UTube Differential Pressure Manometers
Inclined and vertical utube manometers used to measure differential pressure in flow meters like pitot tubes, orifices and nozzles.
Pressure measuring devices using liquid columns in vertical or inclined tubes are called manometers. One of the most common is the water filled utube manometer used to measure pressure difference in pitot or orifices located in the airflow in air handling or ventilation system.
In the figure bellow illustrates the water levels in an utube 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.
Vertical UTube Manometer
The pressure difference measured by a vertical UTube manometer can be calculated as
p _{ d } = γ h
= ρ g h (1)
where
p _{ d } = pressure (Pa, N/m ^{ 2 } , lb/ft ^{ 2 } )
γ = ρ g = specific weight of liquid in the tube (kN/m ^{ 3 } , lb/ft ^{ 3 } )
ρ = Utube liquid density (kg/m ^{ 3 } , lb/ft ^{ 3 } )
g = acceleration of gravity (9.81 m/s ^{ 2 } , 32.174 ft/s ^{ 2 } )
h = liquid height (m fluid column, ft fluid column)
The specific weight of water , which is the most commonly used fluid in utube manometers, is 9.81 kN/m ^{ 3 } or 62.4 lb/ft ^{ 3 } .
Note!  the head unit is with reference to the density of the flowing fluid. For other units and reference liquid  like mm Water Column  check Velocity Pressure Head .
Example  Orifice Differential Pressure Measurement
A water manometer connects the upstream and downstream pressure of an orifice located in an air flow. The difference height in the water column is 10 mm .
The pressure difference head can calculated from (1) as
p _{ d } = (9.8 kN/m ^{ 3 } ) (10 ^{ 3 } N/kN) (10 mm) (10 ^{ 3 } m/mm)
= 98 (N/m ^{ 2 } , Pa)
where
9.8 (kN/m ^{ 3 } ) is the specific weight of water in SIunits.
Inclined UTube Manometer
A 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. Accuracy can be improved by inclining the utube manometer.
The figure bellow indicates a utube where the left tube is connected to a higher pressure than the right tube. Note that the left and the right tube must in the same declined plane for the angle to the horizontal plane to be correct.
The pressure difference in a inclined utube manometer can be expressed as
p _{ d } = γ h sin(θ) (2)
where
h = length, difference in position of the liquid column along the tube (mm, ft)
θ = angle of column relative the horizontal plane (degrees)
Inclining the tube manometer increases the accuracy of the measurement.
Example  Differential Pressure Measurement with an Inclined UTube manometer
We use the same data as in the example above, except that the UTube is inclined 45 ^{ o } .
The pressure difference head can then be expressed as:
p _{ d } = (9.8 kN/m ^{ 3 } ) (10 ^{ 3 } N/kN) (10 mm) (10 ^{ 3 } m/mm) sin(45 ^{ o } )
= 69.3 N/m ^{ 2 } (Pa)
Utube Manometer Calculator
This calculator can be used to calculate the differential pressure measured with an Utube manometer.
(9.8 kN/m ^{ 3 } , 62.4 lb/ft ^{ 3 } default values for water)
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