proudly sponsored by
|
archiving of SOWACS proudly sponsored by |
|
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! 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