit to express pressure. 1 bar is approximately equal to normal atmospheric pressure at sea level. And for every 10 meters, 1 bar of pressure is added. It is also noticeable that the list only goes up to 40 meters. That is not a coincidence: for recreational divers, that is about the maximum depth. Deeper diving falls under technical diving. Technical divers often go into the water with multiple tanks—sometimes three, four, or even more—and with extra equipment. Separate training, specialized equipment, and often other breathing gases are needed for this. Regular recreational diving is much simpler: usually with one tank on your back and regular air as breathing gas.

Boyle: air is compressed

What does that mean in practice? The increasing pressure underwater directly affects the air we breathe. When the pressure increases, air is compressed. A simple rule is: if the pressure doubles, the volume of air halves. That means, for example, that an air bubble at 10 meters depth has only about half its volume. At 20 meters, that's about a third, and at 30 meters, about a quarter of the volume at the surface. This phenomenon is known as Boyle's law: when the pressure on a gas increases, the volume decreases. For divers, this has all kinds of practical consequences. During descent, for example, you must regularly equalize your ears, because the air in your middle ear is also compressed.

The law is named after Robert Boyle, an Irish physicist and philosopher who described this relationship between pressure and volume as early as the 17th century—long before diving with compressed air as we know it today. Boyle conducted this research mainly out of scientific curiosity, but his discovery soon had practical consequences. It helped scientists understand how air pressure works and how gases behave, knowledge later applied in instruments such as barometers and, hundreds of years later, in technologies such as compressors and diving equipment.

Of course, all this doesn't fit on that coaster, and fortunately, you don't have to remember it all. If you want to remember something, just remember the name Boyle. Or even better: remember the principle that the volume of a gas decreases when the pressure increases. That immediately gives us the third column of the coaster. The volume of a gas is inversely proportional to the pressure: the higher the pressure, the smaller the volume—and vice versa: the lower the pressure, the greater the volume.

This also has a very practical consequence for us divers. When we ascend, air expands. Just look closely at the bubbles you or your buddy exhale. As they rise to the surface, they get bigger and bigger. You often even see a bubble split into smaller bubbles, which also get bigger as they rise further. This is also why you should never hold your breath while diving with compressed air. If you hold your breath during an ascent, the air cannot escape from your lungs as it expands. This can cause a lung overpressure injury. As you progress in your diving career, you will learn that there are different forms of lung overpressure injury:

• Arterial gas embolism—air bubbles enter the bloodstream through damaged lung tissue and can block circulation to the brain or other organs (embolism literally means something that is thrown in—so something (like an air bubble) is "shot" into the arteries and gets stuck there).
• Pneumothorax or collapsed lung—air (pneuma) escapes from the lungs into the chest cavity (thorax), causing a lung to partially or completely collapse.
• Mediastinal emphysema—air ends up in the space between the lungs around the heart and the major blood vessels. Doctors call this space the mediastinum.
• Subcutaneous emphysema—air spreads under the skin (subcutaneous simply means "under the skin"), usually around the neck and collarbone.

The word emphysema comes from Greek: em- (ἐν) means "in" and fysein (φυσᾶν) means "to blow". Literally, it means "blown up from within"—exactly what happens when air escapes from the lungs and accumulates in tissue where there is normally no air. But you don't have to memorize all that either. If you remember only one thing, let it be this: never hold your breath while diving. Or even simpler: always keep breathing. That is perhaps the most important rule of diving. And actually, it's a good rule for the rest of your life: whatever happens, always keep breathing. If you do that, chances are you'll live a long life. 😉

Density: why you use more air at depth

The increasing pressure underwater has another important consequence: air becomes denser. This means that with each breath at depth, you inhale more gas molecules than at the surface. This has to do with the relationship between pressure, volume, and density. When pressure increases and volume decreases, density increases. In other words: pressure and density are directly proportional, while volume and density are inversely proportional. You can also see this in the numbers. If you multiply the values from the third and fourth columns of the coaster, you always get 1.

We have also shown this schematically with the balloon on the coaster. The ten yellow and blue balls have much more space in the top balloon, at the surface, than in the balloons below. As the pressure increases, the air particles are forced closer together. That is exactly what we mean by density.

I often explain it even more simply. Suppose you are traveling with a group of friends in a minibus where you all fit comfortably. On the way, the bus breaks down and you are picked up by a small car, for example, a Kia Picanto. If you all want to fit in that smaller car together, it is only possible if everyone sits closer together—literally on each other's laps.

That is exactly what happens to air when the pressure increases: the same amount of air has to fit into a smaller space, so the particles are forced closer together. A direct consequence of this is that you use more air at depth. You use more air when you dive deeper. This is because the air you inhale at higher pressure has a greater density. With each breath, you therefore inhale more gas molecules than at the surface. As a result, your air supply runs out faster.

The effect is easy to understand: at 10 meters you use about 2× as much air, at 20 meters about 3× as much, at 30 meters about 4× as much, etc. You could also say that air becomes "more expensive" the deeper you dive: you get less dive time in return. We can show this schematically on a new coaster. This is of course a simplified example, but it clearly shows how it works: the deeper you dive, the faster your air supply runs out.

When you calculate your personal air consumption, you must therefore link this to the average depth of your dive. Your personal air consumption is often referred to as RMV or SAC. In practice, divers usually mean the same thing: how much air you use per minute, converted to surface conditions. In the metric system, we simply express this in liters per minute.

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