What! No Reflection?

excerpt from the Soil Report Newsletter of Soilmoisture Equipment Corp.

For those of you familiar with TDR, the possibilities of not having a detectable end reflection is not just an imaginary fear but a real and annoying occurrence. This frustration is compounded when you're unable to tell with absolute certainty where and when a TDR waveform will end in a dreaded "flatline" reflection. There are, however, some measuring circumstances that seem to invite the death of a TDR signal through high attenuation levels. These foreboding areas of measurement that gobble up TDR signals include bogs, materials with a high saline content, fertilizers, urea, pore waters having extremely high or low pH, moist dense clays, and other conductive media. In the past, there has been no way to alleviate or mitigate this detrimental effect on end reflections from these harsh environmental conditions.

Now there is help to get a TDR reflection, irregardless of the media. It comes in the form of precision dielectric coated waveguides, both standard and buriable, developed by Soilmoisture. Over the last several years engineers working at Soilmoisture have developed waveguides that enhance the reflective nature of a TDR signal without losing appreciable measurement volume surrounding the waveguides. The result is two types of proprietary coated waveguides both featured in our new TRASE Accessory Brochure, the Coated Connector-type waveguide, model 6008CL, and the CoatedBuriable type waveguide, Model 6005CL2.

For example, let's take a short connector type 15cm long, uncoated (plain stainless steel waveguides). You will notice a dramatic effect of TDR signal loss in mild 4 mmho (0.04 deciSiemens) salt solution. With longer uncoated waveguides, say 60 cm long, there is absolutely no end reflection. With Soilmoisture's "Hi-Reflectance" coated waveguides, however, you can measure very high saline conditions, 50 mmho (.5 deciSiemens) or higher, using a coated 60 cm long waveguide and still get good end reflections. This opens the door to research in media and conditions not available for TDR measurement prior to the advent of these coated waveguides. Coated waveguides are recommended for any TDR installation that might be exposed to high electrolyte pulses such as roads beds salted in the winter or airports requiring deicing processes prior to departure. Coated waveguides also work well in treatment sites that have high residual caustic or electrolyte content, including such interesting locals as leach pits, ore dumps, evaporite ponds, sludge and slurry sites. If you have a need to measure in a highly attenuative condition and have been unhappy finding a "dead" or "absent" end reflection, consider using a product specifically designed and built to find a TDR end reflection.