Forwarded: Re: Airflow measurement - again...

[Stephen Dubovsky]

  Interesting to see the strain gauge idea is getting more airtime again...

>engine before.  If I remember right, it was about 14" H20 at redline. 
>The thing maintained about 5" H20 from about 1000 to 4500 rpm's at full
>throttle where it was fully opened, then sloped up to 14" by 6500 rpm's.
  Sorry todd, but whats the psi/inH20 conversion?  Just dont have a 'feel'
what 14"H20 of pressure is;)  Seems you can get more than a few ponies back
when you change an old 911 w/ K-jetronic to a MAF/MAP EFI system.

  Someone also questioned the restiction/cost issue of strain gauges.  On
the restriction issue, the object can be very small (pea sized).  To measure
the force on an object, it doesn't need to be deflected (well, a VERY small
amount).  You put an object on a square beam of 6160 Al (or some other
material that you know deformation vs force) and glue (using Cya - instant
glue) strain gauges on the front and back of the beam.  When airflow
appllies a force to your object, the beam bends a VERY small amount which
makes one side of the beam longer than the other (the upstram side is in
expansion, and the downstream side is in compression).  A strain gauge is
actually a thin film resistor and when compressed the resistor gets
'shorter' and the R will go down (and of course, goes up when stretched).
This allows you to measure the 'strain' on the beam.  Now, you can use a pea
sized object w/ a very small beam to measure very tiny forces, or use a huge
chunk of Al to measure the weight of a multi-ton dump truck, all w/ the
exact same 2 gauges.  
  On the cost issue, load cells are expensive.  The strain gauges to make
them are very cheap (somewhere in the range of $2-10 each).  An
instrumentation quality load cell might use a little more expensive gauges,
but they are only little Resistors.  I imagine most of the cost is in the
beam material and machining it to shape so the gauge output is linear.  Then
again, there are really nice load cells in these <$60 digital scales on
which we gauge our holiday consumption;)
  Always glad to put in my $0.02.  Just remember, free advice is worth what
you paid for it;)

SMD
--
Stephen Dubovsky
dubovsky at vt.edu

95 Yamaha FZR600
83 Porsche 911SC
84 Jeep Cherokee