equal to atmospheres. In diving we also measure pressure in terms of the equivalent height of a column of fresh water or seawater–meters of seawater (msw), feet of fresh water (ffw), etc. Conversions: equivalents of one atmosphere of pressure 1 atm = 760 mm Hg or torrs (exactly) = 29.92 inches Hg = 101.3 kilopascals (kPa) = 1.013 bars = 1,013 millibars (mb) = 1.033 kg/cm2 = 1,033 gm/cm2 = 14.7 lbs/in2 (psi) = 10.33 meters of fresh water (mfw) = 33.9 feet of fresh water (ffw) – 34 ffw is usually used = 10.08 meters of seawater (msw) – 10 msw is usually used = 33.1 feet of seawater (fsw) – 33 fsw is usually used Note: Ten meters of seawater is very close to exerting exactly 100 kPa or 1 bar of pressure (101.3 kPa/10.08 msw = 100.5 kPa/10 msw), so 10 msw = 1 bar = 100 kPa = 1 atm is commonly used unless exactitude is required. Also, all of the above values are equated to one atmosphere, so it is simple to use the above table to obtain a conversion factor for any combination of units. For example, to convert from psi to bars, multiply by 1.013 bars/14.7 psi. Atmospheric pressure varies around the globe, and it decreases with altitude as the weight of the air above decreases. Unlike water pressure, the decrease is not linear with altitude. Pressure decreases more rapidly near the surface (about 3.5 mb per 30 m/100 feet) and less rapidly with increasing altitude. One must ascend to an altitude of about 5,500 m/18,000 feet to reduce the pressure to one-half atmosphere. Also, as Torricelli discovered, atmospheric pressure varies with the season and the weather. The highest sea-level pressure that has ever been recorded is 1,084 mb, and the lowest is 876 mb. Hydrostatic pressure, like atmospheric pressure, results from the weight of the fluid (water) on the immersed body and is equal in all directions at a specific depth. Some useful equivalents for given depths of water are: 1 meter of seawater (msw) = 0.1 bar or 0.1 atm 1 meter of fresh water (mfw) = 0.097 bar or 0.097 atm (0.1 bar = 1.033 mfw) 1 foot of seawater (fsw) = 0.445 psi 1 foot of fresh water (ffw) = 0.432 psi Absolute Pressure and Gauge Pressure Ordinary gauges show the pressure in excess of atmospheric pressure. This is called gauge pressure to distinguish it from the absolute pressure or total pressure. The best example for divers is a depth gauge. A depth gauge measures the water pressure and displays the value in meters or feet of water (usually seawater). The depth gauge reads zero at the surface even though the pressure there is actually one atmosphere/10 msw/33 fsw. A cylinder pressure gauge also displays gauge pressure. When the gauge reads zero, there is still one atmosphere of pressure in the cylinder, not a vacuum. For most calculations absolute pressures must be used. The absolute pressure at any depth of water is the sum of the atmospheric and hydrostatic pressures. NAUI Master Scuba Diver 82 Diving Physics 12" 12" 12" 144 Square Inches 64 Pounds Total One Square Inch = 0.445 lbs. FIGURE 3-7. ONE CUBIC FOOT OF SEA WATER (64 POUNDS) EXERTS 0.445 POUNDS PER SQUARE INCH (64/144=0.445). THIS PRESSURE MULTIPLIED BY 33 FEET OF SALT WATER EQUALS 1ATM OF PRESSURE
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