where Q = volumetric flow rate in standard cubic feet per minute (scfm), based on 14.7-psi atmospheric pressure and 60°F air temperature.

To simplify the equation, experts typically assume a conservative pressure change between inlet and outlet ports (P1 – P2) of 5 psi. For time or process-critical applications, reduce this to 2 psi. And, in many cases, a pressure drop of 10 psi is not detrimental to the application. A 10-psi pressure drop permits smaller valves that lower costs and require less mounting space.

To simplify calculations, let’s tabulate values for a portion of the equation for 2, 5, and 10-psi pressure changes. Here, we create a constant A, defined as


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