Decompression Sickness

I’ll bet you’ve never heard of Caissons Disease (aka The Bends). A long time ago when the Brooklyn Bridge in New York, was being built, workers would spend long stretches of time in Caissons while working on the bridge underwater.

These Caissons were filled with compressed air that the workers breathed while working underwater near the submerged bridge supports. When the workers came up from underwater, they had abdominal cramps, joint pain and sometimes they died. This illness became known as ‘Caissons disease’.

This illness is now well understood when it comes to divers, and is called Decompression Sickness.  As the diver descends Nitrogen that’s breathed from the scuba tank dissolves into the blood as the diver goes to lower depths. On the other hand, when the diver ascends, the nitrogen comes back out of the blood because of the lowered pressure. We are talking about small bubbles that can turn into big bubbles unless they are re absorbed into the blood or exhaled.

Decompression sickness occurs when these bubbles get to a certain volume and size.

Decompression sickness occurs because of a principle of physics known as Henry’s Law. The technical details of Henry’s Law are taught in all basic diving courses. What happens is very much like what happens when a can of soda is popped open. Dissolved gasses bubble out of the liquid due to a rapid change in pressure. The pressure at each depth point in a dive determines how much gas dissolves and how quickly it’s released.

One of the scary things about decompression sickness (DCS) is that it can destroy your health even if you survive the dive. Decompression sickness is one of the biggest risks associated with scuba diving and is to be avoided at all cost. All dive courses teach about how to avoid decompression sickness. The good news is that decompression sickness can be avoided using dive tables and or a dive computer.

The way divers avoid getting decompression sickness is by coming up (ascending) slowly and in steps. A diver stops on the way back up to the surface about every 30 feet. They stop for roughly between one and three minutes on average. How long a diver stops for depends on how the diver descended. Each of these stops is time for neutral buoyancy to reset and occur.

On shorter dives, divers can come up (ascend) without these intermittent stops as long as they don’t ascend at a rate any faster than 30 feet (10 m) per minute on any dive that was deeper than 20 feet (6 m). However, even in the case of not making any stops on the way up, a safety stop is often made at 10 to 20 feet (3 to 6 meters). The safety stop is just to make sure you don’t get sick on any of these continuous ascent dives.

Deep dives require more care and planning. Longer stops and more frequent stops are required for deeper dives. This is especially true if the diver did not stay at one depth for the majority of the dive. Several factors play into the planning necessary. For example, age, condition, body type (nitrogen is stored more effectively by fat), and water temperature all impact ascent rates and stages.

Ascent and descent calculations get more complicated because rates change when tanks contain differing mixes of oxygen, helium and nitrogen. The good news is that this complexity can be made simple by entering the factors into a dive computer. A dive computer is a small wristwatch that keeps an eye on the dive and tells you at what rates you can ascend safely, how many steps and how long at each stop.

In the old days, before dive computers, divers used tables printed on plastic cards or past experience to determine how to ascend safely. Today, dive computers do this work for divers, mostly. There are still some divers in certain situations that don’t use dive computers and instead do the calculations manually or not at all.  Novice divers should always use a dive computer.

The problem with dive tables is that they assume what divers call a ‘square dive.’ A square dive is when you descend and then stay at one depth for the whole dive until you ascend. The problem is that many dives are not ‘square.’ Many dives involve spending time at a variety of depths. So, unless your dive is ‘square’, dive tables won’t be totally accurate.

The bottom line is that all divers should invest in a good dive computer and use it every dive. The best way to be safe is to use a dive computer and learn best practices from a seasoned dive instructor. Decompression sickness is serious business and it can wreck more than just a dive. Be careful, dive safely and live to dive another day.

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