John Counts

a1 (pipe area at inlet): a2 (pipe area at throat): Q minimum (what minimum flowrate you want to chart from): Q maximum (what maximum flowrate you want to chart to):

The modified equation charted below, having converted to a form using the density of the fluid (rho, p) and assuming equal height of inlet and outlet comes from the flow rate equation:
Q=CA₂sqrt{2(P₁-P₂)/p[1-(A₂/A₁)²]}, and after alteration becomes:
(P₁-P₂)={pQ²[1-(A₂/A₁)²]}/{2C²A₂²}
also charted is the same flow rate and inlet pipe, but with a throat diameter 20% smaller.