Gas flow at small pressure drops in US units

Manufacturers found that equation (A3.1) served well for gas flow at high-pressure ratios as well as for liquid flow. The equation is often quoted with the flow units of standard cubic feet per hour, with the specific gravity standard changed from water to air, which requires the somewhat involved conversion route set down below. The specific gravity with respect to water is related to the specific gravity with respect to air by  

Assuming air can be treated as a perfect gas, the equation of state gives (in US units)  

Using the conversion factor 1 US gallon per minute = 8.0208 cubic feet per hour together with (A3.10) in equation (A3.1) gives the flow in cubic feet per hour as  

We transform this to units of standard cubic feet per hour (scfh) by noting that a mass that has volume V at (p, r ) will have volume V / at p = 14.7 psia, T = 520°R, where  

Applying this to the volume flow rate of equation (A3.11) yields  

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