I'm missing something...

[Tlsalt at aol.com]

According to Bosch, their air flow meter monitor the volumetric flow rate 
(m3/h) of the air being drawn into the engine.  The principle is based on the 
measurement of the force emanating from the stream of air.  The profile of 
the measurement channel inside the meter is designed so that the free cross 
section increases so that there is a logarithmic relationship between the 
flap angle and air throughput.  They state that it cannot compensate for 
changes in density, although there must be some relationship between mass and 
force, the system is not designed to use it.  Perhaps the relationship 
between the change in flap position from heavier air is not equal to a larger 
flow with an equivalent amount of O2.   In contrast, they make a big deal out 
of the ability of the hot wire air mass meters to compensate for variations 
in air density.  An electrically  heated element is mounted in the air stream 
where it is cooled by the incoming air.  A control circuit modulates the flow 
of heating current to maintain a constant temperature differential between 
the heated wire and the air.  The amount of current required provides an 
index for the mass of the air flow (kg/h).  The air mass meter automatically 
compensates for variations in air density, as this is one of the factors that 
determines the amount of warmth the surrounding air absorbs from the heated 
element.

Paul Saltwick

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