Measuring fuel flow and the low rates we use in out Lancairs is difficult at best.  The systems that I have looked at that put the flow sensor between the throttle body and the spider are just pressure sensors. 

Ray,

 

All VM system fuel flow sensors use a turbine (same as JPI, etc.), just like the water wheel in your local grist mill down in the historic district.  If installed properly, it is extremely accurate - assuming the gas pump's gauge used when topping my plane are accurate - I might be off by .3 gallon in 30 gallons.  Since the FF is used to calculate remaining fuel (not the highly accurate fuel probe information), 1% ain't bad.  That is accurate enough for me and always in the right direction - VM says I used more fuel than I actually did.