And if the transmitter , receiver and target are not infinitely small=A0the= re is another problem=A0 ----- Original Message ---- > From: Dave Tweed > To: piclist@mit.edu > Sent: Sunday, August 30, 2009 1:05:52 PM > Subject: Re: [EE] Ultrasound at .0001 inches was: Measuring analog pulses= down to 8 nano-seconds. > = > 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 cou= nt > 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 = -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist