At some point or other on a fine summer day, almost everyone will have tried to momentarily hold their breath, dip their head below water and dive down a metre or so. One of the first things people commonly report is that they experience a somewhat uncomfortable feeling of pressure in their ears (check out the ‘Myth Debunked’ section), which underscores the main topic of the science of diving: pressure and its effect on the human body.
When we walk about on dry land at sea level, the weight of the atmosphere bears down upon us, and we say that we are under a pressure of one atmosphere. However, upon submerging ourselves in water, we have to add (to this one atmosphere) the pressure that the surrounding water exerts on us. Every 10m of seawater adds another atmosphere of pressure, such that at a depth of 10m we experience two atmospheres of pressure (one atmosphere at sea level plus one atmosphere due to 10m of seawater). The deeper we go, the higher the pressure becomes.
Our bodies are adapted to life under just one atmosphere of pressure. On land, when we breathe air, which is roughly composed of 21 per cent life-sustaining oxygen, 78 per cent inert nitrogen and one per cent argon and other trace gases, our bodies metabolise the oxygen but do nothing with the nitrogen.
Upon submerging ourselves underwater, breathing from an air supply that is under pressure (e.g. SCUBA), some changes occur. The various tissues in our bodies absorb the inert nitrogen gas breathed under pressure, much like gas is dissolved in a pressurised soft drink. All is well and good until one remains at depth, but when a diver begins to ascend, the surrounding water pressure starts to drop again, and all the dissolved nitrogen gas starts to come out of solution, again much like what happens when you open up a soft drink bottle and release the pressure.
The problem, as you will surely have had the misfortune of experiencing at some point, is that if the pressure is released suddenly such as when you open the bottle cap too quickly, the dissolved gas forms bubbles and froths out of the bottle. Therefore, it is critical that divers manage their ascent and come up slowly from each and every dive, allowing time for the nitrogen that has dissolved in their body to come out of solution slowly without forming bubbles that can cause myriad problems and, in serious cases, paralysis and even death.
The physics of diving and decompression is a vast topic and the subject of intense research with many questions still remaining unanswered. The above is but a very brief taster of one aspect of dealing with the pressure.
Dr Joseph Caruana is a faculty member at the Department of Physics, Faculty of Science and the Institute of Space Sciences & Astronomy (ISSA) at the University of Malta. In his free time he enjoys researching mathematical models of decompression theory, taking photographs, venturing underwater and playing the piano.
Did you know?
• The first successful SCUBA (Self-Contained Underwater Breathing Apparatus) was designed by Jacques-Yves Cousteau and Émile Gagnan in 1942, and was dubbed the Aqua-Lung.
• Underwater, one can hear sound better than in air, but it is difficult to tell where the sound is coming from.
• The diving reflex, also known as the mammalian reflex, is the process whereby your blood vessels narrow, and your spleen contracts and releases red blood cells, enhancing the blood’s ability to carry oxygen.
• Water removes body heat quicker than air.
• The deeper underwater one goes, the darker it gets. Beyond 200 metres, light is very scarce.
For more trivia see: www.um.edu.mt/think
• For deep, long dives, divers use special machines called rebreathers which recycle the gas breathed by the diver. This means that divers use up just a fraction of the gas they would otherwise consume, which enables them to stay underwater much longer while carrying fewer cylinders. Rebreathers also provide an optimised gas mixture which allows divers to spend much less time decompressing (i.e. getting rid of the excess nitrogen in their bodies). Furthermore, since gas is recycled, no bubbles are released, which allows the divers to get closer to marine life without disturbing it or scaring it away.
• A team of highly trained technical divers are exploring a Phoenician shipwreck from 700BC off the coast of Xlendi. This wreck lies at a depth of 110m, which poses a significant challenge. Due to the risk involved, the planning is meticulous. The divers use rebreathers to extend their bottom time and reduce their decompression obligation. They take about eight minutes to descend to the wreck and spend 12 minutes at a time working on it. This bottom time is then followed by about 2.5 hours of decompression. This expedition to a truly unique wreck is unearthing valuable information about this ancient period of history.
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