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 17th 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.
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 17th 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.
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