Thanks guys...I'm familiar with check valves, I was just curious whether or not a typical 9" membrane diffuser was equipped with a mechanical, stand alone check valve, or if it relied upon the membrane itself to settle down in the "closed" position after air was shut off.

I took a look at some old diffusers I have here, and I can find no valve. I'm assuming that means this particular brand diffuser utilizes the elasticity in the membrane to prevent water from entering.

"Forget pounds and ounces, I'm figuring displacement!"

If we accept that: MBG(+)FGSF(=)HBG(F1)
And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1)
Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT?
PB answer: It depends.