Principles of naval engineering - Historic Naval Ships Association

HChapter 7-PRINCIPLES OF MEASUREMENTabsolutepressure"(PSIA)30PSIA4.7 PSIAPSIAPRESSURE RELATIONSHIPATMOSPHERIC PRESSURE(AVERAGE AT SEA LEVEL)15.3 PSIG GAGE^PRESSURE(PSIG)OPSIGOIn.Hg,29.92 In.g>. VACUUM(In.Hg.)147.181Figure 7-13.— Relationships among gage pressure,atmospheric pressure, vacuum, andabsolute pressure.sufficient to hold a column of mercury at theheight of 76.0 millimeters or 29.92 inches ofmercury. Since a column of mercury 1 inch highexerts a pressure of 0.49 pound per square inch,a column of mercury 29.92 inches high exerts apressure that is equal to 29.92 x 0.49, or approximately14.7 psi. Since we are dealing now inabsolute pressure, we say that the averageatmospheric pressure at sea level is 14.7 poundsper square inch absolute . It is zero on theordinary pressure gage.Notice, however, that the figure of 14.7pounds per square inch absolute (psia) representsthe average atmospheric pressure at sealevel, and does not always represent the actualpressure being exerted by the atmosphere at themoment that a gage is being read.Barometric pressure is the term used todescribe the actual atmospheric pressure thatexists at any given moment. Barometric pressuremay be measured by a simple mercurycolumn or by a specially designed instrumentcalled an aneroid barometer.A space in which the pressure is less thanatmospheric pressure is said to be undervacuum. The amount of vacuum is expressed interms of the difference between the absolutepressure in the space and the pressure of theatmosphere. Most commonly, vacuum is expressedin inches of mercury, with the vacuumgage scale marked from to 30 inches of mercury.When a vacuum gage reads zero, the pressurein the space is the same as atmosphericpressure— or, in other words, there is novacuum. A vacuum gage reading of 29.92 inchesof mercury would indicate a perfect (or nearlyperfect) vacuum. In actual practice, it is impossibleto obtain a perfect vacuum even underlaboratory conditions.Absolute pressure is atmospheric pressureplus gage pressure or minus vacuum. For example,a gage pressure of 300 psig equals anabsolute pressure of 314.7 psia (300 +14.7). Or,for example, consider a space in which themeasured vacuum is 10 inches of mercuryvacuum; the absolute pressure in this spacemust then be 19.92 or approximately 20 inchesof mercury absolute. It is important to note thatthe amount of pressure in a space under vacuumcan only be expressed in terms of absolutepressure.You may have noticed that sometimes we saypsig to indicate gage pressure and other timeswe merely say psi. By common convention, gagepressure is always assumed when pressure isgiven in pounds per square inch, pounds persquare foot, or similar units. The "g" (forgage) is added only when there is some possibilityof confusion. Absolute pressure, on theother hand, is always expressed as pounds persquare inch absolute (psia), pounds per squarefoot absolute (psfa), and so forth. It is alwaysnecessary to establish clearly just what kind ofpressure we are talking about, unless this isvery clear from the nature of the discussion.To this point, we have considered only themost basic and most common units of measurement.It is important to remember that hundredsof other units can be derived from these units,and that specialized fields require specializedunits of measurement. Additional units of measurementare introduced in appropriate placesthroughout the remainder of this training manual.When you encounter more complicated unitsof measurement, you may find it helpful to reviewthe basic information given here previously.PRESSURE MEASURING DEVICESMost pressure measuring devices usedaboard ship utilize mechanical pressure137