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3 Pressure Measurement

3.i OBJECTIVES

In Chapter 1, fluid pressur~ p , was defined as the amount of force, F, on a unit area, A, of a substance. Fluid pressure is computed from p = F/A

(3-1)

The standard unit for pressure in SI units is the .pascal (Pa) or N/m z. Whereas the standard unit for pressure in the V.S. Customary System is lb/ftz, the unit lb/inz (psi) i:- more convenient and is used most often. This chapter will foc s on the measurement of fluid pressure. After completing this chapter, y< u should be able to:

1. Define the relationship ::Jetween absolute pressure, gage pressure l and atmospheric pressure. 2. Describe the degree of ~riation of atmospheric pressure near the earth's surface. 3. Describe the properties of air at standard atmospheric pressure. 4. Describe the properties I)f the atmosphere at elevations from s~~ level to 30000 m. 5. Define the relationship :>etween a change in elevation and the change in pressure in a fluid. 6. Describe how a man on eter works and how it is used J) measure pressure. 7. Describe a V-tube man.meter, a differential manometerf a well-fype manometer, and an inclinoo well-type manometer. 8. Describe a barometer (nd how it indj~e::! the value of the local atmospheric pressure. 9. Describe various types f pressure g~ges and pressure transducers.

3.2 When making calculations involving pressure in a fluid, you must make the ABSOLUTE AND GAGE PRESSIJRE

C

ABSOLUTE AND GAGE PRESSURE

measurements relative to .ome refefenc~ pr ss· r~. N{)TITlally the reference pressure is that of atr.osphere, and the resulting easured pressure is alleT1 gggt! pressure. Pre:sswre measured relative to a penect vacuum is call.erl absolute. {J1"e sare. It is extremely important for you to know the abs = Pgage

+ Patm

(3-2)

43

4

Chapter 3 Pressure Measurement

where

P abs Pgage

P alm

= absolute pressure = gage pressure = atmospheric pressure

Figure 3.1 shows an interpretation of this equation graphically. A few basic concepts may help you to understand the equation. 1. A perfect vacuum is the lowest possible pressure. Therefore, an absolute

pressure will always be positive. 2. A gage pressure above atmospheric pressure is positive. 3. A gage pressure below atmospheric pressure is negative, sometimes called vacuum. 4. Gage pressure will be indicated in the units of Pa(gage) or psig. 5. Absolute pressure will be indicated in the units of Pa(abs) or psia. 6. The actual magnitude of the atmospheric pressure varies with location and with climatic conditions. The barometric pressure as broadcast in weather reports is an indication of the continually varying atmospheric pressure. FIGURE 3.1 Comparison between absolute and gage pressure.

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o (a) Absolute pressure

0 Range .of normal variation in atmospheric pressure: 95-105 kPa (abs) 13.8-15.3 psia

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Perfect vacuum (b) Gagepressul