|I have just completed a telemetry device driven by a PIC
|that can measure frequencies from 0-64kHz in 1Hz steps.

|I now want to use this LM3909 circuit in a pipe to measure
|the level, but I would prefer to know what the pipe sizes
|should be before I start cutting...

|I want to measure a maximum level difference of 2m with a
|maximum frequency deviation of 1 octave. What should the
|diameter be?

If you're after a maximum deviation of an octave with a
length deviation of 2m, then your effective pipe length
will need to vary between 2m and 4m (i.e. 2m above the
water).  Diameter should not be at all critical, and the
pipe need not be straight (provided that the bends are
reasonably nice); the important thing to note is that any
sudden changes in pipe diameter or direction will cause
audio reflections.

Depending upon the phasing of your resonating circuit,
a 2-4m pipe will vary in pitch from 20-40Hz or from 40-80Hz.
Those frequencies are fairly low, so you might want to go
for the higher one (I think it may require using a seperate
speaker and mic, though).  Would be interesting to know how
it works out.


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