1–42

1–42 The water in a tank is pressurized by air, and the
pressure is measured by a multifluid manometer as shown in
Fig. P1–42. Determine the gage pressure of air in the tank if
h1 0.2 m,h2 0.3 m, and h3
0.46 m. Take the densities
of water, oil, and mercury to be 1000 kg/m
3, 850 kg/m3, and13,600 kg/m3, respectively.

ANS

1-42 The pressure ina pressurized water tank ismeasured bya multi-fluid manometer. The gage pressure 

ofair inthe tankis tobedetermined. 

Assumptions The air pressure inthe tankis uniform(i.e., its variationwithelevationis negligible due to its 

low density), and thus we can determine the pressure at the air-water interface. 

Properties  The densities of mercury, water, and oil are given to be 13,600, 1000, and 850 kg/m

3

, 

respectively. 

AnalysisStarting with the pressure at point 1 at the air-water interface, and moving along the tube by 

adding (as we go down) or subtracting (as we go up) the  gh ρ terms until we reach point 2, and setting the 

result equal to Patm

since the tube isopen tothe atmosphere gives