Olin Lathrop wrote:
> Sean Breheny wrote:
> > In other words, the calibration method will probably have
> > to involve multiple known distances at several frequencies, AND this
> > will all probably shift with temperature.
> 
> And pressure and to a lesser extent humidity, basically anything that
> effects the speed of sound in air.

And these calibrations will have to be done to insane levels of precision.
Assuming the span in question is on the order of, say, 10", 0.0001"
accuracy will require knowing the speed of sound under all conditions to
better than 10 PPM. And, of course, the air in question will have to be
perfectly still. Basically impossible.

Seriously, anyone doing this kind of contactless distance measurement is
using optical interferometry. A laser diode, a beam splitter, a corner
reflector and a pair of phototransistors with a quadrature decoder to count
fringes.

Now, the OP did say "resolution" and not "accuracy", so maybe there isn't
a requirement for measuring the total span accurately, just changes in
distance.

-- Dave Tweed
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