Monday, October 20, 2014

1–41E

1–41E A manometer is used to measure the air pressure in
a tank. The fluid used has a specific gravity of 1.25, and the
differential height between the two arms of the manometer is
28 in. If the local atmospheric pressure is 12.7 psia, determine the absolute pressure in the tank for the cases of the
manometer arm with the (a) higher and (b) lower fluid level
being attached to the tank.
ANS
1-41E The pressure ina tank ismeasured witha manometer bymeasuring the differentialheightof the
manometer fluid. The absolutepressure inthe tank isto be determined for the cases of the manometer arm
withthe higher and lower fluid levelbeing attached tothe tank .
Assumptions The fluid in the manometer is incompressible. Properties The specific gravityof the fluid isgiven to be SG = 1.25. The density of water at 32°F is62.4 lbm/ft 3 (Table A-3E) Analysis The density of the fluidis obtainedbymultiplying its specific gravity bythe density ofwater, 3 3 O H lbm/ft 0 . 78 ) lbm/ft 4 (1.25)(62. SG 2 = = × = ρ ρ The pressure difference corresponding to a differential heightof 28 inbetween the two armsof the manometer is psia 26 . 1 in 144 ft 1 ft/s lbm 32.174 lbf 1 ft) )(28/12 ft/s )(32.174 lbm/ft (78 2 2 2 2 3 =             ⋅ = = ∆ gh P ρ Thenthe absolute pressures in the tank for the two cases become: (a) The fluidlevel inthe armattachedtothe tankis higher (vacuum): psia 11.44 = − = − = 26 . 1 7 . 12 vac atm abs P P P (b) The fluidlevel inthe armattachedtothe tankis lower: psia 13.96 26 . 1 7 . 12 atm gage abs = + = + = P P P DiscussionNotethatwe can determine whether the pressure ina tank isabove or below atmospheric pressure bysimplyobserving the side of the manometer armwiththe higher fluid level.

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