Chapter 7-PRINCIPLES OF MEASUREMENT 



absolute 

 pressure" 



(PSIA) 



30PSIA 



4.7 PSIA 



PSIA 



PRESSURE RELATIONSHIP 



ATMOSPHERIC PRESSURE 



(AVERAGE AT SEA LEVEL) 



15.3 PSIG 



OPSIG 



GAGE 

 ^PRESSURE 

 (PSIG) 



OIn.Hg, 



>. VACUUM 

 (In.Hg.) 



29.92 In. H g 



147.181 

 Figure 7-13.— Relationships among gage pres- 

 sure, atmospheric pressure, vacuum, and 

 absolute pressure. 



sufficient to hold a column of mercury at the 

 height of 76.0 millimeters or 29.92 inches of 

 mercury. Since a column of mercury 1 inch high 

 exerts a pressure of 0.49 pound per square inch, 

 a column of mercury 29.92 inches high exerts a 

 pressure that is equal to 29.92 x 0.49, or ap- 

 proximately 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 . It is zero on the 

 ordinary pressure gage. 



Notice, however, that the figure of 14.7 

 pounds per square inch absolute (psia) repre- 

 sents the average atmospheric pressure at sea 

 level, and does not always represent the actual 

 pressure being exerted by the atmosphere at the 

 moment that a gage is being read. 



Barometric pressure is the term used to 

 describe the actual atmospheric pressure that 

 exists at any given moment. Barometric pres- 

 sure 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 

 vacuum. The amount of 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 ex- 

 pressed in inches of mercury, with the vacuum 

 gage scale marked from to 30 inches of mer- 

 cury. When a vacuum gage reads zero, the pres- 

 sure in the space is the same as atmospheric 

 pressure— or, in other words, there is no 

 vacuum. A vacuum gage reading of 29.92 inches 

 of mercury would indicate a perfect (or nearly 

 perfect) vacuum. In actual practice, it is im- 

 possible to obtain a perfect vacuum even under 

 laboratory conditions. 



Absolute pressure is atmospheric pressure 

 plus gage pressure or minus vacuum. For exam- 

 ple, a gage pressure of 300 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 inches of mercury 

 vacuum; the absolute pressure in this space 

 must then be 19.92 or approximately 20 inches 

 of mercury absolute. It is important to note that 

 the amount of pressure in a space under vacuum 

 can only be expressed in terms of absolute 

 pressure. 



You may have noticed that sometimes we say 

 psig to indicate gage pressure and other times 

 we merely say psi. By common convention, gage 

 pressure is always assumed when pressure is 

 given in pounds per square inch, pounds per 

 square foot, or similar units. The "g" (for 

 gage) is added only when there is some possi- 

 bility of confusion. Absolute pressure, on the 

 other hand, is always expressed as pounds per 

 square inch absolute (psia), pounds per square 

 foot absolute (psfa), and so forth. It is always 

 necessary to establish clearly just what kind of 

 pressure we are talking about, unless this is 

 very clear from the nature of the discussion. 



To this point, we have considered only the 

 most basic and most common units of measure- 

 ment. It is important to remember that hundreds 

 of other units can be derived from these units, 

 and that specialized fields require specialized 

 units of measurement. Additional units of meas- 

 urement are introduced in appropriate places 

 throughout the remainder of this training man- 

 ual. When you encounter more complicated units 

 of measurement, you may find it helpful to re- 

 view the basic information given here previously. 



PRESSURE MEASURING DEVICES 



Most pressure measuring devices used 

 aboard ship utilize mechanical pressure 



137 



