The figure bellow indicates a u-tube where the left tube is connected to a higher pressure than the right tube. Accuracy can be improved by inclining the u-tube manometer. P d = (9.8 kN/m 3 ) (10 3 N/kN) (10 mm) (10 -3 m/mm)ĩ.8 (kN/m 3 ) is the specific weight of water in SI-units.Ī common problem when measuring the pressure difference in low velocity systems - or systems with low density fluids - like air ventilation systems - are low column heights and accuracy. The pressure difference head can calculated from (1) as The difference height in the water column is 10 mm. Example - Orifice Differential Pressure MeasurementĪ water manometer connects the upstream and downstream pressure of an orifice located in an air flow. For other units and reference liquid - like mm Water Column - check Velocity Pressure Head. Note! - the head unit is with reference to the density of the flowing fluid. The specific weight of water, which is the most commonly used fluid in u-tube manometers, is 9.81 kN/m 3 or 62.4 lb/ft 3. H = liquid height (m fluid column, ft fluid column) Ρ = U-tube liquid density (kg/m 3, lb/ft 3 ) Γ = ρ g = specific weight of liquid in the tube (kN/m 3, lb/ft 3 ) The pressure difference measured by a vertical U-Tube manometer can be calculated as In the figure bellow illustrates the water levels in an u-tube where the left tube is connected to a point with higher pressure than the right tube - example: the left tube may be connected to a pressurized air duct when the right tube is open to the ambient air. One of the most common is the water filled u-tube manometer used to measure pressure difference in pitot or orifices located in the airflow in air handling or ventilation system. Pressure measuring devices using liquid columns in vertical or inclined tubes are called manometers.
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