by 115-1172523331 » Tue Feb 26, 2008 1:06 am
Maybe I'm missing something here, but if your plate will flow 300CFM at 28 inches, it should flow 320 CFM at 32 inches. If you are not sealed, that 20 CFM could go away. If I am testing right, try this. If this is not right, somebody jump in here! Seal the opening completely then SLOWLY add vacuum to your 28-32 inches. If you are getting flow readings from the pitot, you are leaking. I ran my bench to 44 inches (max for my manometer) and got no flow so I believe that I am sealed.
This may seem silly, but most problems are when you find them! Another thing that popped into my head is the size of the flow element. Are you using 3" PVC, and if so, did you measure the ID? My 1-1/2" is actually 1.57" and the 1-1/4" measures 1.37". A small change at 3" will make a big change in flow (0.1 inch ~ 20 CFM).
Good Luck! -- Doug