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'[EE]Breath airspeed measurement'
2006\02\19@105736 by Mauricio Jancic

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I want to make a alcohol breath analyzer, how can one measure the air speed
when blowing trough the analyzer's tube?



-          I could do it with a small encoder and a propeller (I don't know
for sure if that's the English word.)

-          I can do it with a Pitot tube, but I think its intended for 0.3
Match and above speeds. it that correct?

-          The Venturi tube its intended for non compressible fluids

-          What about a couple of ultrasonic transducers and measuring the
propagation delay?

-          Hot wire? .. Nahhh.



Any other suggestion? Best approaches?



Regards,



Mauricio Jancic

Janso Desarrollos

Microchip Consultant Program Member

spam_OUTinfoTakeThisOuTspamjanso.com.ar

http://www.janso.com.ar

+54 11 4542 3519



2006\02\19@111902 by Metis Adrastea

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2006/2/19, Mauricio Jancic <.....infoKILLspamspam@spam@janso.com.ar>:
> -          The Venturi tube its intended for non compressible fluids

AFAIK, the venturi tube works for both fluids, compressible or not.
The carburators (don't know if this word is correct) take gasoline by
means of an underpressure made by a venturi tube into an air flow.

[spanish version]

Los tubos venturi funcionan en ambos tipos de fluidos, hasta donde yo
sé. Los carburadores (de los motores de gasolina sin inyección)
introducen la gasolina gracias al vacío que se genera en el carburador
(esencialmente un tubo venturi) al pasar a través de ellos una
corriente de aire.

[/spanish]

Regards

Metis Adrastea
--
He comentado ya que tengo un blog? O:-)
http://metisadrastea.blogspot.com/

2006\02\19@122002 by Stef Mientki

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>-          What about a couple of ultrasonic transducers and measuring the
>propagation delay?
>
>
>  
>
should work very good, ...
... you even determine the mollmass.
You know that the expired air is 36..37 C,
100% saturated with water,
contains ...% CO2
contains ...% O2
so you might even detect the alchol directly through mollmass.

Stef Mientki

2006\02\19@133135 by Marcel Duchamp

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Mauricio,

All of the approaches you listed have been tried as well as a few more.
But first off, do you really want "air speed" or "mass flow rate"?

One method to measure flow rate is a resistor (just like current). A
tube with some sort of restriction is used with differential pressure
sensing.  The delta-p is related to the flow rate although it is
nonlinear.  Pressures developed are generally very low.

Google on spirometry, spirometers, pneumotach, etc. Good luck!

Mauricio Jancic wrote:
{Quote hidden}

2006\02\19@140358 by Mauricio Jancic

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Mmm By mollmass I gues (translation issues) that you mean the unit of
concentration. Now, how do you say I should detects CO2 an O2 ?

Mauricio Jancic
Janso Desarrollos
Microchip Consultant Program Member
infospamKILLspamjanso.com.ar
http://www.janso.com.ar
+54 11 4542 3519

> {Original Message removed}

2006\02\19@140631 by Howard Winter

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Mauricio,

On Sun, 19 Feb 2006 12:57:32 -0300, Mauricio Jancic wrote:

> I want to make a alcohol breath analyzer, how can one measure the air speed
> when blowing trough the analyzer's tube?
>
>  
>
> -          I could do it with a small encoder and a propeller (I don't know
> for sure if that's the English word.)

That word's good enough!  :-)

> -          I can do it with a Pitot tube, but I think its intended for 0.3
> Match and above speeds. it that correct?

No!  Light aircraft use Pitot tubes to measure airspeed, and they work from about 40kts upwards.  That's about
0.06 Mach.

> -          The Venturi tube its intended for non compressible fluids

No, you can measure the pressure drop at the narrowest point and calculate the speed - probably not a linear
relationship but it's workable.

> -          What about a couple of ultrasonic transducers and measuring the
> propagation delay?

You haven't got much space in the breath-tube - I doubt if you could get this working.

> -          Hot wire? .. Nahhh.

That would work, but you've got the problem that the temperature of the breath isn't predictable and is
difficult to measure, so probably not a good one.

I think I'd go for Venturi and measuring the pressure.

Cheers,


Howard Winter
St.Albans, England


2006\02\19@145448 by Stef Mientki

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Mauricio Jancic wrote:

>Mmm By mollmass I gues (translation issues)
>
I mean the mass of Avrogado number of molecules ...

> that you mean the unit of
>concentration. Now, how do you say I should detects CO2 an O2 ?
>  
>
When someone is breathing under normal conditions,
there are good estimates for these parameters.
Stef

>Mauricio Jancic
>Janso Desarrollos
>Microchip Consultant Program Member
>.....infoKILLspamspam.....janso.com.ar
>http://www.janso.com.ar
>+54 11 4542 3519
>
>  
>
>>{Original Message removed}

2006\02\19@150038 by Stef Mientki

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>You haven't got much space in the breath-tube - I doubt if you could get this working.
>
>  
>
Well it is done in the medical world ;-)
Some dimensions: 5 cm long, 3cm outside diameter, 1 cm inside diameter.

>>-          Hot wire? .. Nahhh.
>>    
>>
>
>That would work, but you've got the problem that the temperature of the breath isn't predictable
>
It's quiet predictable and constant within 0.1 Celcius.
And measuring it with a very-very small thermocouple isn't too difficult.

>and is
>difficult to measure, so probably not a good one.
>
>  
>

Stef

2006\02\19@174849 by Mike Hord

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> >>-          Hot wire? .. Nahhh.
> >>
> >That would work, but you've got the problem that the temperature of the breath isn't predictable
> >
> It's quiet predictable and constant within 0.1 Celcius.

Actually, I just recently worked with a pneumotach that was
calculating flow rate based using the hot wire method.

I don't know much more than that, but it CAN be done.  The air
was actually directed across a fairly elaborate heat sinking fin
array.

Mike H.

2006\02\20@044153 by Alan B. Pearce

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>I want to make a alcohol breath analyzer, how can
>one measure the air speed when blowing trough the
>analyzer's tube?

I would do it by measuring the pressure drop across a constriction in the
air path - or more correctly the pressure increase due to the person blowing
through the constriction. Use a semiconductor pressure transducer should do
it.

2006\02\20@055129 by Mauricio Jancic

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Ok, thanks to everyone. Now another related question.

If I have this tube and the constriction, I would measure both pressures and
then I would now that:

P1 + Ek1 + Ep1 = P2 + Ek1 + Ep2

Where Px = Pressure
Ekx = Kinetic Energy = 1/2 * Density * V^2
Epx = Potential Energy = Density * g * h

OK, so now, how do I get rid of the density? :) I would spect breath to have
different densities, would I? Or should I try to characterize breath density
and use it as a constant?

In the end, I would need to measure the actual flowrate in liters/second, so
I would need to calculate V1 or V2... right?

Mauricio Jancic
Janso Desarrollos
Microchip Consultant Program Member
EraseMEinfospam_OUTspamTakeThisOuTjanso.com.ar
http://www.janso.com.ar
+54 11 4542 3519
> {Original Message removed}

2006\02\20@070659 by Gerhard Fiedler

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Stef Mientki wrote:

>> That would work, but you've got the problem that the temperature of the
>> breath isn't predictable and is difficult to measure, so probably not a
>> good one.
>
> It's quiet predictable and constant within 0.1 Celcius. And measuring it
> with a very-very small thermocouple isn't too difficult.

Probably better in this temp range than a thermocouple would be a PTC like
Pt100 or Pt1000. You can use two Pt100 elements, one heated (high measuring
current) and one not (low measuring current). The airflow cools the hot
one, you measure the difference.

Gerhard

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