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


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