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Pat-- Your email re the segmented TDR probe reminded me of an exchange I had with Romain Gagnon of SmartRain: >>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 ! > Based on Romain's response, I'm wondering if a more sophisticated program to interpret the data, rather than a segmented probe, is what's needed... Coming from an RF background in the television broadcast equipment industry, I have found it is difficult for even RF-types to understand that is what goes on BETWEEN the two rods of a Campbell Scientific balanced line type probe that counts. Even (especially!) in the microstripline type probe I have proposed, the dielectric between the conductor and the ground plane has the overwhelming effect on what is sensed by the Tek 1502 (or whatever) instrument. The effects of soil outside the conductors or conductor/ground plane of the TDR probe decrease according to a square law function--so 1 cm might be stretching it. The single (but useful) advantage of the microstripline approach I proposed is that the relationship between conductor and ground plane would be absolutely stable, being constructed of printed circuit board material, and avoid anomalous readings caused by a stone or root spreading or narrowing the distance between the rods in a balanced line type of probe. In the broadcast equipment industry, TDR has given way to spectrum and network analyzers--the longer this dialog goes on in SoWaCS, the more I think these more advanced technologies might be appropriate here. Dave Barnett UMass CEE Dept At 05:03 PM 12/20/96 -0500, you wrote: >Return-path: <majordom@aqua.ccwr.ac.za> >Received: from aqua.ccwr.ac.za by ECS.UMASS.EDU (PMDF V5.0-7 #6523) > id <01ID8O19HFYODNY84X@ECS.UMASS.EDU> for dbarnett@ecs.umass.edu; Fri, > 20 Dec 1996 17:03:18 -0500 >Received: by aqua.ccwr.ac.za (AIX 4.1/UCB 5.64/4.03) id AA39416; Fri, > 20 Dec 1996 19:21:10 +0000 (GMT) >Received: from unlvm.unl.edu by aqua.ccwr.ac.za (AIX 4.1/UCB 5.64/4.03) > id AA23536; Fri, 20 Dec 1996 21:20:55 +0200 >Received: from UNLVM.UNL.EDU by UNLVM.UNL.EDU (IBM VM SMTP V2R2) > with BSMTP id 1801; Fri, 20 Dec 1996 13:18:26 -0600 (CST) >Received: from UNLVM (NJE origin AGRO280@UNLVM) > by UNLVM.UNL.EDU (LMail V1.2a/1.8a) with BSMTP id 8860; Fri, > 20 Dec 1996 13:18:27 -0600 >Date: Fri, 20 Dec 1996 12:54:35 -0600 (CST) >From: Pat Mielnick <AGRO280@UNLVM.UNL.EDU> >Subject: TDR >Sender: owner-sowacs@aqua.ccwr.ac.za >To: sowacs@aqua.ccwr.ac.za >Message-id: <961220.131825.CST.AGRO280@UNLVM> >Content-transfer-encoding: 7BIT >Precedence: bulk > >I've been a lurker for a few weeks on the sowacs list and am very >interested in the continuing discussion regarding TDR. I'm wondering >if anyone has had experience using the segmented TDR probes made by >Moisture Point out of California. They use a series of remote shorting >diodes implanted in a resinous matrix that is in a rod shape. You >insert this (with some difficulty in clay soils) in the soil and then >hook up a console that produces the electromagnetic signal which >travels down the length of the probe and back again to give you a >volumetric water content. Each diode gives a soil water content at >the depth in soil where it is located. Problems occur with the probe >bending when inserted into the soil, and with inaccurate soil moisture >measurements in layered soils. Also, recently I've heard that the >sphere of actual soil moisture measurement is less than 1 cm around >the probe. If anyone has used this type of instrumentation, I would >appreciate hearing any comments regarding it. > I've used a "homemade" version of a TDR system where I had short >(< 15 cm) stainless steel rods cut and attached coaxial cable to the >ends. The other end of the cable went into a sort of multiplexer >which then was read (with another short cable) by the Tektronix >cable tester and a Campbell data logger plus batteries (for field >situations). The rods were left in place all season and when >calibrated against gravimetric measurements were within +/- 3% >(note that the gravimetric samples were not pulled from the exact >site of the TDR rods). Problems often occurred when the cable >connections at the top of the rods came loose or when an animal >(i.e. a deer) stepped on a cable. We also had some problems with >wiring in the multiplexer. Thus, the Moisture Point system seemed >like a great idea because everything is incorporated into one >segmented probe and you are not fooling around with miles of >cable. Now, there appear to be some "bugs" with that system >as well (of course!). > Any informative comments would be appreciated. Thanks. >