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Static pressure is expressed as the difference between inside and outside air pressure in inches of water column (wc), and is measured with a manometer. Figures1a and1b show how a manometer works. Both inside and outside air pressure pushes down on a column of fluid within a tube extending through a solid wall. The less pressure that exists on the inside of the building (created by turning on more exhaust fans), the more the outside air pressure will push the liquid toward the inside portion of the open tube.
