Hi there,
Ive been listening in to this...sounds interesting....personally I have no
need for such a unit at present...
but still interesting.
Q. Will ambient air pressure variation affect sound velocity
significantly?

Just asking....

TTYL Paul

>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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