Atmospheric  Pressure

Figure 2-1.—Device for determining the arrangement of the force, pressure, and area formula. Figure  2-1  illustrates  a  memory  device  for recalling the different variations of this formula. Any letter in the triangle may be expressed as the product or quotient of the other two, depending on  its  position  within  the  triangle. For example, to find area, consider the letter A as being set off to itself, followed by an equal sign. Now look at the other two letters. The letter F  is  above  the  letter  P;  therefore, NOTE:  Sometimes  the  area  may  not  be expressed  in  square  units.  If  the  surface  is rectangular,   you   can   determine   its   area   by multiplying its length (say, in inches) by its width (also  in  inches).  The  majority  of  areas  you  will consider in these calculations are circular in shape. Either the radius or the diameter may be given, but  you  must  know  the  radius  in  inches  to  find the area. The radius is one-half the diameter. To determine  the  area,  use  the  formula  for  finding the area of a circle. This is written A = where A is the area, is 3.1416 (3.14 or 3 1/7 for most calculations), and r2 indicates the radius squared. Atmospheric  Pressure The atmosphere is the entire mass of air that surrounds the earth. While it extends upward for about 500 miles, the section of primary interest is the portion that rests on the earth’s surface and extends upward for about 7 1/2 miles. This layer is  called  the  troposphere. If a column of air 1-inch square extending all the  way  to  the  “top”  of  the  atmosphere  could be   weighed,   this   column   of   air   would   weigh approximately  14.7  pounds  at  sea  level.  Thus, atmospheric pressure at sea level is approximately 14.7 psi. As  one  ascends,  the  atmospheric  pressure decreases by approximately 1.0 psi for every 2,343 feet.  However,  below  sea  level,  in  excavations  and depressions, atmospheric   pressure   increases. Pressures  under  water  differ  from  those  under  air only  because  the  weight  of  the  water  must  be added  to  the  pressure  of  the  air. Atmospheric  pressure  can  be  measured  by  any of  several  methods.  The  common  laboratory method uses the mercury column barometer. The height  of  the  mercury  column  serves  as  an indicator of atmospheric pressure. At sea level and at a temperature of 0° Celsius (C), the height of the mercury column is approximately 30 inches, or 76 centimeters. This represents a pressure of approximately  14.7  psi.  The  30-inch  column  is used as a reference standard. Another device used to measure atmospheric pressure  is  the  aneroid  barometer.  The  aneroid barometer   uses   the   change   in   shape   of   an evacuated  metal  cell  to  measure  variations  in atmospheric pressure (fig. 2-2). The thin metal of the aneroid cell moves in or out with the variation of pressure on its external surface. This movement is  transmitted  through  a  system  of  levers  to  a pointer,  which  indicates  the  pressure. The  atmospheric  pressure  does  not  vary uniformly with altitude. It changes more rapidly at lower altitudes because of the compressibility of the air, which causes the air layers close to the earth’s surface to be compressed by the air masses above  them.  This  effect,  however,  is  partially counteracted  by  the  contraction  of  the  upper Figure 2-2.—Simple diagram of the aneroid barometer. 2-2


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