At 08:48 PM 11/11/97 -0500, Sean Breheny wrote:

>I did a little bit of checking and here is how the speed of sound in still
>air varies with humidity and temperature:
>
>At 20deg Cel.
>
>0% Rel Humidity for 40KHz : 1127.188 ft/sec
>100% RH for 40KHz: 1130.963 ft/sec
>
>Not a bit deal at all for RH, would cause less than 1% error.
>
>For Temp:
>
>30 deg Cel (0% RH): 1145.6 ft/sec
>0 deg Cel (0% RH): 1087.4 ft/sec
>
>Here, we have about a 5.3 % variation over the temp range 0 to 30 deg C.
>This is not too bad either because it is unlikely that most of the
>applications mentioned here (with the possible exception of the grain silo)
>would not subject the unit to such a variability in environment. Besides,
>does a farmer really care THAT much if he only knows his grain storage
>amount to 5% on a real time basis?
>
>In your anemometer design, I would think that you were measuring the change
>in the speed of sound in air due to air movement. This is a somewhat more
>pronounced effect, since it is almost like velocity addition. However, even
>extreme wind speeds are only a fraction of the speed of sound.

Sean,

Thanks for the research into the actual numbers.  It doesn't sound like either
 effect will cause much of a problem for us.  I can't imagine that our pulse
 detection circuitry will have an accuracy much better than a few percent over
 short distances, anyway.  As long as we can come "pretty close", I think that
 we will be fine.

 - Rick

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