First of all, we should point out that the pressure on a diver under water is the result of two separate forces which act simultaneously upon him or her. These are:
     1.   The weight of the water
     2.   The weight of the atmosphere over the surface of the water. 

The table on the left provides mathematical equivalents necessary for converting barometric pressure units. The various types of pressure exerted upon divers are summarized further below.
As atmospheric pressure increases, the height of the mercury in the tube also increases and vice versa. That is, the weight of the mercury in the tube always corresponds to the atmospheric pressure.

In the middle of the 17^th century Italian scientist Evangelista Toricelli determined the value of normal atmospheric pressure with the following experiment. A mercury-filled glass tube with a section area of 1cm² and a closed end was vertically immersed in a vessel full of mercury (Hg), the open end pointing downwards. The level of the mercury inside the tube decreased to a certain extent. Further decrease was impeded by the atmospheric pressure that acted upon the surface of the mercury in the vessel. It turned out that the mercury level in the tube measured 760mm and weighed 1033g. If water had been used instead, a different tube would be necessary. It would have to be longer as many times as water is lighter than air. The level of the water in the tube would correspond to the atmospheric pressure and would equal 10.33m. Therefore, at sea level air exerts a pressure of 1033g/cm². Having in mind that the total area of the human body is 17,000–18,000cm², it can be calculated that atmospheric air exerts upon us a pressure of 17 to 18 tonnes!

Scientists have proven that the critical point of the mechanical effect of hydrostatic pressure depends on the evolution level of the organisms. Lower unicellular organisms such as spores, bacteria, and viruses can withstand pressures of thousands of atmospheres. A further increase of pressure causes physical and chemical changes in the cellular structures, thus altering the characteristics of the species.

Divers do not feel the great pressure because the tissues of the human organism contain 65% of liquids that practically do not shrink. In inner cavities, the pressure of the inhaled air counteracts the external pressure. During descent, divers usually do not feel the increasing pressure. They only feel a slight difficulty while breathing because they inhale gases that are under a pressure equal to that of the surrounding water. All underwater diving suits ensure the intake of air held under a pressure that corresponds to the depth at which the diver is. Otherwise, the absence of this condition would cause quick death.

Although divers do not feel the pressure itself, its rapid change may lead to different sicknesses. A quick decrease of pressure during ascent is particularly dangerous and may result in a serious disease called decompression sickness. Read more on that in the Medicine Section.

While under water, a diver feels unequal pressure on the different parts of his or her body. Low parts, if in greater depths than the upper body, endure pressure that is greater by .15–.20x10⁵ Pa than the one on the upper body. 

Atmospheric Pressure is produced by the weight of the gases in the atmosphere, acts on every body and in all directions. Its effects are therefore neutralized. At sea level, it equals 14.7 psi or 1.03 kg/cm²; larger values are often expressed in atmospheres. Atmospheric pressure decreases with the increase of height.

Hydrostatic Pressure is produced by the weight of a fluid, acts upon every body in the fluid, and is one and the same in all directions at a particular depth. Its increase rate is .445 psi/foot (1 kg/cm² per 9.75 meters) when descending in seawter, whereas in fresh water it increases at .432 psi/foot (1 kg/cm² per 10 meters).

Absolute Pressure = Atmospheric Pressure + Hydrostatic Pressure
Its units of measurement are pounds per square inch absolute (psia) or kilograms per square centimeter absolute (kg/cm² absolute).

Gauge Pressure is the difference between the absolute pressure and a specific pressure. It is measured with gauges that read zero at sea level. To convert to absolute pressure add 14.7 to the value in psi or 1.03 to the value in kg/cm².

Partial Pressure is the fraction of the total pressure contributed by a gas in a mixture. It is in direct proportion to the volume percentage of the gas in the mixture.
 

The Buoyant Force is a very characteristic force that acts upon all submerged bodies. This is how Archimedes' Principle explains buoyancy:

The following mathematical equation can be derived from Archimedes' Principle:
the buoyancy of a submerged body = weight of displaced liquid – weight of the body. Therefore, we may conclude that:

     1  The body will float if the buoyancy is positive 
         (body weight < weight of displaced liquid).
     2  The body will be suspended if the buoyancy is neutral
         (body weight = weight of displaced liquid).
     3  The body will sink if the buoyancy is negative
         (body weight > weight of displaced liquid).

The buoyant force of a liquid depends on its density, which equals its weight per unit volume.

• The density of fresh water is  62.4 pounds per cubic foot (28.3 kg/ 0.03 m³).
• Seawater, however, is denser: 64 pounds per cubic foot (29 kg/0.03 m³). 

Lung capacity affects the buoyancy of a person. A diver with full lungs displaces a greater volume of water and, according to Archimedes' Principle, is more buoyant than a diver with deflated lungs. With full lungs, a diver’s relative weight is .96-.99; with deflated lungs: 1.021-1.097. Bone structure, bone weight and body fat are other factors that have an effect on buoyancy and vary from person to person. That is why some people float more easily than others.

Divers who wear wet suits often add diving weights to their weight belts to create the negative buoyancy needed for descent. At the desired depth, they adjust their buoyancy to an appropriate level so that work can be accomplished without extra physical efforts to oppose positive or negative buoyancy.

Usually, a person’s weight is slightly less than the weight of the displaced amount of water. For example, a person who weighs 80kg displaces 79dm2 of water, which weighs 79kg, that is, he has about 1kg of negative buoyancy. Balance under water depends on the location of the center of weight and that of buoyancy. If they are situated on a vertical line (the symmetry axis of the human body) and the center of buoyancy is higher than that of weight, equilibrium will be stable. 

The relative weight of seawater is considered 1. That is why the loss of weight of submerged bodies in Newtons corresponds to the displaced volume in liters. The human body has a relative weight of about 1, which is why it weighs little under water
.
To ensure normal descent, it is not enough to regulate the weight of the diver. It is also necessary that the additional weights attached to the weight belt be situated so as to provide stable equilibrium, on condition that the equipment is intact.

All weight forces can be added and presented as a single force applied to the center of weight. Similarly, buoyant forces correspond to the center of buoyancy, located just above the center of weight. The distance between the two centers must be about 20cm. This fact allows the diver to maintain the erect position of his or her body. 

• If the belt weights are situated too high and the center of weight turns out to be above that of buoyancy, the diver will be rotated upside-down.
• If the belt weights are situated too low, the center of weight would be much lower than that of buoyancy. As a result, the diver will be unable to bend or accomplish underwater work.