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

= ρ g h (1)

where

pd = pressure

γ = specific weight of the fluid in the tube (kN/m3, lb/ft3 )

ρ = density (kg/m3, lb/ft3)

g = acceleration of gravity (9.81 m/s2, 32.174 ft/s2)

h = liquid height (m, ft)

The specific weight of water, which is the most commonly used fluid in u-tube manometers, is 9.81 kN/m3 or 62.4 lb/ft3