Airflow measurement - again...

[Stephen Dubovsky]

At 10:59 PM 12/20/96 -0500, you wrote:
...
>I just finished reading 60 messages.  WOW!
>I like this idea alot.  I agree that using an identical sphere outside the
flow should 
>negate the acceleration of the car effects.  About mass flow measurement, 
>
>  mass flow = density * average air_velocity * projected_area
>
>where as the drag equation is
>
>Drag=Coeff_drag*.5*air_density*air_velocity^2*projected_area
>
>Notice the air velocity^2 term.  If we were to rewrite the drag equation in
terms of 
>mass flow we would get
> 
>Drag = Coeff_drag*.5* mass_flow * air_velocity
>
>It's late at night, so I might be making a mistake, but I think the above
is right.  If 
>that's the case, it means your measurement is mass flow * velocity, which
means if we 
>had the same mass flow rate at a slower velocity (may be due to a lower
temp.), then our 
> reading would be lower.  Right?
>
>Best Regards, Mazda
>

  I agree w/ your algebra.  If you only used the sensor output, it would
seem to give a diff reading at different densities/velocities.  I originally
did not think of measuring mass flow directly w/ the sensor.  You could
measure air press & temp (and then using the ideal gas law), compute density
and then get the velocity term from the sensor.  Then knowing all of the
terms, you could compute mass flow.  You could even go overboard and measure
RH to correct for the 2% or so error that it introduces into the density
calculation (I mention this because the Dec 11 EDN shows a neat RH sensor
that works to 450deg F,  Im asking for more info about it;)
SMD