One question every non-diver asks me is "How long can you stay underwater?", immediately followed by "How deep can you go?" I wish the answers were as simple as "one hour" and "as deep as I want", but they are not. There are a lot of reasons for this, mainly having to with physiology and physics, two of the most hated topics in school.

What happens when you open a bottle of soda? Bubbles form in the liquid the moment pressure is released. This is because the soda was stored under pressure for an extended period of time. A diver's body can react in the same manner. As I'm sure you know, the human body is almost entirely water. When we scuba dive, our bodies are put under pressure. Since we are breathing compressed gas (air) from the scuba cylinder, our body absorbs this gas. The amount of gas we absorb depends on how deep we dive and how long we stay down.

Decades ago, the U.S. NAVY invented the Decompression Tables. These tell us how long we can stay down, and how deep. The tables have undergone several changes, and many variations now exist. These changes were needed as the original tables were designed for the ideal Navy diver - not the average sport diver.

These tables are a little complex, and actually consist several different tables. Plus all the variations - these are usually a modified version of the Navy tables on a plastic card a diver can take in the water.

For example, the tables indicate that a diver can stay at 60 feet for 60 minutes. Science has calculated how much gas the human body can safely absorb, and the tables reflect this. If we spend 60 minutes or less at 60 feet, we are theoretically safe. However, if we stay longer than 60 minutes, we run the risk of decompression sickness. Body weight, stress, exertion, and water temperature can effect our safe bottom time.

Remember that soda bottle? That's basically decompression sickness. Gas bubbles can actually form inside the body. Since the human body is mostly water, the gas can form anywhere. Also called "the bends", decompression sickness can be a life-threatening malady. Treatment involves immediate pressurization in a decompression chamber, basically a cylinder that the diver is placed inside of. The cylinder is pressurized. This pressure causes any bubbles that form to shrink. The diver is then brought up slowly, allowing the gas to dissipate safely.

There is a lot of information available on the Internet dealing with decompression, scuba, and diving in general. There are even web sites dedicated just to diving and decompression. Visit one of the search engines and look for these topics - you'll find literally hundreds of references!

As you can see there are differences between recreational and NAVY time limits. The advent of inexpensive computer chips has also brought about an avalanche of diving computers. These automatically tract the depth and decompression information for you. Worn on the back of a diver's wrist, or as an integrated part of the diver's instrument console, these electronic wonders track depth, time, temperature, air, and much more. Analog gauges will soon be a thing of the part as these instruments continue to drop in price.

So how deep can a diver go, and how long can he stay there?  For maximum safety, stay above 30 feet (that's where most of the coral is anyway).  Going deeper? Consult the tables and add in a safety margin.  Your local dive shop will be able to help.

Richard C. Drew started diving in 1973 and has been teaching scuba diving since 1978.

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