AFMs

[RABBITT_Andrew at mv8.orbeng.com.au]

>>Is it a MASS FLOW measurement? 

NO, vortex shedding frequency is a function of flow velocity, 
therefore it's only volume flow metering.  The pressure and temp. 
sensors are only to do the corrections, which alledgedly leads to 
messy lookup tables and calculations in the OEM ECU. (yuk!)

>>Does a problem occur at low (read idle) flow rates (of resolution)? 

I shouldn't think so, it should be just a matter of sizing the vortex 
shedder and minimum flow velocity appropriately. (my opinion only)

>>All this stems from me being a little nervous about using Bosch 
hotwires in a high vibration offroad application.

If you're worried about vibration on a hotwire, try using the bosch 
hot-film sensor (HFM-2) as fitted to many late model BMW's and 
probably many other European cars.  Take a look at the latest 540i 
(4.4L V8) which should be good for your application

Also, the Hitachi bypass meter fitted to most fords (EEC iv) is an 
analogue output, not frequency.

Also, for everyone's info, Bosch has recently started production on a 
hot-film meter capable of measuring back-flow (properly!)  A neat 
piece of design using micro-machined silicon technology.

Andrew Rabbitt
Orbital Engine Company