Diving Physics

Diving Physics

The term "physics" is intimidating to most people. It can even be intimidating to highly educated people.

So why is "physics" part of every diving class curriculum? That's because the sport of diving involves dealing with an environment that is not natural to us humans. Sure we take showers and bathe and swim (and at some point all mammals emerged from the oceans), but actually being underwater, that's a different thing altogether. We cannot breathe underwater. Water is fundamentally different from air. And if we do not understand the entirely different rules of being, and surviving, underwater, diving can be dangerous or even deadly.

What's so different you might ask? Just take a tank of compressed air with you and you're all set. Not so. Underwater, everything is different. Our senses work differently. We see differently because of the optics of water. We hear differently because sound travels much faster. There is the drag of water and the fact that the laws of gravity no longer seem to apply the way we instinctively know them on dry land. Colours are different because some of them are absorbed by water very differently than by air. Water is a much better conductor of heat than air is. As a result, we lose heat much more quickly in cold water. Most importantly, though, water is much, much denser than air. At sea level, water weighs about 800 times more than air. Which means, that as you dive, you are quickly subjected to very substantial pressure. Since the human body consists mostly of water, you're in no danger of being crushed. However the gasses inside the human body are greatly affected. They get compressed a great deal the deeper you dive, and the impact of that is the most important reason why any diver must be aware of the physics of being underwater.

Buoyancy

The dictionary describes buoyancy, as far as it applies to being in or under water, as "the tendency or capacity to remain afloat in a liquid." That phenomenon is based on something first discovered by the ancient Greek scientist Archimedes, who lived over 2,200 years ago. What he found was that an object that is partly or fully immersed in water is pushed up by a force equal to the weight of the fluid displaced. That, obviously, is of significant importance to divers. In diver speak, "neutral buoyancy" means you (your body and whatever gear you have on you) is exactly as dense as the water. Applying Archimedes' finding, that means you just float in place and neither rise nor sink. If the sum total of your body is less dense than the surrounding water, you are "positively buoyant" and you rise. If you and your gear are denser than the water, you are "negatively buoyant," and you sink. So if you ever thought diving simply meant swimming down or coming back up, not so. Proper diving means you constantly have to control your buoyancy.

How does one control one's buoyancy? There are three major ways. The first is by adding weights. Those can be attached to the diver's body in various ways. As a rule of thumb, they generally amount to about 10% of body weight. The second way is by using the Buoyancy Compensator (BC), also called a Buoyancy Control Device (BCD). The BC gets its air from the scuba tank and is used to adjust buoyancy at various depths. It is, of course, also a safety device, should a diver need to ascend quickly. The third, and (for beginners) least effective, buoyancy control is your lungs. Most people are positively buoyant in water with their lungs full of air, but negatively buoyant once they breathe out.

Keep in mind that overall buoyancy depends on a lot of things: The size and weight of the diver, the type of diving suit, the weight belt, the design and weight of the scuba unit, the air in the BC, the air in your lungs, the remaining air in the tank, all the stuff you carry with you, and also whether you are diving in fresh or salt water. Salt water contains dissolved minerals, mostly salt. In fact, we're talking about around 30 to 35 kilograms of salt in a thousand kilograms of water, which means seawater is denser and thus exerts a bit more upward buoyancy onto your body.

Gassy matters

The major issue here is that gasses are very low density compared to water. For all practical purposes, water does not compress at all, even at the bottom of the ocean (well, it does, but not enough to matter for diving). Gas, on the other hand, can easily be compressed, and things happen when it does. That is why it is essential to have a basic understanding of the laws that govern the behaviour of gas.

Now what are we concerned about? Is it just air in general, or the different components of air? Both. What we call "air" actually consists of a number of gasses but two make up almost all of it.

The first is oxygen. Scientists know it as a chemical element in the periodic table that lists all chemical elements known to man. It is known by the symbol "O" and carries the atomic number 8. Oxygen is the second most common element on earth and makes up about 21% of the air we breathe. Oxygen easily bonds with other elements and thus carries nutrients around our body and thus enables life. Oxygen easily bonds with virtually all other elements, which makes it a tremendously useful element. Of course, it's also the reason why things rust, and too much oxygen can actually be poisonous for the human body.

The second is nitrogen, another element in the period table, carrying the letter "N" and the atomic number 7. Nitrogen is by far the largest component of our air, 78%. In the human body it doesn't really do anything. Above water, whatever nitrogen we breathe in, we also breathe out. Some nitrogen gets absorbed by our bodies and just stays there.

The remaining one percent of gas in the air consists of a variety of elements such as Argon, Neon, Helium, Methane, Krypton, Hydrogen, Carbon dioxide and others. With the exception of Carbon dioxide, those trace gasses don't matter when you're diving.

Oxygen and nitrogen, however, do matter. Oxygen, because you need it to live and Nitrogen because it can do bad things to your body. The effects of nitrogen are, in fact, the primary reason why divers must know about the physics of gasses.

The problem with Nitrogen

So what makes nitrogen a bad guy? The problem is that when you dive, the increased pressure of the water compresses nitrogen and more of it dissolves into your body. Just as there is a natural nitrogen saturation point at the surface, there are saturation points under water. Those depend on the depth, the type of body tissue involved, and also how long a diver is exposed to the extra pressure. The deeper you dive, the more nitrogen your body absorbs. Worse, the deeper you go, the larger the difference between the nitrogen already absorbed in your body and what it now can hold, which means more nitrogen gets absorbed ever faster. Absorbing nitrogen is not really the problem, at least not to a certain point (if you dive deeper than 30 metres or so, nitrogen can result in nitrogen narcosis - more on that later).

The problem is getting rid of the nitrogen once you ascend again. As the pressure diminishes, nitrogen starts dissolving out of the tissues of the diver's body, a process called "off-gassing." That results in tiny nitrogen bubbles that then get carried to the lungs and breathed out. However, if there is too much nitrogen and/or it is released too quickly, small bubbles can combine to form larger bubbles, and those can do damage to the body, anything from minor discomforts all the way to major problems and even death.

Nitrogen off-gassing is an exceedingly complex issue that currently still only relies on models and approximations derived from decades of observation and testing. And as if the underlying physics were not complex enough, each individual diver is different and will absorb and release nitrogen in a different way. Different body tissues, for example, have widely differing absorption/diffusion properties. Cartilage and bone are very slow to absorb and release nitrogen, lung tissue very fast. Body fat, which very widely varies among individuals, has a much greater affinity for nitrogen absorption and can thus impact decompression significantly.

Sound

Sound travels much faster in water than in the air, about four times as fast. Sounds also travels farther in the water. (This is why the military is so concerned about having silent submarines.) And sound underwater is affected by pressure, water temperature, and salinity. This can produce interesting results when underwater sound meets two layers of water with different temperatures.

All this means that our hearing, which is designed to interpret sound in the air, can easily get confused underwater. Our brain detects the source, and to some extent the distance, of a sound by the difference between its arrival in the right and left ear. With underwater sound so much faster, our brain cannot process direction easily, and underwater sound often comes across as being all around a diver.

Light and vision

We see differently underwater, and the reason, once again, is physics. The diving mask itself, with its flat glass lens or lenses, bends light so that objects underwater are magnified by about a third. I say "about" because the magnification factor depends both on how far the lenses are from the eyes, and also how far away an object is. For some reason, inexperienced divers tend to underestimate the distance of close objects, and overestimate the distance to objects farther away. There are special diving masks that attempt to eliminate this distortion, but they have their drawbacks and require getting used to.

Colours are different underwater as well. Colours are really nothing more than different wavelengths reflected by an object. Underwater, waves travel differently, and some wavelengths are filtered out by water sooner than others. Lower energy waves are absorbed first, so red disappears first, then orange, then yellow. Green stays longer and blue the longest, which is why things look bluer the deeper you go. As long as the water is clear, that is. In murky water there is less light penetration and things tend to look greenish-yellow.

Even under the best conditions, colours can only be seen down to a depth of a bit over 30 metres. Below that things look black or grey. The way colours are absorbed has an impact on underwater photography: a good light source is needed for vibrant images.

But wait, there's more. Or rather, less. See, the water surface is reflective and can act a bit like a mirror. At high noon, with the sun being right overhead, that's no big deal and almost all of the sun's light enters the water. However, early and late in the day when the sun's rays hit the water at shallow angles, almost all light is reflected away and doesn't make it into the water. So be aware that it can get dark sooner than you think.