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The effects of Pressure whilst Scuba diving

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Scuba diving is a sport that many people enjoy but very few understand the physics behind. Physics is vital for divers, whether recreational or professional, as without an understanding of physical principles, diving would become unimaginably dangerous. Lots of interesting physics is involved in diving, the effects of buoyancy must be taken into account by divers to ensure that they have control of their bodies during a dive.

The phenomena of light and sound under the sea are also very different to those at the surface, Light is gradually filtered out as a diver descends, starting with red light each colour of the visible spectrum is lost, the last colour to be filtered out by the water column is blue (hence why many underwater pictures appear to be blue) until at around 70m all visible light has gone.

Sound travels much faster and further in the sea, this confuses the brain and means that sounds can appear to be all around a diver as the brain can only detect distance and direction of sounds by the time difference that they are detected in each ear, as the sounds travel much faster, the brain thinks that the sound must be all around, sounds also travel much further in water that in air, so quiet sounds are amplified and can lead to disorientation. (this is also why it is important for submarines to be quiet to avoid detection).

I have chosen to focus on the effects of the changes in pressure experienced by divers, as I feel this is the most important aspect of physics that needs to be appreciated for safe Scuba diving. A brief history of diving The first recorded incidence of diving comes from ancient Greece (1), divers jumped into the water carrying a 15kg stone, sank to around 30 metres and cut the sponges away from the sea bed. Divers were capable of holding their breath for a maximum of 5 minutes.

Later, the ancient Greeks developed a crude system using a submerged cauldron that maintained a bubble of air when sunk and could allow for extended but still very limited periods underwater. Diving helmets connected to surface air supplies, appeared in the early 19th century. A frenchman called Paul Lemaire d’Augerville invented and made a diving apparatus with a copper backpack cylinder, the first sucessful attempt to dive with a personal air supply like modern divers. It was used down to 15 or 20 meters for up to an hour in salvage work.

In around the 1930’s, the first instance of people swimming with Scuba equipment occurs, before this all systems used had been to heavy to allow people to swim whilst using them. During the second world war there were rumours about several nations, using ‘frogmen’, soldier who would use Scuba type systems so avoid detection. During the second world war the US navy also produced the first dive tables, a safety measure to account for the absorption of nitrogen (see Henry’s law). In the 1950’s recreational diving starts as diving systems become more readily available.

In 1965, Scuba diving is shown in the James Bond film Thunderball, this caused interest in diving to increase dramatically. And finally in 1966 PADI (professional association of dive instructors) is formed, the international governing body of recreational diving. What effects does pressure have on divers? As a diver descends through the water, the pressure increases on their body, as pressure changes, the volume of gases in the diver’s body and soft equipment changes too. Pressure needs to be taken into account as even small increases in depth can lead to drastic changes in buoyancy and air consumption.

The main purpose of a Scuba system is to supply air to a diver without the need to connect themselves to something on the surface. One difficulty encountered is how do we carry enough air, to make a dive worthwhile. The answer comes in the shape of compressed air, which allows the diver to have a much larger supply of air than if it was uncompressed. Remarkably the science behind the air supply is not hugely complicated. The problems encountered are that the nature of gases under pressure is completely different to at the surface.

In fact at certain pressures usually harmless gases can become deadly. The problems with gas The gas problems that scuba divers encounter, have nothing to do with the gas problems of everyday life. In fact when underwater, the characteristics of gases becomes a deadly serious matter. If the behaviour of gases is not properly understood, the consequences can be catastrophic. Surprisingly, the actual supply of gas is not the major issue during diving. The physics required to compress the air into a tank is relatively simple and safe.

The major issue with gases is that they, in comparison to water have much lower densities. For all practical purposes, water does not compress under pressure,(it does, but not enough for Scuba divers to need to worry about), conversely, gases are easily compressible, and their properties change as pressure increases. As a result divers need to be aware of the basic physical formulae that relate to gas. (explained below. ) But, what is to worry about? Scuba tanks are filled with normal air aren’t they?

For the most part, scuba tanks are filled with normal air. a few special high oxygen mixtures are used by professional divers so that divers can spend many hours underwater but the principles and dangers remain the same) The gas that we call ‘air’ is actually a mixture of many gases; the two main components are Oxygen (21%) and nitrogen (78%). Usually the most important gas is oxygen. Obviously humans must have a constant supply of oxygen in order to survive. Oxygen bonds easily with other elements and is thus able to aid the body in carrying nutrients around the body, thereby enabling life.

In spite of its abundance, at surface pressures, Nitrogen has almost no effects on the human body. All the nitrogen we breathe in, is exhaled without any changes to the gas, or the human body. A small amount is absorbed by the body, but at the surface it is not noticeable. The remaining 1% of the air is made up of a variety of other gases, (e. g. methane, hydrogen, carbon dioxide and the noble gases) with the exception of carbon dioxide, these trace gases have little effect divers and don’t really matter, so far as divers are concerned. Oxygen and Nitrogen however do matter.

Oxygen, because without out it, you would die, and Nitrogen, because it can cause lots of serious problems when inhaled under pressure. The effects of nitrogen in fact, are the main reason divers must have a knowledge of the physics of gases. The problems with Nitrogen When diving, the increased pressures cause more of the nitrogen gas to be absorbed by the body. The amount of nitrogen absorbed increases the deeper a diver goes, this is because as pressure increases, the amount of nitrogen atoms present in the gas being inhaled increases, in addition so does the amount of gas that the body tissues can absorb (see Henry’s law below).

The differences in concentration of nitrogen in the air to that of the nitrogen in the body tissues also increases as a diver descends, therefore as well as more nitrogen being absorbed, it is also absorbed much faster. The presence of nitrogen in the blood is not that much of a problem, (Nitrogen narcosis can sometimes occur, this is intoxication of the brain due to an excess of nitrogen, similar to the effects of alcohol.

This intoxication can sometimes be dangerous but a short ascent usually eliminates the effects with no lasting effects. The main issues appear during ascent as nitrogen is forced out of the body. As the pressure decreases the nitrogen gradually dissolves out of the body tissues, being carried to the lungs in small bubbles in the blood and being exhaled normally. If an ascent is made to quickly, lots of bubbles get released into the blood, these can combine to make large bubbles, causing divers to develop a condition known as decompression sickness or ‘the bends’.

These bubbles cause lots of problems ranging from minor discomfort to death, as a result it is very important for divers to ascend slowly so that as much nitrogen as possible is released. The gas laws Due to the dangers of gases under pressure (as shown above) it is now apparent why divers must have a comprehensive knowledge of the physics of gases in order to prevent the serious problems that can happen. There are several laws that govern the nature of gases (a combination of these form the ideal gas equation, no gas is ‘ideal’ however so each law must be applied independently in real life situations).

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