MAF vs AFM my take

[Chris Guy <Cguy1@xxxxxxxx>]

I think Jim C and Mark K. are both wrong on this one (though Jim is right 
99% of the time).

Dyn Press = 1/2 * air density * velocity^2. 
Volume flow rate = Velocity * Area of duct.
Mass flow rate= density * velocity * area


Jim is wrong:
>
>  THE AFM measures VOLUME flow rates.. which must be corrected for 
>  Pressure and Temperature to become "mass equivalent"

I assume the AFM works basically on the dynamic pressure of the incoming 
air deflecting the flapper. 

Looking at the formula for Dyn Press, density does have an effect in 
addition to velocity (and velocity is another word for flow rate). If I 
took it up to Mt Everest (or higher) and flowed 300 CFM of ambient air 
into the inlet of the air inlet duct the flap would deflect less than at 
than a similair flow at sea level. Look at the below example for 
different flow rates with same Dyn Press.

As Jim mentions Temp and Press can be used to correct this.

[Jim my 86 535i does not have a pressure sensor like the 528e does, so 
how does it handle the density calc?]




Mark is wrong too. 
>The AFM DOES measure mass flow . Basic aerodynamics say that I sent air
>over an air foil (which the flapper is a crude air foil)  it will move
>according to mass flow over it .

Most aerodynamic properties are related to dynamic air pressure (ie Lift 
= CL * Dyn Press * Wing Area), not mass flow rates.

The AFM will read less at altitude, but not in proportion to the change 
in mass flows over different speeds.

Numbers: 
    At 91.5 ft/s at Sea Level (.00237 sl/ft^3) Dyn Press = 9.93 lb/ft^2
       Mass flow = .217 slugs/second (for 1 ft^2 duct)
    At 100 ft/s at 6000 ft (.00198 sl/ft^3) Dyn Press = 9.93 lb/ft^2
       Mass flow = .198 sl/s (same duct)

So with the same Dyn Press (and thereby same flapper position) you will 
get different mass flow rates with different densities.




This is all theoretical and there is a _LOT_ of assumptions here, but it 
does show that in theory the flapper does not measure mass flow rate 
directly.
    


Chris Guy - CGuy1@domain.elided
86 535i 5sp
Denver, CO (amb press is around 11.8 psi, not 14.7!) 

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