Forwarded: Re: Airflow measurement - again...

[Stephen Dubovsky]

>  . . . . .
>>Someone else on this list posted a message suggesting use of a strain 
>>gauge for flow measurement.  I've been thinking about that some more from 
>>time to time and it seems like a good idea.  His idea was basically based
>
I was the fellow who proposed the idea about the strain gauges.

>How do MAF meters work?  I understand they typically use heated wires??
>Why not use thermistors (slower response?? -- but larger R change and
>easier to instrument .. ??).  I suspect that they have long ago been
>tried and discarded -- true??
>
>
>Tom Cloud <cloud at peaches.ph.utexas.edu>

 Ok, MAFs work on a heat loss principle.  If you take a wire and heat it,
then there is a loss due to convection (the forced loss far outweighs the
natural convection component usually).  (There is also a loss due to
radiation).  With most things neglected, the loss depends on flow^2 (I
think).  Now the problem is that different ambient temps will affect loss so
they typically use two wires.  Dont heat the first, just use it to find
ambient, and heat the second to some fixed delta-T abouve ambient.  The
power required (you cam measure the current and voltage applied to the wire)
is now a square or square-root looking funcion of air flow.  This method
doesn't actually measure MASS flow, but some other terms as well.  If I ever
dig up my flow book from Omega, I would tell you what it DOES measure.  You
usually use very thin Pt wires as they are corrosion resistant, have low
thermal mass (for fast response), and are relatively linear.  

  Another way to really measure mass is to place a heated wire upstream
(dumping some fixed P into the fluid) in a tube, and measure the delta-T it
produces downstream.  The Omega flow book also explains why this is the
case.  Omega doesn't sell any of these sensors for airflow measurements, but
I dont know why.  It may take a prohibitive amount of power to produce a
reasonable delta-T or something.  Some automotive MAFs look like they have
little sensing tubes that hold the actual sensor, which leads me to believe
this is how they do it.  I have seen MAF replacements for Porsches and they
simply have a sensor stuck out into the airstream, which seems like they are
using the first method.  Don't know for sure.

  Now that we are again mentioning MAFs and strain gauges in the same mail,
why not use a simple strain gauge to determine the flow direction and
corrent the 'flow rectification' the MAF returns.  Simply switch in a -1
amplifier when the flow is negative.  All flames appreciated.

SMD
--
Stephen Dubovsky
dubovsky at vt.edu

95 Yamaha FZR600
83 Porsche 911SC
84 Jeep Cherokee