• Home
  • Download PDF
  • Order CD-ROM
  • Order in Print
PRESSURE  DEFINITIONS
PRINCIPLES  OF  HYDRAULICS

Fireman - Navy Firefighter, Fireman training manual
Page Navigation
  34    35    36    37    38  39  40    41    42    43    44  
Figure  2-11.—Relationships  among  gauge  pressure,  atmos- pheric pressure, vacuum, and absolute pressure. the word  pressure,  the  relationships  among  gauge, atmospheric, vacuum, and absolute pressures are shown  in  figure  2-11. GAUGE PRESSURE is the pressure actually shown  on  the  dial  of  a  gauge  that  registers pressure  relative  to  atmospheric  pressure.  An ordinary  pressure  gauge  reading  of  zero  does not  mean  there  is  no  pressure  in  the  absolute sense;  rather,  it  means  there  is  no  pressure  in excess  of  atmospheric  pressure. ATMOSPHERIC PRESSURE is the pressure exerted by the weight of the atmosphere. At sea level, the average pressure of the atmosphere is sufficient  to  hold  a  column  of  mercury  at  the height of 76 centimeters or 29.92 inches. Since a column of mercury 1 inch high exerts a pressure of  0.49  pound  per  square  inch  (psi)  at  its  base, a  column  of  mercury  29.92  inches  high  exerts  a pressure that is equal to 29.92 x 0.49 or about 14.7 psi. Since we are dealing now in absolute pressure, we say that the average atmospheric pressure at sea level is 14.7 pounds per square inch absolute (psia). It is zero on the ordinary pressure gauge. Notice, however, that the figure of 14.7 psia represents the average atmospheric pressure at sea level;  it  does  not  always  represent  the  actual pressure being exerted by the atmosphere at the moment a gauge is being read. Since fluctuations from  this  standard  are  shown  on  a  barometer (an   instrument   used   to   measure   atmospheric pressure),  the  term  barometric  pressure  is used to Figure 2-12.—Typical barometer. describe the atmospheric pressure that exists at any  given  moment.  Figure  2-12  shows  the operating  principle  of  a  typical  barometer. BAROMETRIC PRESSURE is the term used to describe the actual atmospheric pressure that exists at any given moment. Barometric pressure may be measured by a simple mercury column or by  a  specially  designed  instrument  called  an aneroid  barometer. A  space  in  which  the  pressure  is  less  than atmospheric pressure is said to be under partial vacuum. The vacuum is expressed in terms of the difference between the absolute pressure in the space and the pressure of the atmosphere. Most commonly,   vacuum   is   expressed   in   inches   of mercury,  with  the  vacuum  gauge  scale  marked from 0 to 30 in.Hg. When a vacuum gauge reads zero,  the  pressure  in  the  space  is  the  same  as atmospheric pressure—or, in other words, there is no vacuum. A vacuum gauge reading of 29.92 in. Hg would indicate a perfect (or nearly perfect) vacuum.  In  actual  practice  a  perfect  vacuum  is impossible   to   obtain   even   under   laboratory conditions. A reading between 0 and 29.92 in.Hg is a partial vacuum. ABSOLUTE  PRESSURE  is  atmospheric pressure plus gauge pressure, or absolute pressure minus vacuum. For example, a gauge pressure of 300 pounds per square inch gauge (psig) equals an absolute pressure of 314.7 psia (300 + 14.7). Or,  for  example,  consider  a  space  in  which  the measured  vacuum  is  10  in.  Hg;  the  absolute 2-16






Western Governors University

Privacy Statement
Press Release
Contact

© Copyright Integrated Publishing, Inc.. All Rights Reserved. Design by Strategico.