PRINCIPLES OF NAVAL ENGINEERING 



102. 20X 



Figure 7-12. — Optical pyrometer. 



high, or that a column of water in a U-tube 

 would be displaced 1 inch by the pressure being 

 measured. Similarly, a reading of 12 inches of 

 mercury (12 in. Hg) means that the measured 

 pressure is sufficient to support a column of 

 mercury 12 inches high. What is really being 

 expressed (even though it is not mentioned in the 

 pressure unit) is the fact that a certain quantity 

 of material (water, mercury, etc.) of known 

 density will exert a certain definite force upon 

 a specified area. Pressure is still force per 

 unit area, even if the pressure unit refers to 

 inches of some liquid. 



It is often necessary to convert from one 

 type of pressure unit to another. Complete con- 

 version tables may be found in many texts and 

 handbooks. Conversion factors for pounds per 

 square inch, inches of mercury, and inches of 

 water are: 



1 in. Hg= 0.49 psi 

 1 psi = 2.036 in. Hg 



1 in. H2O 

 1 in. Hg 



0.074 in. Hg 

 13.6 in. HgO 



in. H2O: 

 1 psi ■ 



0.036 psi 

 27.68 in. HoO 



In interpreting pressure measurements, a 

 great deal of confusion arises because the zero 

 point on most pressure gages represents at- 

 mospheric pressure rather than zero absolute 

 pressure. Thus it is often necessary to specify 

 the kind of pressure being measured under any 

 given conditions. To clarify the numerous mean- 

 ings of the word pressure, the relationships 

 among gage pressure, atmospheric pressure, 

 vacuum, and absolute pressure, is illustrated 

 in figure 7-13. 



Gage Pressure is the pressure actually shown 

 on the dial of a gage that registers pressure at 

 or above atmospheric pressure. An ordinary 

 pressure gage reading of zero does not mean 

 that there is no pressure in the absolute sense; 

 rather, it means that there is no pressure in 

 excess of atmospheric pressure. 



Atmospheric pressure is the pressure exert- 

 ed by the weight of theatmosphere. At sea level, 

 the average pressure of the atmosphere is 



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