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November 11, 1996 To: SOWACS@aqua.ccwr.ac.za On 11 Nov 1996, Romain Gagnon <RG@smartrain.com> wrote: > Also, do you have any equations or empirical relationships >relating soil salinity and soil conductivity at a certain >frequency ? No, but you should read Rhoades, et al., 1989, Soil electrical conductivity and soil salinity: new formulations and calibrations, Soil Sci. Soc. Am. J., 53:433-439 *** On a separate matter - several of the manufacturers of new sensors claim them to be "frequency domain" or "velocity domain" by virtue of letting the reflected pulse set the frequency of oscillation. This supposedly avoids the expense of time domain measurements by measuring frequency rather than time delay. The speed of an electrical pulse in wire is about a nanosecond per foot. For a one-foot sensor, the return of the reflected signal would then be about two nanoseconds. If it truely set another pulse immediately in motion, the repetition rate of the pulses in the sensor would be about 500 million per second. The reduction of pulse velocity with the addition of a propagation medium of higher dielectric constant can be only a few percent. That's why TDR setups are so costly - the measurement of delays in fractions of nanoseconds. If a sensor does not radiate at about 500 megahertz and the frequency varies over more than ten or twenty percent with water content, it's not measuring pulse "velocity" in the probe. It might instead be a simple astable multivibrator, with the probe as the external resistor or capacitor. If so, that's why there can be no patent. Any 2-year electronics technician student can make one. It's not a bad method, just not dependent on the velocity of the signal in the probe. It would also account for the loss of signal if the salinity gets too high - it would short out the timing capacitor formed by the probe. Don Baker dbaker@comp.uark.edu