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Flow rate is directly proportional to the square root of pressure. So for any given orifice, all else being equal, $\frac{GPM}{\sqrt{PSI}}$ is constant. Technically PSI here is the pressure difference across the orifice, but with no backpressure only the input pressure matters. For an orifice rated 1.8 GPM at 90 PSI, $\frac{GPM}{\sqrt{PSI}} = 0.1897$. To ...

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If you know the external diameter and orifice diameter, From Wikipedia the orifice plate equation : $$q_m =\frac{ C_d}{\sqrt{1-\beta^4}}\epsilon\frac{\pi}{4} d^2 \sqrt{2\Delta p \cdot \rho_1}$$ where: $C_{d}$ = coefficient of discharge, dimensionless, typically between 0.6 and 0.85, depending on the orifice geometry and tappings $\beta$ = diameter ...

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Here is a reference from RoyMech, see section under "Orifice Flow Meter". The setup has the orifice placed in a pipe. Below is a similar, perhaps simpler, expression, from Jobson 1955, part of the way between equations (4) and (5) of that paper. (The paper is really about incompressible flow, this is just a warmup for the author). \dot{m} = {{\...

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