Air flow benches are devices that measure the volume or mass of air that will pass through a bounded path or conduit in relation to an applied pressure differential. The pressure differential is usually relative to ambient air pressure and is often produced by some mechanical means such as an electric blower or at least one electric vacuum motor. The flow bench air source creates a pressure differential across the test object that causes air to flow through not only the test object but through the entire flow bench. At some point in the flow bench, the air flow velocity is measured. From the measured air velocity, the volume or mass of air that is flowing through the test object can be calculated.
A flow bench with an air source that can apply a pressure differential to the object being flow tested is referred to as an active flow bench. A passive flow bench has no air source, the object being tested supplies its own pressure differential to promote measurable air flow, such objects would be electric blowers or fans and vacuum motors.
It is often believed that flow benches are only used for flow testing internal combustion engine cylinder heads, but in fact, many other items like carburetors, intake manifolds, exhaust manifolds, turbocharger housings, air filters, pneumatic couplings and fittings, AC grills and condensers, and nozzles of different types often find their way onto a flow bench. Early flow benches were mostly used to test aircraft carburetors.
The pressure differential applied to the test object is important to know since air flow is relative to the applied pressure differential. This pressure is usually measured in Inches of Water Column, or "WC. If you place a drinking straw into a glass of water and draw the water up the straw 1 inch above the water level, that would be 1"WC pressure less than ambient air pressure, or -1"WC. If you apply pressure to the drinking straw that causes the water level in the straw to be below the water level in the glass by 1 inch, that would be 1"WC greater than ambient or +1"WC. 28"WC is about 1 PSI (Pounds per Square Inch) pressure. Internal combustion engine parts are often flow tested at 28"WC.
Air flow through a bounded path or conduit is often measured as a volume of Cubic Feet per Minute, or CFM.