Re: Aquaflex soil moisture sensor

[John R Johnston <john_johnston@juno.com>]

John R Johnston <john_johnston@juno.com> writes

Essentially this is a time domain based electromagnetic energy
transmission technique with all of the inherent advantages, and
presumably many of the environmental disadvantages of TDR. The principal
advantages of this technique appear to be that it doesn't have to deal
with the interpretation of a degraded signal at the receiving end because
it:

(a) can use a low loss cable, instead of a high loss parallel bar or rod
transmission line,
(b) has only half the distance to travel (compared to a TDR with the same
"region of influence"), and,
(c) it doesn't lose energy at the open segment end (a high loss
discontinuity TDR requires to create the reflection for an open ended
system approach).

The above provide application advantages because:

(a) low loss transmission techniques enable a longer transmission line,
therefore a larger "region of influence", hence a 3 meter long sensor
cable.....versas the typical TDR basic rod length of 30 cm. The larger
region of influence presumably average variations in the environment that
is being measured;......if properly installed without voids or excessive
soil compaction, and
(b) less expensive components can be used in the sensor and instrument,
minimizing the cost of a system.

To gain the above advantages you must sacrifice the insertion advantage
of a rigid rod, and accept a disturbed environment by:

(a) burying the cable, or
(b) cutting a slice in the soil to a target depth and inserting the
cable, then recompressing the edges of the slice, or
(c) burying the cable when a new location, or planting is initially
prepared.

All of the calibration issues (variations in bulk density, variations in
partical size and distribution, etc.) effecting TDR also presumably
effect this technique, however they are to some extent exacerbated by the
fact that the cable must be buried, complicating any calibration.

The technique doesn't actually measure water content, it is inferred by a
related measurement of the overall environment. So, if the application is
one where relative change is of interest, then this soil moisture tool
works as well as any, and better than some. If absolute water contents
are necessary, the requirement to calibrate is slightly more or less
challenging than any other technique.

Is the above summary generally correct?

Thanks, John



 At 07:03 PM 8/11/98 GMT, you wrote:
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>
>>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.