The pressure in a pressurized water tank is measured by a multi-fluid manometer. The gage pressure of air in the tank is to be determined. Assumptions: The air pressure in the tank is uniform (i.e., its variation with elevation is 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/m3, respectively. Analysis: Starting 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) th e ρgh terms until we reach point 2, and setting the result equal to Patm since the tube is open to the atmosphere gives: P1+ρ gh(water) +ρgh(oil)−ρgh(mercury)=Patm I cant figure out the: moving along the tube adding (as we go down) or subtracting (as we go up) part. This is an exercise problem from This text book: Thermodynamics: An Engineering Approach Seventh Edition Yunus A. Cengel, Michael A. Boles McGraw-Hill, 2011 Problem: 53 chapter 1.