Flowrate in a suction line

[burnerman]

I hope this hasn't been covered yet. I tried the search feature with no luck...

I'm trying to measure flowrate (either mass or volumetric, I don't care) for a ring blower. The blower is controlled with a VFD, and will have varying backpressure. I'd like to use a venturi/nozzle/orifce (still working that out) and measure differential pressure. The calculator won't work with negative values. I would assume the theory still applies (???).

What I know
D1: 4"
D2: 3"
P1: -8 inWC
P2: -21 inWC
Dry Air @ 70F.
Max Blower flowrate is 706 scfm (says the manufacturer)

Anybody have any help?

I'm still designing the system, so D2 can change if it seems out of line. I'd like to put as little restriction on the blower as possible.

[PipeFlowCalcs]

When you say negative - do you mean pressure under atmospheric value?
Calculators are working with absolute pressure value so you should be able to calculate it.

[burnerman]

OK...

Using a U-tube to measure suction pressure. It reads 8.1 inchesWC on the large diameter (4" pipe) and 21.2 inchesWC on the venturi throat (3.025"). Should I be using 398.9 inchesWC for p1 value and 385.8 inchesWC for p2?

Tried this, and the flow still calcs to 21% over manufacturers rated output.

I'm going to try to get my hands on a velocity transducer and see what that tells me.

Thanks for the help. I'll keep trying, and let you know what I find out. If anybody has any other ideas, I'd be glad to hear.