Hello Martin,
To make it very scientific, why not go for a differential pressure setup, i.e. flow across an orifice - either use a dp transmitter, or better still, incline manometers? That way you don't really have to worry to much about air straighteners et al? I a sure we could find a nice, small dp transmitter somewhere, and design an orifice for your pipe-dia?

Would you be interested in something like this, or......?

Have a good one, and look forward to your tests though

Thanx for your efforts!