a HFM or what kind of airflow metering device do you mean? A HFM usually gives either a analog output 0-5v or a frequency (TTL) output. to read frequency you have to sacrifice one of the RPM inputs on the standard kit, if the HFM is analog it should just hook up to a spare analog input. The output is usually not linear so youll need to somehow source the linearisation from somewhere and dial it into the 'curve' option tab on either the RPM or AUX input setting page.

hi, i have a superflow air flow sensor from the sf 901.  https://www.popscreen.com/prod/MTE0MDQ2MTY0/SuperFlow-4-Air-Flow-Measurement-Turbine-eBay

@jeppen, hook it up to power (12V if I remember right?). Signal and 0V go to Freq2 for example. Configure the Freq2 to be a flow meter. Blow into the flowmeter and you shall see the channel record. You need to know the pulses vs flow, or you can calibrate it.