Although dive computers are now considered an indispensable piece of equipment, they simply do not, and should not, replace the roll of tables in diver education. Dive tables illustrate the physics behind decompression theory in a way that computers simply cannot. Only with an adequate knowledge of the rules of safe diving, and more importantly why these rules exist, can any diver be expected to be self-sufficient underwater.
One of the first things that every student diver should learn is a law of physics called known as Boyle’s Law. Boyles Law states that the volume of a gas is directly relative to the pressure applied to it. In other words, if you increase pressure exerted on a gas, the volume of the gas decreases. This is expressed as the gas’s partial pressure. Inversely, if you decrease the pressure exerted on a gas, the volume of the gas will increase, thus lowering its partial pressure. Boyles Law affects gases in a free gaseous state, as well as gases dissolved within another substance. When gases at a higher partial pressure come in contact with a substance at a lower partial pressure, they have a tendency to equalize. That is to say the higher-pressure gases will migrate to areas of lower partial pressure, until the partial pressures in each substance are equal. When a diver enters the water, their body is put under pressure by the surrounding water. This pressure compresses the molecules in a diver’s breathing air, (primarily oxygen and nitrogen), allowing a greater amount (in mass) of these gases to dissolve into the body then possible at the surface, due to the gas’s reduced volume relative to its mass. However our body tissues can only hold a certain volume of these gases in a dissolved state. This is not unlike a glass of water that can only hold a certain amount dissolved salt. So it’s important for a diver to monitor “decompression status” to prevent exceeding this limit and causing a life threatening condition referred to as decompression sickness.
Unlike oxygen, which is metabolized by the body, the nitrogen in our breathing air is inertand can’t be used by our body, so once absorbed it must leave the body the way it was introduced; by way of our respiration. If you decrease the pressure on a diver (ascending from a dive) you will increase the volume of the nitrogen dissolved in your tissues. If the volume of that dissolved nitrogen exceeds the carrying capacity of your body, it will return to its gaseous state while still in your body, causing decompression sickness.
To better illustrate this point, consider a can of soda. Carbon dioxide is dissolved into the soda at a high pressure. When sealed in a can, that pressure remains constant and the carbon dioxide remains dissolved. However when you open the can, you are releasing that pressure. The carbon dioxide expands, exceeds the carrying capacity of the soda and returns to its gaseous state, causing bubbles to form. The same happens to a diver when they stay too long at depth. Once they ascend and lower the pressure exerted on their bodies, the dissolved nitrogen in their tissues expands and can potentially exceed their body’s carrying capacity. Therefore it’s important to monitor the time spent at certain depth/pressure by way of a dive table or dive computer to prevent too much nitrogen from dissolving into the body.
The first dive tables were organized in 1906, by conducting simple decompression experiments on lab animals, and charting the results. Later, the United States Navy revised the tables and printed them for use by Navy and commercial divers. Over the years the dive tables have evolved into increasingly more conservative models to accommodate recreational divers, however the principals behind them remain the same.
A basic dive table displays a list of depths on one axis, and a list of times on the other. Additionally each depth is noted with a maximum time, known as the bottom time. This is the time (in theory) it takes to absorb the maximum amount of nitrogen that once returned to the surface, would not exceed the body’s capacity. The deeper the depth on the table, the shorter the time allowed. On the flip side of a dive table, is another chart allowing a diver to calculate needed information to perform multiple dives in succession. The key benefit of these tables is that the diver must stop, think and take the time to work out their no decompression status in order to dive safely.
A dive computer is an electronic device that tracks depth, time and decompression status automatically. It uses the same mathematical models as found on a dive table, but does these calculations for the diver and presents them with a simple descending timer to guide them on their dive. A diver simply activates the computer and follows the guidelines offered on the computers display. Although convenient and accurate, it does not require the diver to actually understand the information given, it only requires them to obey it. In recent years, dive computers have become so popular, that many training organizations now consider dive tables to be obsolete and are pushing to have tables eliminated from diver education altogether. This would be a great loss to the industry.
In all fairness, dive computers are popular for a good reason. They eliminate human error; allow for a more detailed dive profile, and they are easier to use than dive tables. I use a computer on every dive I make. Because of these benefits and their seeming universal popularity, some believe that teaching dive tables is now unnecessary. However, I reject this philosophy and believe that the dive tables must remain a required part of diver training. The fundamental knowledge of why a dive computer is telling a diver something is just as important to that diver’s safety as what the computer is telling them.
As an example, if someone learns to drive a car and is simply told to “stop at red lights”, but is never explained why, it is likely that the first time they come up to a red light, they will ignore the rule and cause an accident. However if that person was told to stop at the light, because there is cross traffic, that driver will likely stop, because they understand the reasoning behind the red light. When diving, it’s important for your safety, as well as your enjoyment of the sport, to understand decompression theory, what a computer is for, why you should use it, and how it works.
Divers recreate in a hostile environment, and eliminating decompression theory from basic training, would be inviting disaster. For our sport to survive, grow, and remain relatively unregulated, we must insist that our fellow divers are conducting themselves in a safe, responsible manner, and the only way to insure that is to insist upon an adequate education. Otherwise we are encouraging divers to blindly follow the advice of a digital display, taking no responsibility for their own safety and in the end, this can only damage our beloved sport.
Scott Shenton
www.scottshenton.com
info@scottshenton.com
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