Re: Aquaflex soil moisture sensor

[Marty Grogan]

>Steve Evett    srevett@ag.gov writes:
>
>For Tom Reynolds and all:
>
>I believe that Robert Lascano has done some work on comparison of the
>theta probe to gravimetric water contents (r-lascano@tamu.edu).  Contact
>him to see if he has published.  I doubt if anyone has compared the theta
>probe to neutron scattering.  The measurement volumes and depths of
>application are so different it wouldn't make a lot of sense to directly
>compare these two (or am I missing something).  I'm thinking of the "theta
>probe" with pointed rods that one presses into the soil.
>
>About the AquaFlex sensor, I've been following the discussion with
>interest.  But in all of it I've not yet found a good technical
>description of how it works.  That is much needed in this discussion. 

Basically, electromagnetic waves propagate at a velocity directly related to
the dielectric properties of the surrounding medium.  In the case of TDR and
cable, both techniques use timing measurements.  TDR uses round trip
times--cable uses single trip timimg.  The timing can be measured using many
techniques, i.e., variable oscillators, phase references or counters.

TDR requires access to only one end of a conductor pair, cable methods
require access to both ends.  The time delay along the conductors varies
according to the volumetric percentage of water adjacent due to the fact
that water presents a much higher dielectric than either air or soil.

Implementing circuits and techniques offer many different tradeoffs
regarding cost, convenience and accuracy.  A simple demonstration can be
made using a length of 300 ohm twin lead, a signal generator and an
oscilloscope:

1.  Make a coil of 10' of the 300 ohm twin lead about 6" diameter.
2.  Attach the signal generator to one end of the twin lead.
3.  Attach one channel of the oscilloscope to the signal generator.
4.  Attach a 300 ohm resistor and a second channel of the oscilloscope to
the other end of the twin lead.
5.  Measure the difference between the two signals (x1-x2) on the oscilloscope.
6.  Place the coil of wire into a bucket of water--do not immerse the
connections.

The oscilloscope will display the phase change that results from the change
in effective dielectric--and yes, a portable oscilloscope used in this
fashion could make an effective soil moisture monitor.

M. G.