WHY ?? do we need DIVE TABLES ??!!!
Before we can discuss dive tables, we must first understand why we need them. Henry’s Law tells us that the amount of gas that will dissolve into a liquid at a given temperature is almost directly proportional to the partial pressure of that gas. What does this mean?
Basically, liquids have a certain amount of gas dissolved in them. For example, a glass of water sitting on your kitchen table will have gasses dissolved in it. The atmosphere is comprised primarily of nitrogen and oxygen in a mixture of about 78% nitrogen and 21% oxygen (with the balance being comprised of argon, carbon dioxide, neon, helium, etc). Henry’s Law then tells us that of the gas dissolved in our glass of water, about 78% is nitrogen and 21% is oxygen.
Under our normal atmospheric pressure of 1 bar, or 14.7 pounds per square inch, we have a fixed amount of nitrogen and oxygen in our glass of water. If, however, we double the pressure of the gas in our house, twice as much nitrogen and oxygen would dissolve into our water. Three times the pressure would force three times as much gas into the water, etc.
Well, the pressure in your house will probably not double or triple, but as you learn in your basic open water scuba class, as we descend in the water, the pressure on our bodies increases greatly.
We also know that our bodies are primarily liquid. This means that as we increase the pressure on our bodies, and we breath air that is also under pressure, our bodies will absorb more gas, just like our glass of water.
For example, if you descend to a depth of 10 metres, or 33 feet, you are under twice as much pressure as you are at the surface, and in time you would absorb twice as much nitrogen as you presently have in your body now.
You notice that we said "twice as much nitrogen..". What about the oxygen? Our bodies use oxygen - we metabolize it, so at the depths we dive to as recreational scuba divers, we don’t have to worry about the oxygen in our air, just the nitrogen. Since our bodies don’t use the nitrogen, it can accumulate or build up in time.
We have stressed "in time" because this is not an instant process. The gas exchange in our bodies happens in our lungs with each breath, and then the excess gas is carried to our body tissues by our circulatory system, by our blood.
The same is true when we lessen the pressure on our bodies. The excess nitrogen is slowly eliminated by our bodies as we breath out the nitrogen over a period of time.
Not only time under pressure, but how much pressure we are under is an important factor in how nitrogen is absorbed or eliminated by our bodies. It stands to reason that if you are under higher pressure, the nitrogen is pushed into your body faster. If we are under extreme pressures, as say 39 metres or 130 feet of depth, it does not take long for our body to absorb quite a bit of nitrogen.
It turns out that having too much excess nitrogen can cause problems for us when we reduce the pressure on our bodies by surfacing after a dive. Much like a bottle of soda, if kept under pressure, the gas will stay dissolved in the liquid, but when you reduce the pressure rapidly, bubbles can form. If you look in a sealed bottle of soda, you see no bubbles, but pop the lid and it can bubble over.
As scuba divers, we want to avoid this "bubbling over". When we absorb too much nitrogen in our tissues, and/or ascend too rapidly, we can form bubbles of significant number and size to cause pain or permanent damage to our body. This is commonly referred to as "The Bends" since it would cause the sufferers to contort and bend in attempts to alleviate the pain.
The more technical term is decompression sickness (DCS). It was first noted in men working in pressurized coal mines in the early 1840’s when they would leave the pressurized area and return to standard pressure.
Research on DCS started in the mid to late 1800’s but it was not until 1907 that a physiologist named Dr. John Scott Haldane published the first dive table. His table was developed from experimentation with the Royal Navy Helmet divers and research with goats.
Research has advanced by leaps and bounds since these early experiments. Scientists now have doppler bubble detectors that can "hear" tiny bubbles in our bodies. Computers can calculate theoretical models to approximate how fast different tissues in our bodies might be absorbing or eliminating nitrogen.
Even though the technology has advanced, the primary goal is the same as it was in 1907. To develop and use a table that would allow us to dive safely by giving us time and depth limits that would stop us from getting DCS.
It is important to note that although technology, research and computers have all advanced dramatically in the past years, we are still dealing with the human body. No dive table or computer can guarantee that you will not suffer from DCS, but by knowing how to use the tables properly, and by diving conservatively within the tables, one can greatly reduce the chances on DCS.