I would like some ideas for a self powered hotwire sensor or even using a easily available automotive mass flow sensor if viable. We currently are testing a unit that ranges from 45cfm to 110cfm through a 6" smooth metal duct. I am working on a solar heat project where Eventually I will use a controller to set flow for best output. Initially using this blower, Nidec Beta V TA600DC 24 VDC .82 AMP A34457-10 FAN 172MM, and a pwm circuit such as the one attached. The regulator is LM217, opamp is LM324 and the drive for the fan will be an IRFZ34N with Sr56 as D1. I would use a 11pos stepped attenuator at VR1 to get feedback from RPM out to build a flow table. I have a brand new VW/Audi massflow sensor but the schematic in service manual doesn't explain how it works.