U-Tube Manometer
The inclined and vertical u-tube manometers are inexpensive and common in differential pressure measurements of flow meters like pitot tubes, orifices and nozzles
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Pressure measuring devices using liquid columns in vertical or inclined tubes are called manometers. 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.
Vertical U-Tube Manometer
The pressure difference in a vertical U-Tube manometer can be expressed as
pd = γ h (1)
where
pd = pressure
γ = specific weight of the fluid in the tube (kN/m3, lb/ft3 )
h = liquid height (m, ft)
The specific weight of water, which is the most commonly used fluid in u-tube manometers, is 9.8 kN/m3 or 62.4 lb/ft3.
Example - Differential Pressure Measurement in an Orifice
A water manometer connects the upstream and downstream of an orifice located in an air flow. The difference height of the water column is 10 mm.
The pressure difference head can then be expressed as:
pd = (9.8 kN/m3) (103 N/kN) (10 mm) (10-3 m/mm)
= 98 N/m2 (Pa)
where
9.8 (kN/m3) is the specific weight of water in SI-units.
Inclined U-Tube Manometer
Common problems when measuring pressure differences in low velocity systems as air ventilation system are the low column heights and satisfying accurately.
The pressure difference in a inclined u-tube can be expressed as
pd = γ h sin(θ) (2)
where
θ = angle of column relative the horizontal plane
Inclining the tube manometer will increase the accuracy of the measurement.
Example - Differential Pressure Measurement with an Inclined U-Tube manometer
We use the same data as in the example above, except that the U-Tube is inclined to 45o.
The pressure difference head can then be expressed as:
pd = (9.8 kN/m3) (103 N/kN) (10 mm) (10-3 m/mm) sin(45)
= 69.3 N/m2 (Pa)
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Related Topics
- Fluid Flowmeters - Flow metering basics - Orifice, Venturi, Flow Nozzles, Pitot Tubes, Target, Variable Area, Positive Displacement, Turbine, Vortex, Electromagnetic, Ultrasonic Doppler, Ultrasonic Time-of-travel, Mass Coriolis, Mass Thermal, Weir V-notch, Flume Parshall and Sluice Gate flow meters and more
- Fluid Mechanics - The study of fluids - liquids and gases. Involves various properties of the fluid, such as velocity, pressure, density and temperature, as functions of space and time.
- Ventilation - Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more
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Related Documents
- Density, Specific Weight and Specific Gravity - An introduction and definition of density, specific weight and specific gravity - formulas with examples
- Pitot Tubes - Pitot tubes can be used to indicate fluid flow velocity by measuring the difference between the static and dynamic pressures in fluids
- Water - Density and Specific Weight - Density and specific weight of water at temperatures ranging 0 - 100 oC (32 - 212 oF) - Imperial and SI Units
