TORRICELLI ‘S LAW
A gas layer known as atmosphere surrounds the earth, which can reach width of over 20,000 metres. Given that all gases have a weight, this layer of air exerts a pressure on the earth’s surface. This is known as barometric or atmospheric pressure, and its unit of measurement is the atmosphere (1 atm = 1 kg/ cm2).
This pressure varies according to the height of the gas layer and is at its highest at sea level. Since air can be compressed it is most dense in its lowest layer where it is weighed upon by the total height of the atmosphere. Variations in pressure in the atmosphere are not consistent in that as the height of the air varies so does its density, and therefore its weight. Descending underwater, which is not compressible, the increase in pressure is constant at 1 Atmosphere (1 kg per cm2) every 10 metres because there will be no change in density. For the purposes of diving, therefore, atmospheric pressure is considered separate from hydrostatic pressure, and when making calculations it is the sum of the two, known as absolute pressure, that needs to be taken into account.
PASCAL’S LAW
“ If pressure is applied to a non-flowing fluid in a container, then that pressure is transmitted equally in all directions within the container”.
If we transfer this principle to diving we can see the same situation in the human body as the pressure of the air breathed from a cylinder is transmitted in all directions and to all cavities with the same intensity. A body immersed in a fluid is not crushed by the column fluid above it: instead pressure tends to envelop it exerting the same force on its whole surface.
For this reason it is possible to bear this degree of pressure, which otherwise would be unbearable. Given that the human body is subject to a pressure of 1 kg per cm2 on the surface of his body. If it were a solid substance exerting an equivalent pressure on the divers body it would be crushed by a weight of several hundred kilograms. When a diver descends the water presses evenly over the body, allowing him to survive to pressures of several kg per cm2.
ARCHIMEDE ‘S PRINCIPLE
Archimede’ s law states that “ a body immersed in water will receive an up thrust equal to the weight of water it displaces”. This law is at the base of one of the most important procedures that must become automatic for a diver: the assessment of the hydrostatic equilibrium between the body, the equipment and the weight.
Doing this, is important for getting as near neutral balance as possible and therefore more safety and comfort. In order to evaluate this balance is important to consider the relation between volume and weight of:
a) the human body
b) the density of the liquid (fresh water, sea- water)
c) the neoprene suit (specific weight lower than normal body weight)
BUOYANCY IN WATER
Buoyancy in diving refers to the tendency for the body to float caused by the push described in Archimede’s law.
Some common terms to describe it are:
Positive Buoyancy: the tendency of the body to float
Neutral Buoyancy: the body neither float nor sink
Negative Buoyancy: the tendency of the body to sink
The diver may change his buoyancy in different ways. By adding weights to the weight belt, the specific weight is increased causing him to sink. Breathing in and out changes the lung capacity, which in turn changes the volume of the water displaced resulting in an increased likelihood of either floating or sinking. Even small changes of volume can result in considerable change from one type of buoyancy to another. If we apply Archimede ‘s law we can see that an increase in the volume of the lungs of about 3 litres (the equivalent of a deep inhalation) can cause an up thrust of about 3 kg. Generally the divers try to obtain neutral buoyancy by regulating the amount of air in the BCD so that they can swim or dive deeper more easily, or in order to float without too much effort.
BOYLE’S LAW
The Boyle‘s law states: “At a constant temperature the volume of a gas varies inversely with absolute pressure while the density of a gas varies directly with absolute pressure”.
One way to demonstrate this law would be to put an inverted glass filled with air and take it to a depth of 10 metres. The total pressure is 2
bar absolute and the glass will appear half full of air, as water enters the glass filling it halfway thus reducing proportionally the quantity of air.
Given that, when diving each 10 metres adds 1 atm to external pressure, the variation in pressure will be very high at low depth, progressively reducing towards the surface. Variations in the volume are inversely proportional to the increase in pressure. 
Therefore the diver must be careful when approaching the surface because of the even greater increase in the volume of gas inside his body. For this reason the diver must continue to breathe regularly during ascent and never hold the breath. In the case of loss of regulator it is necessary to continue to exhale to keep the respiratory airways open.
NEVER HOLD
THE BREATH!
SIGHT UNDERWATER
Sight doesn’t present particularly problems in diving.
However when the eyes are in direct contact with water the vision is unfocused, everything appear blurred and indistinct because the rays of light coming from the viewed objects undergo a index of refraction different to that of the air, the images is no longer being formed on the retina.
To be able to see clearly underwater it is therefore necessary for the eyeballs to be in contact with air. The clear vision thus obtained can however suffer from slight alteration which is caused by different density in the materials that light rays have to go through to reach the eye:
water, lens of the mask, and air.
Objects appear closer and bigger by about 1/3, water also absorbs light, an effect that increases with wather depth. For example the colour red at 5 meters loses its brightness at 15 metres it appears very dark.the next colour to be afected is orange, then yellow, green, and blue. Therefore in order to able to see the true colours underwater a diving torch must be used.
SOUND UNDERWATER
Underwater sounds are transmitted much more quickly because water has a greater density than air. While the speed of sound in air is 330/ mt per second underwater is 1500 m/ s. The diver will therefore be able to perceive sounds much more clearly and from a greater distance than through air, but it is difficult for him to identify the direction and the distance of the source. This is caused by the considerable difference in time that it takes for the sound to reach the ear. This result in a sensation of being completely surrounded by sound and we are therefore unable to establish the origin.
COMPENSATION
If we subject a non ridged containerfilled with gas to a higher external pressure in order to keep the volume unchanged, it is necessary to increase the internal pressure so that an equilibrium with the former can be achieved. This is called compensation.
Our body contains cavities filled with gas that subjected with pressure are compensated for in two ways. Both of them follow Boyle and Mariotte’s law.
1.INCREASE IN PRESSURE
If the external pressure progressively increases and gas at ambient pressure is introduced into the organ, to compensate, the volume of the organ doesn’t change. When the pressure is reduced this greater quantity of gas must be able to be easily released, in order to avoid an increase of volume. The respiratory airways are compensated automatically as they are in contact to air at ambient pressure through the air supplied by the regulator.
2.REDUCTION OF VOLUME
When the rising external pressure crushes an empty organ such as stomach, intestine, and the middle ear it causes a reduction in their volume. The gases they contain proportionally increase their pressure and the external and internal pressure are always in equilibrium. In the stomach and intestine this variation in volume doesn’t normally entail any problems but in the ear it causes the crash of the Eustachian tube; the connection with the airways is severed causing the inward bending of the ear drum and consequent pain.
Making it necessary to introduce air to bring back the volume of the middle ear to its original dimension and the eardrum to its natural position.
The most common and effective methods for achieving compensation are VALSALVA (forced exhalation with intermittent and simultaneous closure of both nose and mouth) and Freznel manoeuvre (compression of air at the back of the pharynx).
The Valsalva manoeuvre is easy to learn but can be difficult in that it requires a contraction of the thorax which use all the muscles employed for exhalation.
Closing the nose and moving the tongue upwards does the Freznel manoeuvre and backwards; it is a very effective manoeuvre that causes a movement of the pharyngeal walls, which facilitates the open of the Eustachian tube. This is achieved with the use of few major muscles, with minimum effort and without interfering with the circulation. The only drawback is that it is not an easy manoeuvre and it requires some practice.
Whichever technique is chosen, compensation must be carried out before the ear start to hurt: too long a wait can result in reduction in the volume of the air inside the middle ear and that can make the manoeuvre difficult. In addition, it can cause introflexion of the eardrum that may result in damage to the ear.
COMPENSATING THE EQUIPMENT
It is always necessary to compensate the mask, as you go deeper underwater the volume of the air inside the mask is reduced and it is pressed into the soft tissues of the face. This can be avoided by blowing air through the nose into the mask to maintain a constant internal volume. The volume of air in the BCD is also reduced through the increase of pressure and must be continually readjusted in order to maintain neutral buoyancy.
