Giant Steps

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.

Physics of Diving. The Diving Environment - Water. Conduction & Sound.

Conduction

Water has a colossal capacity for conducting heat away from the body. The high heat capacity and rate of conductivity of water are such that thermal protection is needed in all but the warmest tropical waters. An unclad diver in waters of less than about 21C will lose heat faster than his body can replace it, and he will become chilled. In extreme cases, hypothermia will follow. Protective clothing is necessary to avoid chilling.

Sound

Because water is such a dense medium, sound travels more than four times faster in water than in air. It is anything but a silent world. Since sound travels so quickly it is difficult to determine the exact sound source. However, sound signals made by rapping a stone or knife handle against an aqualung cylinder in a distinct code is a popular way of attracting a dive partner’s attention.

Sounds made above water will not penetrate the surface, and neither will underwater sounds pass through into the air.

Physics of Diving. The Diving Environment - Water & Buoyancy

ARCHIMEDES’ LAW states that any object immersed in a fluid suffers an upthrust equal to the weight of fluid it displaces, i.e., whose volume it occupies. If the immersed body is that of a diver, he has the facility to vary the volume he displaces by breathing. With full lungs he will displace more water than his body weight and he would be positively buoyant. When he breathes out, he may displace less water than his body weight and will sink, being negatively buoyant. Somewhere between the two is the desired state of neutral buoyancy.

A diver seeks to adjust his buoyancy to suit the varying requirements of his diving. In the vast majority of situations he will try to attain neutral buoyancy, i.e. a precise equality between his total weight and the upthrust due to the displaced water. This is achieved in a simple way. The diver, kitted up as the dive demands, launches himself into the water and exhales hard. By emptying his lungs he is reducing his body buoyancy and he should sink. He inhales from his aqualung, increases his buoyancy and floats upwards. He adds or subtracts weights from his weightbelt until the normal span of breathing bridges the gap between sinking (negatively buoyant) and floating upwards (positively buoyant). He is now neutrally buoyant.

However, there are other factors which will affect his state of neutral buoyancy while he dives. These are dominated by two effects, which act quite differently from each other:

CHANGE OF WEIGHT

A 1700 litre compressed air bottle contains about 2 kg of air when full and much less than 1% of this figure when empty. Thus, a diver starting with 1700 litres of air will end his dive some 2 kg lighter. This excess buoyancy can be a considerable embarrassment at the end of a dive, especially if decompressing or returning along the bottom to avoid heavy waves on the surface.

CHANGE OF VOLUME

A rubber diving suit, whether it is a foam wet suit or a dry suit covering woollens, reduces heat loss from the body by interposing an insulating layer of air between skin and water. The volume of this trapped air varies in inverse proportion to the hydrostatic pressure acting on it (see Hydrostatic Pressure and Boyle’s Law) so that at a depth of 30 m the air will occupy only one quarter of its volume at the surface. The average 5 mm thick neoprene suit contains about 6 litres of nitrogen bubbles which makes the diver considerably buoyant on the surface. At a depth of 20 m these bubbles will have been compressed to about 2 litres with a corresponding reduction in buoyancy which will, however, be regained on ascent.

The free diver has a variety of methods of countering these inevitable changes in buoyancy. By swimming downwards if too light, or upwards if too heavy, he can overcome an imbalance of several kilogrammes, but this is extremely tiring and is to be strongly discouraged. A less tiring method of balancing changes of buoyancy is provided by controlling one’s breathing. The human lungs contain on average about 6litres when fully extended and a residual volume of about 1.5 litres after complete exhalation. Thus, the diver can vary his volume by as much as 4.5litres simply by forcibly breathing in and out; this is equivalent to a change of 4.5 kg of displaced water. The range of normal breathing covers only the middle 20% of this range, so that a neutrally balanced diver breathing normally will experience a regular change of buoyancy from about 0.5 kg too light, when he breathes in, to 0.5 kg too heavy when he breathes out. But by controlled breathing he can maintain an average change in his buoyancy of up to 1.5 kilogrammes.

Really deep breaths retained for all but brief periods of exhalation followed by immediate inhalation will make the diver about 1 kg more buoyant than when he breathes naturally. The converse, short shallow breaths designed to reduce one’s buoyancy is less easy and may lead to panting, which for a diver is a very inefficient and possibly hazardous way of breathing.

The simplest means of adjusting buoyancy while diving is to use an ABLJ or other buoyancy aid, which can be inflated - by direct feed, cylinder or mouth- to restore neutral buoyancy at will. On ascent, the air can be vented as it expands, thereby avoiding the dangers of a rapid ascent. The use of buoyancy aids for maintaining neutral buoyancy during a dive should be looked upon as a sensible practice. However, it requires a full appreciation of the possible dangers and good technique in handling your equipment. It should NOT be used as an excuse for not being correctly weighted at the start of a dive.

So far, we have considered methods for adjusting buoyancy continually during a dive. Now we must consider how much constant ballast should be carried in the form of lead weights on a quick-release belt. While the basic technique of achieving neutral buoyancy has been explained above, there are occasions when it is desirable to be slightly otherwise than neutrally buoyant. This is really a matter of philosophy based on physics: in general, it is more convenient to be slightly overweight during the early stages of a dive (both to assist the initial descent and to help keep on the bottom once there) than to be too light at the end of a dive, which will probably be in shallow water. So one carries sufficient ballast to ensure neutral buoyancy at a depth of, say, 5 m with empty cylinders. To achieve this, the diver with a 2000 litre cylinder should aim to be about 1.5 kg heavy when on the surface at the start of his dive. Carry out a normal buoyancy check at the surface, then add an extra 1.5 kg to the weightbelt.

THE USE OF BUOYANCY FOR LIFTING

One of the most convenient ways to lift a heavy object from the seabed is to fill one or more plastic drums with the exhaled air from one’s aqualung. This system has the particular merit of affording a constant-buoyancy system at low cost. An air-filled 12.5 litre drum displaces 12.5 kg of water, so if its mass is 2 kg, the net buoyancy will be 10.5 kg. As the object rises, the air in the drum will expand and the excess will flow freely from underneath, leaving the displacement, and hence the buoyancy, constant. In general, it is best to use slightly too little buoyancy when raising a heavy object by this method, the remainder being supplied by pulling on a rope from the surface. Otherwise, if the buoyancy exceeds the object’s weight, it will rise up with increasing speed until the drums break surface, overturn and fill with water, with perhaps disastrous results.

If, on the other hand, it is decided to use closed bags or balloons, they must be provided with an exhaust valve to allow the expanding air to escape as the object rises. These bags should always be blown up taut on the bottom; if an oversize, partially filled bag is used, its buoyancy will increase as it approaches the surface, giving a spectacular, but quite uncontrolled ascent.

Physics of Diving. Our Normal Environment - Air

The diver is affected by increasing water pressure as he descends and this manifests itself in several ways. Some will be noticed quickly: others will take longer to become apparent. Both the diver’s body and his equipment will be affected. Divers should have a clear understanding of how the laws of physics apply to them and to their equipment. Without this knowledge they put themselves at risk.

Before considering the diving environment, it is necessary to look at the atmosphere in which we normally live and the gases which make up the air we breathe.

ATMOSPHERIC PRESSURE. The earth is surrounded by an envelope of air which we call the atmosphere. Air is a mixture of gases, and like all matter, it has mass. A mass exerts a force on those things which lie beneath it, and at sea level the atmosphere presses down with a force of approximately 1 kilogram for every square centimetre of the earth’s surface. Gas pressure is commonly measured in units of bar and our own atmosphere exerts a pressure at sea level of approximately 1 bar.

Atmospheric Pressure = 1.02 bar (1 bar approx.)

Atmospheric pressure varies slightly with changes in weather and diminishes with altitude until it reaches zero at the extreme limit of the atmosphere. At about 5000 m above sea level, for example, the atmospheric pressure is about 0.5 bar.

Our bodies do not suffer in any way from this pressure which is applied to every square centimetre of their surface-we are born to it!

GAUGE PRESSURE. When a pressure is to be measured, it is normal practice to relate it to ambient pressure. Thus a simple gauge would read zero at an atmospheric pressure of 1 bar. An aqualung contents gauge would perhaps read 200 bar, but this really means 200 bar above the normal atmospheric pressure of 1 bar. Such a recording would be known as a Gauge Pressure.

ABSOLUTE PRESSURE. If the above gauge were related to true zero as found in a vacuum it would read 201 bar - the extra 1 bar being atmospheric pressure.
Such a gauge reading would be termed an Absolute Pressure.

Absolute Pressure = Gauge Pressure + Atmospheric Pressure

In diving physics, it is normal to work in absolute terms, and the reasons for doing so will be soon apparent.

COMPOSITION OF AIR. The air we breathe is a mixture of gases comprising:

Nitrogen (N2) approx. 79% (say, 4/5)
Oxygen (02) approx. 21% (say, 1/5)

There are traces of Carbon Dioxide (C02) and other rare or inert gases, but in such small quantities that they can be ignored. All gases are compressible, having neither shape nor volume.

On the other hand, liquids have a definite volume and mass and may be considered to be incompressible at the pressures we are to consider.