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

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

p_d = γ h (1)

where

p_d = 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:

p_d = 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

p_d = 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 45^o.

The pressure difference head can then be expressed as:

p_d = 9.8 (kN/m³) 10³ (N/kN) 10 (mm) 10^-3 (m/mm) sin(45)

    = 69.3 N/m² (Pa)

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