Peter Parfitt said:
There is a relationship between static pressure and CFM for specific fans/setups. As, in this case, the fan (extractor) is the same then the static pressure can be assumed to be linked to potential CFM.
[member=41421]Peter[/member]: Having dealt with vacuums (industrial grade) in static and dynamic setups I must side with [member=8955]Coen[/member] here, measuring static pressure like you did is only good to detect leaks.
CFM is about
flow (not pressure) between two pressure levels (room and DE intake) and is highly dependant on the geometry (lenght, width, form) of the airway between these two levels as that determines the way the air moves (laminar vs. turbulent) and thus the internal resistance of the airway.
Possibly, with a spring of carefully selected tension (so you get good readings) your setup could be used to measure the pressure near the DE intake, which would measure the dynamic situation (the DE sucking at the hose vs. the air coming through it) so the effect of putting different things on the other end of the hose (nothing, another hose, cyclones of certain types) could be estimated:
Should the pressure near the DE intake fall one would know that what's currently at the end of the hose poses a greater resistance to the airflow (as the DE is able to evacuate the beginning of the hose to a lower pressure level as less air is able to travel through the setup). A good cyclone should have only very little resistance, thus should result in only a small pressure drop at DE intake.
Possibly you have a Y connector (to go directly into the DE), so you could attach your test rig to one connector of it, then a normal hose (for reference, then the cyclones) at the other and redo the test? It would suffer from venturi effect (as of the geometry of the Y connector) a little, but that shouldn't be
that relevant for this kind of test.
An airflow meter (basically an unpowered fan with a RPM readout, with a little electronics to multiply with the known fan characteristics and the area of it) measuring what's going into the end of the hose would give way more accurate results though...
Back on topic, I would suggest (should the vac mainly be ment for stationary use) to keep it and invest in a DD style cyclone, would be cheaper and solve the 'full bag' problem in an (relatively) inexpensive way.