Pressure head

When considering dynamic (flowing) systems, there is a third term needed: velocity head.

The venturi meter in the diagram on the left shows two columns of a measurement fluid at different heights.

Such barometers are an enclosed column of mercury standing vertically with gradations on the tube.

The lower end of the tube is bathed in a pool of mercury open to the ambient to measure the local atmospheric pressure.

The reading of a mercury barometer (in mm of Hg, for example) can be converted into an absolute pressure using the above equations.

See the torr, millimeter of mercury, and pascal (unit) articles for barometric pressure measurements at standard conditions.

The reading of these meters (in inches of water, for example) can be converted into a differential, or gauge pressure, using the above equations.

A Venturi meter with two pressure instruments open to the ambient air. ( and ) If the meter is turned upside down, we say by convention that and the fluid inside the vertical columns will pour out the two holes. See discussion below.
A flow of air through a venturi meter , showing the columns connected in a U-shape (a manometer ) and partially filled with water. The meter is "read" as a differential pressure head in centimeters or inches of water.