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At 15:25 96-11-12 -0500, you wrote: >Finding such a lively dialog re SOil WAter Content Sensors was very lucky >for me. Hopefully SoWaCS will continue to thrive when Bruce leaves. The >subject seems well irrigated--and not over-fertilized! > >This posting is in response to email I received most recently directly >and/or via SoWaCS. Commercial and industrial production often leads academic >research. Even "intuitively obvious" ideas need to be verified and >quantified. Having instruments available off-the-shelf can save much money >and time. Changing from working for industry to academia reminds me of the >line, "Where you stand on an issue depends on where you sit." > >Romain Gagnon's comments and questions have been thought provoking and the >SmartRain website is very informative and user-friendly. > >My concern that while a TDR probe shorted at the end would be stronger than >an unterminated one, PUSHING it into the soil might create a cavity that >would collect water and give misleading readings may have been misplaced. >The Smart Rain sensor is apparently designed to be BURIED in the soil rather >than pushed into it. (The Environmental and Geotechnical people here push a >lot of cones instrumented with strain gages and pore pressure transducers, >etc. into the ground using drilling rigs and hydraulic rams.) It seems >ironic that the more rugged probe typically is used more gently. (My pet >project still is to make a fully insulated uStripline probe approximately 30 >cm [11.8 in.] long x 2.54 cm [1 in.] wide x 0.476 cm [3/16 in.] thick!) > >According to Smart Rain, their sensors are very advanced, providing soil >water content, fertilizer level and temperature and the system is >bi-directional. (While a centralized weather station is not necessary, >connecting one should allow you to control the weather! Sorry, Romain, I >couldn't pass that one up.) > >Sam Hokett and Don Baker's comments on insulating TDR probes and using >longer vs. shorter rod lengths bring up several points: >1. Conductive soils (higher saline or other ionic compound >concentrations) would be more "lossy" to the RF signal than typical soils. >Except that the Campbell Scientific and Smart Rain probes use the signal >reflected off the end of the probe for timing purposes, (It's not easy to >measure minute changes in a nominal 2 nanosecond pulse!) the probes could be >almost any length. The Tektronix 1502C TDR can measure metallic cable >defects up to 50,000 FEET! The TDR probes used here are intentionally >designed to be susceptible to their environment, so we can't just run down >to Radio Shack and buy a long roll of 300 Ohm twinlead TV wire to instrument >our whole site with one probe--but the idea is tantalizing! By the way, this idea does work ! >2. Sine wave oscillators should be used whenever possible. One of my >first projects here was to build an updated version of a 4 Electrode Soil >Conductivity Monitor. (Austin and Rhodes, Soil Sci, 1979) To avoid plating >the electrodes from one to the other and/or out into the soil requires two >operating conditions: a) an alternating current, and split power supplies >with the center referenced to earth ground. Two 9 Volt batteries easily >provided the split power supplies, but I made the mistake of using my >favorite CMOS IC, the CD4093 to generate a square wave instead of op-amps to >create a sine-wave. The 4ECM worked fine on the bench measuring a resistor, >but adding even a moderate length of cable to connect to electrodes on a >cone caused the signal coming back from the sample to be out of phase with >the reference voltage. I'm sure this can all be settled down with a couple >of small capacitors, but I haven't gotten to it yet. >3. The "salinity" of soil may not always be due to "Na+" ions. >Fertilized fields and golf courses, etc. might have a higher concentration >of ammonium nitrates, etc. Because the response of saline soils is >different at different frequencies, making soil conductivity measurements at >a minimum of three frequencies would indicate a curve that could be >characteristic of moisture, ion concentration and types of ions. Probably >some or all of this work has already been done and I just haven't had time >to research and read the literature. >4. Regarding patented vs. proprietary technology--If you could find a >novel way to use a wheel, even though you didn't invent the wheel, per se, >that "utility" could be patented. If a company develops novel technology, >by all means they should patent it and benefit fully from their inspiration >and perspiration. If not, a company should succeed by having the highest >quality product at a reasonable cost and with great customer service, rather >than hermetically sealing the product in a lot of mumbo-jumbo. > >Finally, it would be useful to hear more about situations, such as eucalytus >roots making cracks in clayey soils, that pour cold water in the face of >tidy technological answers that didn't hear the whole question... > >Dave Barnett >UMass CEE Dept > > > 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