jim is wrong and Chris is right, well look below.

[mark kibort <mkibort@xxxxxxxxxxxxxx>]

Chris,
Look below to my posts to your comments
with the>>>>>>>>>>>>>>>>>>next to them
mark



Message text written by INTERNET:bmw@domain.elided
>From: Chris Guy <Cguy1@domain.elided>
Subject: MAF vs AFM my take

I think Jim C and Mark K. are both wrong on this one (though Jim is right=
 =

99% of the time).

Dyn Press =3D 1/2 * air density * velocity^2. =

Volume flow rate =3D Velocity * Area of duct.
Mass flow rate=3D 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 =

=3D CL * Dyn Press * Wing Area), not mass flow rates.
>>>>>>>>>>>>>>>>air pressure is directly proportionate to Mass flow.
Presure on the front of the Vain vs the rear of the
vain will tell and measure mass flow.  However, in extreme cases when the=

air gets too thin, the 2 order equation relating to =

momentum comes into play.  the meter becomes less effective as the air
desity aproaches altitude.

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 =3D 9.93 lb/ft^2=

       Mass flow =3D .217 slugs/second (for 1 ft^2 duct)
    At 100 ft/s at 6000 ft (.00198 sl/ft^3) Dyn Press =3D 9.93 lb/ft^2
       Mass flow =3D .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.
>>>>>>>>>>>>>DID YOU READ YOUR FORMULAS, THEY ARE GREAT, BUT MY POINT IS
WELL MADE BY YOUR STATEMENTS.    YOUR SPEED IS DIFFERENT!!!!!!!   THIS
PROVES THAT AT THE SAME SPEED WHAT WOULD HAPPEN??
You got it, at the same speed , more mass flow would be moving thru the A=
FM
   (slugsper 3 feet is 15-16% higher at  sealevel vs 6000 feet right??)




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.
>>>>>>>>>>>>>>>>It does and you just proved it above.   you see my respon=
se
to Jims comments and some quotes out of some books.  Air flow meters are
Mass flow meters although not called such.
    =



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

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