Manometer for DC CFM measurement

[James Hebert]

If I build this to measure CFM on my dust collector:

http://www.science-house.org/student/bw/flight/manometer.html

........do the opening on each end need to be identical?

Id like to insert a metal tube connected to npt through the PVC duct and then connected to the slack tube manometer but the opening of the metal tube would be different dia then the tube opening on the other end.

Any thoughts?

[Mark Engel]

Stick a piece of the metal tube in both ends. I think the openings at both ends of the tube need to be the same.