Time Domain Reflectometry Measurement of Water Content and Electrical Conductivity Using a Polyolefin Coated TDR Probe

TDR;Coated probes;Geography

McIsaac, Gerald

University of Waterloo

关键词: TDR;    Coated probes;    Geography;   

Others  :  https://uwspace.uwaterloo.ca/bitstream/10012/5213/1/McIsaac_Gerald.pdf

瑞士|英语

来源: UWSPACE Waterloo Institutional Repository

【 摘 要 】

The use of time domain reflectometry (TDR) to determine water content (θv) from themeasurement of the apparent dielectric constant (Ka) or the square root of the apparent dielectricconstant (Ka0.5) in highly saline environments has been limited due to the dampening effect thatelectrical conductivity (EC) has on the TDR signal. The objective of this research was to evaluate theuse 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 CS605TDR probe increased the capability of the probe to measure θv at EC levels as high as 1.06 S m-1compared to 0.132 S m-1 for a NC CS605 probe. The CCRC probe was found to be incapable ofdetermining any difference in EC levels. A 0.01 m long section or ;;gap” at the center of thepolyolefin coating on the center conducting rod (GAP probe) was cut from the polyolefin coating toexpose a section of the stainless steel center-conducting rod to allow direct contact with the materialbeing sampled. The GAP probe was found to be capable of measuring θv and EC at EC levels as highas 0.558 S m-1.Using a water-air immersion method, a comparison between the NC probe and the CCRC andGAP probes was undertaken. The correlation between θv vs. Ka0.5 was found to be linear for all threeprobes with the slope (m) of the regressed equation for the NC probe (m = 7.71) being approximatelytwice that of the CCRC probe (m = 4.25) and the GAP probe (m = 4.36). The intercept values wereequivalent for all three probes. The linearity between θv vs. Ka0.5 for the NC and CCRC probes usingthe water-air immersion method was also observed when the probes were used to measure Ka0.5 ofdifferent sand-water mixtures. The slope of regressed equation for the NC probe in the sand-waterivmixtures (m = 7.69) was equivalent to the water-air immersion slope for the NC probe, however theintercept values for the sand-water mixtures was lower than the intercept values for the water-airimmersion 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 Ka0.5 values using a waterairdielectric-mixing model (WAMM) were equivalent to measured Ka0.5 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 ofhomogenous water contents should be completed to ascertain the reliability of the water-airimmersion method.The straightforward relationship between the inverse of TDR measured impedance (ZL-1) and ECprovided 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 EClevel of 0.132 S m-1 for the NC probe. The use of the cell constant method was considered to beunacceptable as a means of developing a calibration curve due to the fact that the cell constant K wasnot a constant value.Ka0.5 values for the CCRC and GAP were consistently less than Ka0.5 values for the NC probeat all qv levels except θv = 0.000 m3 m-3 or 100% air. The difference in Ka0.5 (DKa0.5) between the NCprobe and the CCRC and GAP probes was seen to increase with increasing water content. Similarly, ameasurable effect was found between the TDR waveforms for the NC probe when the probe head wassurrounded completely by air when compared to the TDR waveforms for the NC probe when theprobe head was completely surrounded by water. Modeled electrostatic fields for the NC and CCRCCS605 TDR probes displayed a decrease in the electric potential and electric field intensity in theregion outside of the polyolefin coating of the CCRC probe compared to the NC probe. The decreasevin potential and electric field intensity became greater when the dielectric constant of the materialsurrounding the CCRC probe increased.The use of a polyolefin coating on the center-conducting rod with a small section of thecoating removed at the midsection of rod provides an effective means of extending the application ofTDR θv and EC measurement in saline environments where standard TDR probes cannot be used.