Hmmm, this seemed so less complicated when I was first thinking of it. I diameter of the cylinder shouldn't be a factor as one foot of head will be the same amount of pressure in a 1" or a 10" diameter column. Two problem areas come to mind tho. First would the suction of the pumps intake throw this test out of whack? Would it be better to have the test piece dumping into an open res and the pump intake also drawing from same res? Something like this?
The other possible problem that comes to mind, as the level in the cylinder rises to indicate the amount of head added by the test piece, the performance of the pump drops. I'm not sure this would effect anything other than the flow.
The more I ponder this, the more apparent te limits of my knowledge becomes. I'm hoping some of the guys more versed in this area will chime in, explaining it better. Hopefully in layman's terms so I can grasp it 8)