At 12:03 PM 11/12/97 +1100, you wrote:
>Hi Rick and other interested parties,
>
>I don't want to be seen to be raining on your parade, .... but has anyone
>thought to look at the sensor science behind this project yet?
>
>Back in the mid '70s I fleshed out the design for an ultrasonic anemometer
>based upon an orthogonal array of 40KHz transducers. As I recall it, the
>speed of sound is a function of air temperature and moisture content. If you
>ignore these, as it appears is your intent from your pin assignments below,
>you are likely to reap the consequential accuracy and stability limits.
>Perhaps "y'all" need to decide what your project objectives are in terms of
>performance and then look at what units, baud rates etc can be applied.
>
>Ross "Often the realist" McKenzie
>Melbourne Australia

Ross,

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


Sean Breheny,KA3YXM
Electrical Engineering Student