The use of time domain reflectometry (TDR) to determine water content (θv) from the
measurement of the apparent dielectric constant (Ka) or the square root of the apparent dielectric
constant (Ka
0.5) in highly saline environments has been limited due to the dampening effect that
electrical conductivity (EC) has on the TDR signal. The objective of this research was to evaluate the
use of a three-rod TDR probe with a polyolefin coating on the center-conducting rod (CCRC probe)
to simultaneously measure θv and EC in saline conditions where standard, non-coated TDR probes
(NC probe) are ineffective.
The application of a 0.00053 m thick polyolefin coating on the center-conducting rod of a CS605
TDR probe increased the capability of the probe to measure θv at EC levels as high as 1.06 S m-1
compared to 0.132 S m-1 for a NC CS605 probe. The CCRC probe was found to be incapable of
determining any difference in EC levels. A 0.01 m long section or “gap” at the center of the
polyolefin coating on the center conducting rod (GAP probe) was cut from the polyolefin coating to
expose a section of the stainless steel center-conducting rod to allow direct contact with the material
being sampled. The GAP probe was found to be capable of measuring θv and EC at EC levels as high
as 0.558 S m-1.
Using a water-air immersion method, a comparison between the NC probe and the CCRC and
GAP probes was undertaken. The correlation between θv vs. Ka
0.5 was found to be linear for all three
probes with the slope (m) of the regressed equation for the NC probe (m = 7.71) being approximately
twice that of the CCRC probe (m = 4.25) and the GAP probe (m = 4.36). The intercept values were
equivalent for all three probes. The linearity between θv vs. Ka
0.5 for the NC and CCRC probes using
the water-air immersion method was also observed when the probes were used to measure Ka
0.5 of
different sand-water mixtures. The slope of regressed equation for the NC probe in the sand-water
iv
mixtures (m = 7.69) was equivalent to the water-air immersion slope for the NC probe, however the
intercept values for the sand-water mixtures was lower than the intercept values for the water-air
immersion method. Similarly, the slope of the CCRC probe in the sand-water mixtures (m = 5.00)
was equivalent to the CCRC probe water-air immersion slope. Calculated Ka
0.5 values using a waterair
dielectric-mixing model (WAMM) were equivalent to measured Ka
0.5 values for the NC probe.
The water air immersion method was found to provide a suitable methodology for TDR research,
however a more definitive test of the coated probe response in a series of soils with a range of
homogenous water contents should be completed to ascertain the reliability of the water-air
immersion method.
The straightforward relationship between the inverse of TDR measured impedance (ZL
-1) and EC
provided an effective calibration method for both the NC and GAP probes. The use of the Giese-
Tiemann method to establish a calibration curve for EC measurement was limited to a maximum EC
level of 0.132 S m-1 for the NC probe. The use of the cell constant method was considered to be
unacceptable as a means of developing a calibration curve due to the fact that the cell constant K was
not a constant value.
Ka
0.5 values for the CCRC and GAP were consistently less than Ka
0.5 values for the NC probe
at all qv levels except θv = 0.000 m3 m-3 or 100% air. The difference in Ka
0.5 (DKa
0.5) between the NC
probe and the CCRC and GAP probes was seen to increase with increasing water content. Similarly, a
measurable effect was found between the TDR waveforms for the NC probe when the probe head was
surrounded completely by air when compared to the TDR waveforms for the NC probe when the
probe head was completely surrounded by water. Modeled electrostatic fields for the NC and CCRC
CS605 TDR probes displayed a decrease in the electric potential and electric field intensity in the
region outside of the polyolefin coating of the CCRC probe compared to the NC probe. The decrease
v
in potential and electric field intensity became greater when the dielectric constant of the material
surrounding the CCRC probe increased.
The use of a polyolefin coating on the center-conducting rod with a small section of the
coating removed at the midsection of rod provides an effective means of extending the application of
TDR θv and EC measurement in saline environments where standard TDR probes cannot be used.
| Identifer | oai:union.ndltd.org:WATERLOO/oai:uwspace.uwaterloo.ca:10012/5213 |
| Date | 18 May 2010 |
| Creators | McIsaac, Gerald |
| Source Sets | University of Waterloo Electronic Theses Repository |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
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