Physics of Diving. The Diving Environment - Water & Gases
HYDROSTATIC PRESSURE
Water is a dense medium and, therefore, exerts a noticeable pressure upon anything which is immersed in it. Water pressure increases rapidly with depth and a cubic metre of water (1000 litres) has a mass of 1000 kilograms or one tonne. Some fairly simple arithmetic will reveal that, if our cubic metre is divided up into one metre high columns, each of one square centimetre cross section, that the mass of water in each column is 0�1 kg. If each 1 cm2 column were extended to 10 m in length, the mass would be 1 kg and the pressure exerted by the column would be 1 kgf/cm2.
But 1 kgf/cm2 = 1 bar (approx). So, at 10 m beneath the surface the water pressure or hydrostatic pressure is equal to the atmospheric pressure at the surface. 10 m of water is equal to 1 bar gauge pressure or 2 bar absolute, and for every further descent of 10 m beneath the surface, the hydrostatic pressure increases by another bar. Thus at 30 m the absolute pressure is 4 bar.
In a fluid, pressure has the particular property of acting in all directions: thus, 30 m down the body is subjected evenly to 4 bar absolute all over and in all directions. The reader will recognise that this is so when he considers the pressure of the water inside an underwater cave: although it may be largely covered with rock, not water, the pressure inside will exactly equal that of the open sea at the same depth, the pressure being transmitted horizontally.
As the human body consists largely of liquid, it takes up the ambient hydrostatic pressure without any decrease in volume, but the spaces that contain air (for example the lungs) will be compressed unless they are artificially filled with air of pressure equal to that of the surrounding water. The aqualung demand valve will supply the diver with air at ambient pressure, but this subject should be studied further because it affects the body in many ways. The behaviour of gases under pressure needs to be considered.
PRESSURE/VOLUME CHANGES
When a gas is compressed, its volume varies in inverse proportion to the absolute pressure. This is the basis of BOYLE’S LAW-a relationship first recorded by the early physicist of that name.
Thus, an inverted bucket which is full of air at the surface where the pressure is 1 bar will be only half full at a depth of 10 m, where the total pressure is 2 bar, and only a quarter full at 30 m (4 bar absolute). Here we see that the fractional change in the gas volume for a given change of depth decreases with depth. Thus, a change of 10 m near the surface halves the volume, while the same 10 m drop at 40 m only reduces the volume by a factor of one-sixth.
Divers will encounter the effects of this relationship during training, and several times - and in several ways - on every dive, whether snorkelling or aqualung diving. Ear clearing, mask squeeze, loss of buoyancy, function of a demand valve, ascent risks, air consumption, decompression - ALL are governed and affected by Boyle’s Law. Any compressible air space, be it in the diver’s body or in his equipment, will change its volume during descent and ascent, and if not equalised or controlled, damage of some sort can occur. The term barotrauma is used to describe injuries which result from sudden changes in air pressure: in other words, from failure to allow Boyle’s Law to happen safely.
PARTIAL PRESSURES
It was explained earlier that nitrogen makes up approximately four-fifths of the atmosphere and oxygen the other fifth. If the atmospheric pressure is 1 bar, is it not reasonable to assume that nitrogen is responsible for 0.8 bar and. oxygen for 0.2 bar? Correct, and these are known as the Partial Pressures. DALTON’S LAW of Partial Pressures states that the total pressure of a gas is equal to the sum of the partial pressures which each member gas has and would alone have if the others were absent. Thus, while at sea level the partial pressure of oxygen is approximately one-fifth bar and nitrogen is approximately four-fifths bar, the air breathed by a diver 40 m (5 bar absolute) below the surface contains nitrogen at 4 bar and oxygen at 1 bar, the total pressure being 5 bar. The importance of this Law lies in the fact that the physiological effect of a gas depends upon its pressure or, when in a mixture such as air, upon the partial pressure.
Dalton’s Law reveals itself in such conditions as oxygen poisoning, carbon dioxide and carbon monoxide poisoning, and nitrogen narcosis. An understanding of partial pressures also helps in the study of circulation, respiration, hypoxia and decompression.
SOLUBILITY OF GASES
When a gas is brought into contact with a liquid (e.g. when the air in the lungs comes into contact with the blood) then some of the gas will dissolve in the liquid. The amount that will dissolve and the rate at which this takes place is dependent upon several factors-the pressure of the gas, the contact area between gas and liquid, the temperature, the maximum solubility of the gas in the liquid. As the gas nears saturation level, so the rate of solution decreases. If gas has dissolved in a liquid, and if the prevailing conditions are varied, then the amount of dissolved gas may also vary.
This relationship was established by yet another learned scientist of old, and is known as HENRY’S LAW. The fact that gas will dissolve into the bloodstream and be released again when the ambient pressure is reduced, gives rise to the problems of decompression sickness.
TEMPERATURE OF GASES
Temperature affects both Boyle’s and Henry’s Law, but since temperature variations encountered in diving are very limited, for simplicity, these effects have been ignored. One other gas law which is of interest and which involves temperature is CHARLES’ LAW. The volume of a gas varies directly as its absolute temperature if the pressure remains constant. Usually, it is the volume which is constrained to remain constant, while the pressure goes up! For example, an inflatable boat, left in the hot sun, could suffer from expansion of the contained air to the point of explosion. Keep in the shade or the boat partly deflated when not in use.
Water has several other properties: buoyancy: conduction of heat: and transmission of sound. These will now be considered.