The following active chart will show either the flow rate, the pressure drop, or the orifice diameter when a gas is forced through an orifice. Pictured below are the four orifice configurations having different impacts on the results. The default gas is air, but if the specific gravity of another gas is known, that information is one of the variable that can be changed in the charts.
Chose between metric and US Customary units and change any of the colored cells to see the results.
