## Physical Principles

Many adverse physical conditions are encountered in the underwater environment. These include cold, wetness, changes in light and sound conduction, increased density of the surrounding environment, and increased atmospheric pressure. Of these, the indirect or direct effects of pressure account for the majority of serious diving medical problems.

PRESSURE Pressure is force per unit area and is measured in a number of different units (T.a.ble...„.1..92.-.3). The weight of air at sea level is equal to 14.7 lbs/in2 (psi) or 1 atm absolute (ATA). Under water, pressure increases because of the weight of the water. Because water is much denser than air, large changes in pressure will accompany small fluctuations in depth. Thus, at a depth of 33 ft of seawater (fsw) the pressure is 2 ATA, and at 165 fsw it is 6 ATA ( Table 1..?.2,:4).2 The proportionate change in pressure per unit depth is greatest near the surface and progressively diminishes with increasing depth. Because fresh water is less dense than saltwater, it takes a depth change of 34 ft of fresh water (ffw) to change the pressure 1 ATA. Scuba diving is generally done at pressure of less than 6 ATA, with the overwhelming majority in the 2- to 4-ATA range.

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