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Posted by Robert Scott on December 7, 2007, 2:16 pm
 
On 7 Dec 2007 08:52:47 -0500, nicksanspam@ece.villanova.edu wrote:


A techique in automotive airflow sensors is to use a hot wire in the air stream
and compare the resistance of that wire with an unheated reference wire that is
also in the airstream.  But this technique is best suited to higher airflow
rates.  At very low airflow rates (such as what you might get in
thermo-siphoning systems), the localized thermo-siphoning created by the sensor
itself will dominate the flow rate that you are trying to measure.




Robert Scott
Ypsilanti, Michigan

Posted by Morris Dovey on December 7, 2007, 4:02 pm
 
Robert Scott wrote:
| On 7 Dec 2007 08:52:47 -0500, nicksanspam@ece.villanova.edu wrote:
|
||
||| ... are you aware of any off-the-shelf "one-wire" devices that
||| can be used to measure airflow?
||
|| I'm not, but it's an interesting idea. We might automate airflow
|| measurement through a vent by soldering 20 $ DS18B20s with 1/8W
|| resistors in parallel on a rectangular grid of wires and multiplex
|| the resistors with 2406 drivers to heat each 1820 a bit, and then
|| measure the cooling off rate, which should be proportional to the
|| airflow. Some foil around the sensor/resistor could add mass and
|| shade it from stray sunlight.
|
| A techique in automotive airflow sensors is to use a hot wire in
| the air stream and compare the resistance of that wire with an
| unheated reference wire that is also in the airstream.  But this
| technique is best suited to higher airflow rates.  At very low
| airflow rates (such as what you might get in thermo-siphoning
| systems), the localized thermo-siphoning created by the sensor
| itself will dominate the flow rate that you are trying to measure.

IanaEE, and I'm grasping at straws here - but extracting what I think
I understand <g> from both Nick's and Rob's response, would it be
possible to use the wire as a variable resistor in part of an RC
circuit and measure the time constant using a Schmidt(sp?) trigger and
counter clocked by a crystal-controlled oscillator?

If so, would it need to be a "round" wire - or could the wire be a
trace on a PCB?

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/



Posted by Robert Scott on December 7, 2007, 3:54 pm
 


A round wire has the advantage that the airsteam can touch all sides of it,
while a trace on a circuit board can only conduct heat to the air on one side.
Also it is important to use a metal that has a high sensitivity of resistance
change to temperature change.   The copper on a trace is not optimal in this
respect.  It is also too low in resistance.  Wires used in airflow sensors are
made from NiChrome or something like that which has a fairly high resistance to
begin with.

I think a cost-effective approach might be to purchase an mass airflow sensor
from an auto parts store and figure out how to interface to it.  I suspect that
the output is already converted to a frequency, which travels undistorted
through long wires.  I've got an old one here from a project I worked on years
ago and that's how it works.


Robert Scott
Ypsilanti, Michigan

Posted by Morris Dovey on December 7, 2007, 6:14 pm
 Robert Scott wrote:
| On Fri, 7 Dec 2007 10:02:09 -0600, "Morris Dovey"
|
|
|| IanaEE, and I'm grasping at straws here - but extracting what I
|| think I understand <g> from both Nick's and Rob's response, would
|| it be possible to use the wire as a variable resistor in part of
|| an RC circuit and measure the time constant using a Schmidt(sp?)
|| trigger and counter clocked by a crystal-controlled oscillator?
||
|| If so, would it need to be a "round" wire - or could the wire be a
|| trace on a PCB?
|
| A round wire has the advantage that the airsteam can touch all
| sides of it, while a trace on a circuit board can only conduct heat
| to the air on one side. Also it is important to use a metal that
| has a high sensitivity of resistance change to temperature change.
| The copper on a trace is not optimal in this respect.  It is also
| too low in resistance.  Wires used in airflow sensors are made from
| NiChrome or something like that which has a fairly high resistance
| to begin with.

Gotcha - that makes sense (even to me).

| I think a cost-effective approach might be to purchase an mass
| airflow sensor from an auto parts store and figure out how to
| interface to it.  I suspect that the output is already converted to
| a frequency, which travels undistorted through long wires.  I've
| got an old one here from a project I worked on years ago and that's
| how it works.

Ok. I'm planning a visit to may favorite automotive guru for a confab,
but I think I'll also dig out my scope and soldering iron. I have a
notion that I might be able to cycle a single NiCr wire between warm
and cool and compare the warm and cool resistance readings. Something
like an 80C51 (or more recent equivalent) should let me interface to
the one-wire protocol net.

I ran across a critter labeled the NSLU2 that looks like might make a
good one-wire net controller and logging device.

Thanks!

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/



Posted by Robert Scott on December 7, 2007, 5:54 pm
 


OK, but remember that any electronics that you put out into a collector has to
survive the stagnation temperature.  The usual commercial temperature range for
semiconductors is 0 to +70C.  The extended/military range is -40C to +125C.  I
can see how in some collectors, even that may not be high enough to withstand
the stagnation temperature.

A better solution may be to keep all active semiconductors on the other side of
some insulation to protect it from collector stagnation temperatures.


Robert Scott
Ypsilanti, Michigan

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