Airflow measurement - again...

[John Hess]

Actually, most thermistors will have a larger cross-sectional area, 
thus create more drag and turbulence in the intake path.  The "hot 
wire" creates enough to be highly disruptive (note that REALLY high 
performance engines are MAP, not MAF).



----------
From:  Darrell Norquay[SMTP:dnorquay at iul-ccs.com]
Sent:  Friday, January 10, 1997 10:47 PM
To:  diy_efi at coulomb.eng.ohio-state.edu
Subject:  Re: Airflow measurement - again...

At 02:44 PM 1/10/97 -0800, Tom Cloud wrote:

>> 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>

And Mazda replied:

>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.

Thermistors don't need to be slow.  It depends a lot on the size and 
thermal
mass of the thermistor body.  Fenwal and YSI sell lots of units that 
are
only 1mm in diam, very fast response.  In any case, a hot wire 
anemometer is
just a more rugged type of thermistor, which uses more current and has 
less
sensitivity.  Also, a little slower response may give better averaging 
of
transients and a smoother signal...

Thermistor mass flowmeters are very common in the instrumentation 
world, and
can be very accurate indeed.  Usual method is to have two identical
thermistors in a "bridge" configuration, one in the air stream and one 
out.
This compensates for any ambient temp effects.  The thermistor in the
airstream is purposely heated to raise it's temp, and the drive 
current is
"servoed" to maintain a constant voltage across the sensor.  As 
airflow
increases, it takes more current to keep this voltage (and therm temp, 
BTW)
constant, and the change in drive current is simply measured to 
determine
airflow.  Exactly the way a MAF works, but a MAF is a mass produced, 
limited
accuracy unit made to be cheap and not particularly accurate.  (BTW, 
there
was some discussion I saw somewhere on how to make a hot-wire 
anemometer
from a couple of #52 panel lamps, one with the glass envelope broken 
off...)


regards
dn
dnorquay at iul-ccs.com