Air Flow Measurement

[Stephen Dubovsky]

  Thank you for your feedback, very interesting reading, but of course I now
have other questions/comments.

>The response to a step increase is faster than a step decrease in airflow.
>This means that if you have pulsating airflow, the signal is skewed upwards,
>including the average.

  Ok, I though about it alot, and finally talked to all of my ME/ESM buddies
and none of us can think of why this happens.  If you have two Pt elements,
one to measure ambient, and you heat the other to some constant delta-t
above ambient and use the energy lost to find mass flow, there is no
physical phenomenon to say this should work asymetrically with a step change
in the airflow.  I CAN believe someone didn't do a good job w/ the control
electronics trying to regulate the delta-t, but this doesn't have to be the
case.  We just think someone should have failed EE 101.

>A lot of this went over my head, but 2MHz is out of the question for an
>automotive spec ECU.  I'd be lucky to sample at 1kHz, but I'd prefer to sample
>at a lot less.  Remember you've got to do more than just measure airflow :)

you computationally challenged people;) I agree that 2MHZ is a little fast
(as I mentioned before), but 10-100kHz data collection and computation is
not out of the question w/ a $30 DSP.  I just don't understand why
aftermarket ECU's still use older technology (I understand why saving $25 is
important in mass production though).

Tnx, SMD



--
Stephen Dubovsky
dubovsky at vt.edu

95 Yamaha FZR600
83 Porsche 911SC
84 Jeep Cherokee