The bends, or Decompression Sickness, occurs when nitrogen bubbles in the blood lodge in certain parts of the body. A bubble caught in a vital organ can be fatal. Before much was known about decompression sickness, divers got bubbles lodged in their joints forcing them to “bend” over in pain.
Nitrogen In the Blood
A diver breathes air from the scuba tank that contains approximately 79 % nitrogen and 21 % oxygen. The pressure of being underwater forces the nitrogen into the body’s fatty tissues. The longer and deeper the dive, the more nitrogen is forced into the tissues. For example, at 10 metres the partial pressure of each gas is doubled, so twice as much nitrogen is absorbed into the tissues as on the surface. At 30 metres, four times as much nitrogen is absorbed.
When the diver comes to the surface; that is, moves from a greater pressure to lesser pressure, the nitrogen comes out of the tissues back into the blood stream. This is like a bottle of beer being opened. The gas is kept in solution by the pressure under the cap; when this is removed the gas bubbles out.
Getting this nitrogen out of the tissues at the correct rate is the challenge for divers.
How to Prevent the Bends
To prevent being “bent”, a diver must slow the rate of nitrogen coming out of the tissues. This is achieved by reducing the change in pressure by slowing the ascent, even stopping part of the way.
With the beer bottle; if the cap is opened a fraction and the pressure equalizes slowly, there is little bubbling. It is this slow equalizing of the pressure, or decompression, the diver must go through during ascent to the surface.
Decompression Stops
Most decompression stops are done three metres below the surface, but if a diver had been very deep or underwater for a long time, then the stops might be ten metres, six metres, and then three metres. The longest decompression stop is at three metres because the greatest pressure change occurs in the top three metres of water.
The time spent at each stop depends on the length and depth of the dive. A dive to thirty metres for thirty minutes would require a stop of three minutes at three metres, but a dive to forty metres for the same length of time would require a stop of fourteen minutes at three metres.
This time spent decompressing can be unnerving for divers, as the surface is so close but the diver is not allowed to go up: the natural environment could kill!
Planning To Eliminate The Bends
Divers plan their dives by using a dive computer or decompression tables. These provide the time and depth for a decompression stop for the particular dive.
Dive computers are obviously the optimum device, as they do all the calculations and can instantly cater for changes in a dive plan. That is, for extra depth or times. Dive computers can be incorporated into the regulator’s gauges, a separate unit or within a dive watch.
Dive tables are useful for a diver to gain an understanding of the times and depths.
As each individual body will absorb nitrogen at different rates, tables and dive computers should be used only as a guide. Even when well within the parameters specified by the tables, cases of the bends have occurred. It is good diving practice to add in a safety margin by decreasing the duration of the dive and increase the length of the decompression stop.
The bends is a serious diving illness but one that can be avoided by taking the correct precautions. Like other sports, the danger exists but can be reduced by thoughtful planning.
Bruce Iliff - Hello from an Australian freelancer! I’ve been scratching articles together in Australia in a range of subjects for a number of ...
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