Pressure_measurement

Pressure_measurement - Pressure Measurement Author: John M....

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Pressure Measurement Author: John M. Cimbala, Penn State University Latest revision: 14 April, 2006, 5:30 p.m. Introduction Pressure is defined as the normal force exerted by a fluid per unit area . In this learning module, we discuss various ways to measure pressure. Dimensions, units, and conversion factors for pressure The dimensions of pressure are {} { } 2 2 force mL/t pressure area L P ⎧⎫ == = ⎬⎨ ⎩⎭ , or {} 2 m Lt P = ⎨⎬ . In the metric (SI) system, the pressure unit is the pascal (Pa), where 2 1 Pa 1 N/m = . But since a pascal is a very small unit, the kilopascal (kPa) is more popular. 1 kPa 1000 Pa = . In the English system, the pressure unit is pound-force per square inch (psi), where 2 1 psi 1 lbf/in = . In either unit system, another popular unit of pressure is standard atmosphere (atm), defined as the pressure at standard temperature and pressure at sea level, 1 atm 101,325 Pa 14.696 psi = = . Pressure is often expressed as a head , which is pressure expressed as an equivalent column height of a liquid , as discussed below. For example, it is common to hear someone say something like “The pressure is 3.2 inches of water.” What this means is that the pressure is high enough to push a column of water up 3.2 inches. Here are some unity conversion factors for pressure: 1 psi 1 6.89476 kPa ⎛⎞ = ⎜⎟ ⎝⎠ , o 1 atm 1 760 mm Hg at 0 C = , 2 1 psi 1 144 lbf/ft = , o 0.1333 kPa 1 1 mm Hg at 0 C = , o 1 atm 1 29.92 in Hg at 30 F = , and o 2 0.2490 kPa 1 1 in H O at 20 C = . Absolute, gage, and vacuum pressure The pressure you are used to using in thermodynamics is the absolute pressure ( P or P abs ), defined as pressure measured relative to absolute vacuum (absolute zero pressure). Some pressure measuring devices measure absolute pressure. (Absolute vacuum) P abs < P atm P vac = P gage Many pressure measuring devices, however, measure instead the gage pressure ( P gage ), defined as pressure measured relative to the local atmospheric pressure ( P atm ) . An alternative definition of gage pressure is the difference between absolute pressure ( P abs ) and local atmospheric pressure ( P atm ) . P atm P abs = 0 In equation form, the relationship between absolute and gage pressure is gage abs atm P PP =− . Gage pressure can be positive or negative, depending on whether the absolute pressure is, respectively, greater than or less than the local atmospheric pressure. When the absolute pressure is below atmospheric pressure, some engineers use vacuum pressure ( P vac ), defined as the difference between local atmospheric pressure ( P atm ) and absolute pressure ( P abs ) . In equation form, vac atm abs P . Thus, comparing P gage and P vac , vac gage P P = − . Absolute, gage, and vacuum pressures are illustrated in the sketches to the right for two cases: The top case is for P abs < P atm , and the bottom case is for P abs > P atm . For the bottom case ( P abs > P atm ), P vac is generally not defined since it would be negative. Gage pressure is positive in this case.
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Pressure_measurement - Pressure Measurement Author: John M....

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