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At 12:49 96-11-08 -0800, you wrote: > > Romain Gagnon wrote the following > >+Why would you want to add heat-shrink tube to TDR or FDR probes ? > >+It will not prevent the signal from being attenuated by the saline soil even >+though there is no direct contact between the probe and the soil. Also it >+would lower the sensitivity therefore the precision of the sensor even after >+calibration. > >It has been our experience that the TDR can be used in much more conductive >soils by insulating the rods of the probe with fiberglass resin, or any other >insulator. In cases where no visible reflection is seen from the probe end, >the insulation enables a strong reflection to be recorded from the probe end. >You are correct that the sensitivity of the system is decreased, but good results are still obtainable. This loss in sensitivity can be compensated for >to some degree by using a longer probe length. (shorter un-insulated probe >lengths would be the traditional approach to using tdr in more conductive >media. Naturally sensitivity is also lost by shortening the probes. The obvious problem is that the system has to be >calibrated for each conductive soil it is to be used in. Also insulating the >probes is difficult and it is unknown how long such coatings will last in the >field. In cases where we have measured the amplitude of the reflected TDR signal, amplitude remains constant even as conductivity increases. Ofcourse in >the case of an uninsulated probe amplitude of signal decreases with increasing >conductivity and this fact has been used widely to calculate soil >bulk conductivity. It seems to me that if an insulated probes signal amplitude >does not decrease with increasing conductivity that in effect signal attenuation >has been prevented? > >Sam Hokett >Desert Research Institute >Las Vegas, Nv >Sam@snsc.dri.edu > > > If you insulate TDR probes, the reflected signal will obviously be less attenuated. However, as you say, the sensitivity will decrease accordingly. Now, if you make the probe longer to compensate for sensitivity loss, then your signal should be as much attenuated as the initial shorter and non-insulated probe...at least in theory. The fact that there is no contact has an impact on low frequencies but not on high frequencies. If you do observe a difference, then the frequency spectrum of the reflected pulse is different from the initial one. Therefore, the calibration required is more complex than a mere linear one based on probe length. Does that match your observations ? Romain Gagnon Eng Smart Rain Corporation inc 1505, Place de l'Hotel de Ville, suite 102 St-Bruno, Quebec, Canada --- J3V-5Y6 E-mail: Info@SmartRain.com Phone: 1-514-441-4289 FAX: 1-514-441-2147 WEB: http://www.SmartRain.com