The Virrib sensor

Dear soil water content buffs: At the sowacs home page under the category of "other" methods the phase transmission approach (the virrib sensor) is discussed with a big question mark as to how it works. So, here is my best hypothesis: The phase shift of a sinusoidal wave having traveled a fixed distance, relative to the phase at the origin, depends on the length of travel, the frequency, and most importantly the velocity. Thus for fixed frequency and travel path, a measurement of this phase shift will give a measure of water content. This has been known for some time, but had not evolved into a practical instrument. We had attempted (with very little practical success) to measure this phase shift for antennae transmissions through undisturbed soil thereby measuring a large volume of soil. The problems we had were with unacceptibly low power throughput, spurious pathways of travel, and transmission line induced phase errors. The use of waveguide type transmission with the phase measuring electronics mounted to the waveguide and the waveguide beginning and ending at the electronics block could presumably solve all of those problems. i.e. something that looks very much like the virrib sensor. This may be the idea with the virrib phase transmission method. If so, it seems to me like a particularly ingenious idea. When I was working with this type of phase transmission, I was calling it "phase domain" and would defer from calling it frequency domain or time domain (for fear of starting another controvercy) and it does not even fit with traditional definitions of reflectometry. We at the UW are particularly interested in testing out any type of new inexpensive sowac instruments and do appreciate peoples input and ingenuity. Thanks, g.s. ************************************************ * Gordon Starr * * Dissertator * * University of Wisconsin, Madison * * Soil Science Department - Soil Physics * * 1525 Observatory Drive * * Madison, WI 53716 * * 608-262-0415 * * gcstarr@facstaff.wisc.edu * * * ************************************************

We have tested FDR, TDR and PDR (phase domain reflectometry) and really
  beleive that the latter is the best.  However, Virrib's sensor is using a
  pulse instead of a single tone signal.  Therefore, they suffer from the
  same limitation than time-domain: the pulse gets distorted when transmitted
  reducing precision. Also, Virrib's sensor is limited to 3dS/m which is too
  low to be used in south-west usa.

  We are willing to pre-sell samples of our new sensor to scientists only in
  May, that is 2 months earlier than official release date.  If anyone
  interested, please send your inquiry via E-mail.

Romain Gagnon, Eng
  President & CEO
  Smart Rain Corporation
  1505, Place de l'Hotel-de-ville, suite 102
  St-Bruno, Quebec
  Canada, J3V 5Y6

  tel:   (514)441-4289
  fax:   (514)441-2147
  cell:  (514)893-6506
  E-mail:RGagnon@SmartRain.com
  WEB:   http://www.SmartRain.com

 The advantage found for using the cable tester for estimating the impedance
  of wave guides buried in moist soil was and continues to be the fact that
   the dielectric constant of water is nearly independent of dissolved salts in
   the 1 to 3 GHz range. Using frequencies in the more convinient (sp?) MHz
  ranges will
   create some strange dependencies between soil type, salinity, texture, and
  ogranic matter. But so does oven drying.

  Perhaps the use of several discrete frequencies could be used and the
  overall pattern of responses could be used to estimate texture, BD, OM,
  water, and salinity?? At one time the UofNEbraska was doing work in this
  area but I have not heard from Dr.Skopp for sometime.

Nearly is the operative word here. In agricultural settings i suppose,
  salinities are kept pretty low by irrigation and subsequent flushing out
  of collected salts. however, our work in desert soils has found many locals
  where TDR was completely inoperable, (all signal lost to the conductive 
  soil), or even worse, where the conductivity was not so high as to completely
  attenuate the signal but caused a significant over estimation of water 
  contents. Electrical insulation of the rods of the tdr probe extends the
  operation range but requires an individual calibration for each soil, which
  makes the whole technique much less friendly, not even to mention that the
  salinity level could change at a given local and invalidate the calibration.
  I have observed the same problem with salinity with the FD tool sold by
  troxler, actually even worse. I believe that soil moisture and campbell 
  scientific both no offer insulated tdr probes for just this purpose. I am
  very interested in this smart sensor, smart probe (excuse me i cant remember 
  the name of the probe) which has been discussed so much, prehaps it is the
  silver bullet for conductive soils as reported? 
  Sam Hokett
  Desert Research Inst

In case recent SOWACS participants missed the following posts last year,
  here they are again.  

  This discussion led to the formation of HydroTek and development of the
  WAVE-1 sensor.  See WWW.HYDROTEK.COM for further information.

  >
  >Posted 03/12/96
  >
  >To: IRRIGATION-L@LISTSERV.GMD.DE (AND OTHERS)
  >From: ges@oneworld.owt.com (Marty Grogan)
  >Subject: Do-It-Yourself TDR, Really Cheap!
  >
  >Use a square pulse generator, a length of 300 ohm TV (flat
  >twin lead) antenna wire and an oscilloscope.  Connect the
  >oscillator to the TV wire, a length of which is buried in
  >the ground to be measured.  The longer the buried wire, the
  >more sensitive the measurement will be.  Measure the time
  >delay along the wire using the oscilloscope by comparing an
  >entering pulse with an exiting pulse.  Be sure to match the
  >impedance at both ends of the TV wire to 300 ohms.  The TV
  >wire will perform as a variable time delay line providing an
  >indication of the dielectric changes in the surrounding dirt
  >due to changes in moisture.  The measurement will represent
  >an average over the length of wire buried.  An interval
  >measuring counter can be used instead of the oscilloscope.
  >Experiment with the pulse repetition rate and pulse width
  >until you get the best results.  Calibrate this system as
  >you would any other.  Higher rates and shorter pulses will
  >probably work best.  Both ends of the TV wire must be
  >accessible, so the wire should be buried in a circular
  >pattern.
  >
  >Integrated circuits are available to construct this setup
  >for less than twenty dollars.  A simple voltmeter could be
  >used for the indicator.
  >
  >Posted:  3/16/96
  >
  >From: ges@oneworld.owt.com (Marty Grogan)
  >To: Multiple recipients of list 
  >  (and others)
  >Subject: Re:  Really Cheap TDR
  >X-Listprocessor-Version: 6.0c -- ListProcessor by Anastasios Kotsikonas
  >X-Comment:  Precision Agriculture Mailing list
  >
  >Because of the apparent interest in transmission line TDR, I will build
  some experimental devices.  If you are interested, send EMAIL describing any
  special geometry requirements (circumference, length), dynamic response
  needed, mechanical issues, soil depth, sensitivity (relative H2O by volume),
  quantity needed, sample 
  >spacing, number of samples, etc.
  >
  >I expect to provide 4-20 ma, line powered devices at about one-third the
  regular TDR probe cost.
  >
  >If production units result, the experimental units will be replaced with
  new units in return for information about successful use.  (Postable, of
  course.)
  >
  >I am also interested in any problems you may be having using available
  equipment to see if I might be able to address solutions with the
  experimental units.
  >
  >Experimental units will include complete drawings, electrical schematics
  and a liscense to make all of the units you want for your own use.
  >
  >Some price and delivery advantage will be available for quantity orders and
  prepayment.

  I have completed a short run of experimental devices which are being
  evaluated by several individuals.  Full scale production will begin very soon.

  Although the present single unit price is around US$300, volume production
  and purchasing can easily cut production costs and prices in half.

  Any opinions participants would care to offer concerning decision-price
  points are quite welcome.

  There have been many requests for a low-power, voltage output device.  This
  type of device has been designed and will soon become available at a
  slightly lower cost than the industrial device.

  Marty Grogan, BSAE, MSEE
  Owner, Grogan Engineering Services
  President, HydroTek, Inc.
  1328 Rathwood Ave.
  Richland, WA  99352
  (509)627-3083 (voice and FAX)
  ges@oneworld.owt.com

  Specializing in Systems Integration Services for Agriculture:  Automation,
  Telemetry, Programming, Maintenance, Remote Sensing and Project Delivery.

               GES--The technology buckstopper.

I think we have established that the CS615 is TRUE TDR, but not FULL BLOWN
  TDR.  Each technology has its limitations and--fortunately--overlaps others.
  With the application of Network Analyzers, the final chapter obviously has
  not been written on SoWaCS.  Just add money--a lot in the case of "FULL
  BLOWN TDR" (Tek 1502C) or Network Analyzers. And calibrate often.

  Thanks for everyone's contributions while I watched SoWaCS for Bruce
  Metelerkamp until Mark Summerton comes on board as of next week.  If you
  have had any problems, email me here at:  I will try
  to take care of them so they won't be a burden to Mark.


  Dave Barnett
  UMass CEE Dept