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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.