>
> On Feb 4, 2010, at 10:02 AM, MCH wrote:
>
> Good info, but I won't be controlling the fan speed - I'm just looking
> for when it slows down or stops (due to age or what have you).
>
> IOW, I need to make sure the fan is running and close to rated speed  
> for
> the (nearly constant) voltage provided. If it's normal RPM is 1300  
> RPM,
> I want to sound an alert when it drops below 1000 or 800 RPM.
>
> BTW, this will be done with two (redundant - running side-by-side)  
> fans,
> so I'm not sure if the watchdog idea would work or not.
>
> For this project, it is essential that the "wind keep blowing" - hence
> the redundant fans and the alarm if one of them fails or slows.
>
> Joe M.


You can always build something that detects the wind is blowing.   
Depending on your orientation of the fans, you can put a "flag" in the  
airflow and optically / magnetically / capacitively / e-field  detect  
that it is waving.   Or measure a temperature differential that  
indicates that air is flowing.

Gus
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