Themes > Science > Physics > Mechanics > Fluid Mechanics > Worked Examples > Manometers Pressure and Manometers 1.1 What will be the (a) the gauge pressure and (b) the absolute pressure of water at depth 12m below the surface? rwater = 1000 kg/m3, and p atmosphere = 101kN/m2. [117.72 kN/m2, 218.72 kN/m2] a) b) 1.2 At what depth below the surface of oil, relative density 0.8, will produce a pressure of 120 kN/m2? What depth of water is this equivalent to? [15.3m, 12.2m] a) b) 1.3 What would the pressure in kN/m2 be if the equivalent head is measured as 400mm of (a) mercury g=13.6 (b) water ( c) oil specific weight 7.9 kN/m3 (d) a liquid of density 520 kg/m3? [53.4 kN/m2, 3.92 kN/m2, 3.16 kN/m2, 2.04 kN/m2] a) b) c) d) 1.4 A manometer connected to a pipe indicates a negative gauge pressure of 50mm of mercury. What is the absolute pressure in the pipe in Newtons per square metre is the atmospheric pressure is 1 bar? [93.3 kN/m2] 1.5 What height would a water barometer need to be to measure atmospheric pressure? [>10m] 1.6 An inclined manometer is required to measure an air pressure of 3mm of water to an accuracy of +/- 3%. The inclined arm is 8mm in diameter and the larger arm has a diameter of 24mm. The manometric fluid has density 740 kg/m3 and the scale may be read to +/- 0.5mm. What is the angle required to ensure the desired accuracy may be achieved? [12 39']

 Volume moved from left to right = The head being measured is 3% of 3mm = 0.003x0.03 = 0.00009m This 3% represents the smallest measurement possible on the manometer, 0.5mm = 0.0005m, giving 1.7 Determine the resultant force due to the water acting on the 1m by 2m rectangular area AB shown in the diagram below. [43 560 N, 2.37m from O] The magnitude of the resultant force on a submerged plane is: R = pressure at centroid area of surface This acts at right angle to the surface through the centre of pressure. By the parallel axis theorem (which will be given in an exam), , where IGG is the 2nd moment of area about a line through the centroid and can be found in tables. For a rectangle As the wall is vertical, , 1.8 Determine the resultant force due to the water acting on the 1.25m by 2.0m triangular area CD shown in the figure above (with question 1.7). The apex of the triangle is at C. [43.5103N, 2.821m from P] For a triangle Depth to centre of gravity is . Distance from P is Distance from P to centre of pressure is Information provided by: http://www.efm.leeds.ac.uk