Physics of scuba diving

The effects of Pressure whilst Scuba diving

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?

This is a preview of the whole essay

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).

The main laws which a diver must consider are as follows:

Boyle's law-

For a fixed amount of an ideal gas kept at a fixed temperature, P [pressure] and V [volume] are inversely proportional (while one increases, the other decreases)

Sir Robert Boyle. c.1662

Charles' Law-

At constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature on the absolute temperature scale (i.e. the gas expands as the temperature increases).

Jacques Charles c.1780

Dalton's law-

For a fixed amount of an ideal gas, the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture.

John Dalton c. 1801

Henry's law-

At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

William Henry c. 1803

How do divers use the gas laws?

Boyle's law

Boyle's law describes how gas gets compressed as pressure increases. For every 10metres that a diver descends, the pressure increases roughly twice. By applying Boyle's we see that this means that the volume of the gas must have halved. (as volume x pressure = a constant.) because the volume of air would be half, the air in the tank would last half the amount of time it would at the surface, as the amount of gas inhaled is always the same. At 20m, the pressure is three times that of the surface, hence the gas is one third of the volume and therefore lasts a third of the time etc. generally, For each 10m interval through which a diver descends, the amount of air the diver uses increases by the same factor.

An extension of Boyle's law is as such: Imagine a diver taking a breath of air at 10m under the surface, the diver then ascends to the surface, using Boyle's law, we can deduce that the pressure would halve and thus the volume would double. This means that an excess volume of air would now be trying to fill the lungs, this condition (known as 'pneumothorax') can lead to the lungs collapsing or puncturing. It can also lead to a condition known as 'arterial air embolism', where air bubbles are forced into the blood stream and can cause blockages similar to heart attacks.

Charles' Law

Charles' law describes why when a Scuba tank is filled, its temperature is increased, the same way in which a bicycle pump get hot when it is used. When the tanks are warm, just after being filled at a dive centre, the tanks show a high pressure on the gauges (Pressure gauges are used to show how much gas is left inside a Scuba tank) when the tanks come to be used however, we notice that the pressure in the tank has decreased, this does not mean that the tank is leaking and air has escaped, just that the temperature has lowered and that the overall pressure has decreased.

Dalton's Law

Dalton's law explains how the partial pressures of individual gases, make up the pressure of the gas as a whole. This means that as pressure increases, the ratio of the gases as a whole remains the same. As we can see from Boyle's law, we can see that under higher pressures the amount of gas molecules that are inhaled by a diver increases. If a faulty compressor has pumped a small amount of toxic gas into the tank (for example carbon monoxide) it may have little or no effect at surface pressures, but when at 40metres five times the amount of carbon monoxide molecules are inhaled in one breath, enough to damage the body. Likewise, normally safe gases like oxygen can become toxic and high pressures. (This is why nitrogen-oxygen mixtures as in the air are used in Scuba tanks, as opposed to pure oxygen which would be deadly below certain depths.)

Henry's law

Henry's law is extremely important as it describes how much gas is absorbed into liquid (and due to the large amounts of water in the body, also into body tissues) The problems encountered with excess nitrogen in the body, come from the uncertainty of how fast the gases are expelled from the body. This is different sorts of tissue expel air at different rates, it is very hard to calculate the correct amount of time to ensure safe ascents however, through years of observations, dive tables (and more recently dive tables) give estimates on the amount of time required, these are usually overestimates of the time required but in some cases it is still not enough. The dive tables also give recommended times to leave between dives, to ensure that nitrogen remaining in the body from previous dives is also taken into account.

Other effects of pressure experienced by divers

Another extension of Boyle's law, is that as a diver descends, the gas inside the body gets less dense, as a result the deeper a diver goes, the more able he is to sink. A diver needs to control their buoyancy however, so a scuba system usually incorporates a way to fill a suit with some air, to stop the rate of descent (usually in the form of an inflatable jacket called a buoyancy control device or BCD). As shown above, it is important that when ascending the diver releases this air from the BCD, as the gas inside will increase and cause rapid ascent, a slow controlled descent is required to allow nitrogen to be released from the body.

Conclusion

As I have shown, the effects of pressure that are experienced during Scuba diving are vitally important, and if not taken into account, could cause serious problems.

Without physics, Scuba diving would be impossible!