cdb wrote:

>Option 2. Use either a differential or maybe two pressure sensors 
>inserted into pipe, difference in air pressure = rate of flow - not 
>too sure on this.
>  
>
Seems to me this might work using the Venturi and *Bernoulli's 
principles*.  Depending on the flow rate, this could generate a good 
pressure.  Wikipedia claims there are two different equations for 
compressible and non-compressible fluids. 

See: <http://en.wikipedia.org/wiki/Venturi> and 
<http://en.wikipedia.org/wiki/Bernoulli%27s_principle>

Cheers,
David


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