Forwarded: Re: Airflow measurement - again...

[kleenair at ix.netcom.com]

Tom Cloud wrote:
> 
>   . . . . .
> >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
> 
> 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>

I don't have the response curve to prove it, but thermistors are 
generally much too slow for such an application.  Also, a thermistor 
changes resistance with temperature, so you cannot use it by itself for 
flow measurement.  You would have to have a mass and a heater to heat it, 
so you can measure a temperature.  After all that you end up doing what 
the hot wire does, except much slower and at higher cost.

Regarding the method of measurement using the strain gauge, the common 
way is to use a wheatstone bridge.  Maybe both strain gauges (the one in 
the flow and the one outside the flow) can be wired into the same bridge. 
I'll have to look at that, but it seems to me this method makes use of 
VERY robust hardware.

Best Regards,
Mazda