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'[EE]: 0-5V Air Flow Sensor'
2001\04\04@102401 by Montaigne, Mike - NRC

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       Hi:

       I am using a PIC for a simple gas flow check on an air sampler.  My
crude first run at this was to use a Radio Shack 271-110A 10K@25 degrees C
Thermistor, 5 min epoxied to a 82 ohm resistor which was connected to 5
volts.  The thermistor was connected in series with a 5K resistor to 5V and
the middle junction point connected to a meter (eventually a a/d ip on a
PIC).  I got 1.72V (a/d 88) at 25C room temp, 3.28V (a/d 168) with no
airflow (63C), and 2.78V (a/d 142) with some airflow (50C).  The a/d numbers
are what I figure I'll get in my PIC with 256 bit resolution.

       So my questions............

       I can't find a cheap airflow sensor.  A pressure switch won't work
as it could still read OK if there was a blockage.  This sensor would cost
me about $3. and I need 20 or so.

       My airflow is 20L/min and I have not tested at that exact airflow
yet.  I'm hoping there will be enough cooling to give me a significant
change in my a/d reading.  How many counts change between no airflow and
20l/min should I attempt to design for to give me reliable operation.  Is
there another approach I should be looking at?

       I know there are lots of gurus out there who are a lot smarter at
this than I am.  I'm just trying a seat of the pants approach and this is
where I am at, after my first run at this.

       tks

       Mike Montaigne
       Montaigne Audio
       Pembroke Ontario
       spam_OUTmontaigmTakeThisOuTspamnrtco.net

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2001\04\04@104043 by Spehro Pefhany

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At 10:11 AM 4/4/01 -0400, you wrote:

>
>        My airflow is 20L/min and I have not tested at that exact airflow
>yet.  I'm hoping there will be enough cooling to give me a significant
>change in my a/d reading.  How many counts change between no airflow and
>20l/min should I attempt to design for to give me reliable operation.  Is
>there another approach I should be looking at?

Yes, you should try to temperature compensate it. Try making a bridge
with two thermistors and heaters but with one shielded from the air flow.

You'll then get a differential reading that is independent of ambient
temperature. You may be able to do this without additional parts, just
going directly into the PIC A/D input.

Best regards,

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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2001\04\04@113239 by James Paul

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What about a Mass Air Flow Sensor for a car?  I'm not sure of the
cost, but that may be the killer.  Anyway, just a thought.

                                           Regards,

                                             Jim


On Wed, 04 April 2001, "Montaigne, Mike - NRC" wrote:

{Quote hidden}

.....jimKILLspamspam.....jpes.com

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2001\04\04@182252 by Spehro Pefhany

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At 12:52 PM 4/4/01 -0500, you wrote:

>
>There's an exceedingly complex formula describing the relationship between
>the airflow and the voltage output, which we won't delve into here.

A very good approximation can be made by King's law.

It's like one line of C.

Best regards,

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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
EraseMEspeffspam_OUTspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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2001\04\05@092908 by Lawrence Lile

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<Drum roll = on....>   And the one line of C code is.....????  </DrumRoll>

<Grin = Big> Lawrence Lile </Grin>



{Original Message removed}

2001\04\06@080113 by Peter L. Peres

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> <Drum roll = on....> And the one line of C code is.....???? </DrumRoll>
>
> <Grin = Big> Lawrence Lile </Grin>

(taken from:)
From: "Mark Folsom" <folsommanspamspam_OUTredshift.com>
Newsgroups: alt.sci.physics,sci.physics
Subject: Re: Question about Specific heat and conductivity experiment.
Date: Tue, 20 Mar 2001 16:51:38 -0800

Here's what I posted before on the topic:

"In the range of Reynolds numbers from 3 to 100, the empirical correlation
between the Nusselt modulus and the Reynolds and Prandtl numbers for cross
flow over a tube is (Principles of Heat Transfer 3rd ed., Frank Kreith)

Nu = 0.82*Pr^.3*Re^.4

where

Nu = hc*D/k, k is thermal conductivity, hc is the film heat transfer
coefficient, D is the diameter of the tube,

Pr = Cp*mu/k, Cp is specific heat, mu is kinematic viscosity,

Re = V*D*rho/mu, V is fluid velocity, rho is fluid density.

This can be reorganized to give you the film heat transfer coefficient in
terms of properties we are familiar with:

hc = 0.82 * Cp^.3 * k^.7 * rho^.4 * V^.4/(D^.6 * mu^.1).

The properties of the oil are Cp, k, rho and mu.  Heat capacity per unit
volume doesn't vary much unless there are phase changes or reactions
involved.  Reduced viscosity could help a little."

The above indicates that thermal conductivity is more important than heat
capacity, but that could change at higher flow rates.

Mark Folsom

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