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What is a flow bench ? How it works ?
  • A flow bench is a device that measures the volume or mass of air flowing through a conduit relative to an applied differential pressure. The pressure difference is usually relative to the ambient air pressure (local atmospheric pressure) and is often produced by mechanical means such as a fan or a vacuum motor. The test bench air source circulates air not only through the object being tested but also through the entire test bench. This creates a pressure difference through the tested object and through the primary flow meter element which may be a gate or diaphragm, a Venturi tube or a nozzle.

  • The difference in air pressure is measured at the orifice sides in the test stand. From the pressure difference, it is possible to calculate the volume or mass of air passing through the tested object and even the gas velocity in the geometric cross-section of the tested object.

  • It is often thought that flow benches are only used to test the flow rate of internal combustion engine cylinder heads, but in fact many other components such as carburettors, intake manifolds, exhaust manifolds, turbocharger casings, air filters and nozzles of various types are often found on a flow test bench.

  • It is important to know the pressure difference applied to the object being tested because the air flow rate is relative to the pressure difference applied. This pressure is generally measured in inches of water column ("H2O) in standard "Imperial" units or in hPa (1 hPa = 1 mbar) in "International" units.

If you place a tube in a glass of water and suck the water up to 1 inch above the water level, this corresponds to a pressure of 1 "H2O or 2.5 mbar lower than the ambient air pressure (-1 "H2O or -2.5 mbar). If you apply a pressure to the tube that causes the water level in the pipe to be 1 inch lower than the water level in the glass, this corresponds to a pressure 1 "H2O or 2.5 mbar higher than the ambient air level (+1 "H2O or +2.5 mbar).​​

Internal combustion engine parts are often tested at 10 "H2O in Europe and 28 "H2O in America. 10 "H2O and 28 "H2O represent about 25 and 70 mbar pressure respectively.

Air flow through a conduit is often measured in cubic feet per minute (CFM) or l/s (liter per second).

For indication, 1 l/s = 0.472 CFM.

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