ASTM D6565-00
(Test Method)Standard Test Method for Determination of Water (Moisture) Content of Soil by the Time-Domain Reflectometry (TDR) Method
Standard Test Method for Determination of Water (Moisture) Content of Soil by the Time-Domain Reflectometry (TDR) Method
SCOPE
1.1 This test method covers the determination of water content (or moisture content) in soil by the use of the electromagnetic technique called Time-Domain Reflectometry (TDR).
1.2 This test method was written to detail the procedure for conventional TDR measurements of soil. Other TDR applications exist for the purpose of quantifying water content in soil and are not covered here, such as flat probe technologies and wetting front advance methods.
1.3 Commercial TDR applications exist which automate the TDR methodology and are not detailed in this test method. It is likely that overlap exists in the automated commercial systems versus this applied method, and the user is encouraged to adhere to this test method when applicable.
1.4 This test method is one of a series on vadose zone characterization methods. Other standards have been prepared on vadose zone characterization techniques.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D 6565 – 00
Standard Test Method for
Determination of Water (Moisture) Content of Soil by the
Time-Domain Reflectometry (TDR) Method
This standard is issued under the fixed designation D 6565; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope Pressure Tester Method
D 5220 Test Method for Water Content of Soil and Rock
1.1 This test method covers the determination of water
In-Place by the Neutron Depth Probe Method
content (or moisture content) in soil by the use of the
electromagnetic technique called Time-Domain Reflectometry
3. Terminology
(TDR).
3.1 Definitions:
1.2 This test method was written to detail the procedure for
3.1.1 time domain reflectometry (TDR)—anelectromagnetic
conventional TDR measurements of soil. Other TDR applica-
method used in the determination of water content of soil.
tions exist for the purpose of quantifying water content in soil
3.1.2 Definitions of other terminology used in this guide
and are not covered here, such as flat probe technologies and
may be found in Terminology D 653.
wetting front advance methods.
1.3 Commercial TDR applications exist which automate the
4. Summary of Test Method
TDR methodology and are not detailed in this test method. It is
4.1 Aspeciallyconstructed,multi-wave-guideTDRprobeis
likely that overlap exists in the automated commercial systems
inserted into the soil. The electronic cable tester (or automated
versus this applied method, and the user is encouraged to
commercial TDR electronics) is used to send a pulsed wave-
adhere to this test method when applicable.
form to the probe.The cable tester then receives a return signal
1.4 This test method is one of a series on vadose zone
which was influenced by the dielectric constant of the soil,
characterization methods. Other standards have been prepared
which in turn is a function of water content.An analysis of the
on vadose zone characterization techniques.
waveform trace supplies the necessary information to calculate
1.5 This standard does not purport to address all of the
the water content of the soil.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
5. Significance and Use
priate safety and health practices and determine the applica-
5.1 The determination of the water-content, or moisture
bility of regulatory limitations prior to use.
content, of soil is one of the fundamental needs in the soil
physics and hydrology disciplines. The need arises from
2. Referenced Documents
requirements for defining the optimal time for irrigation, the
2.1 ASTM Standards:
infiltration rate, the soil-moisture flux, contaminant transport
D 653 Terminology Relating to Soil, Rock, and Contained
rates, and evaluating the potential for leakage from a waste site
Fluids
or a surface or subsurface barrier.
D 1452 Practice for Soil Investigations and Sampling by
5.2 The TDR application covered in this test method is that
Auger Boring
used for point measurements of moisture content in soil. The
D 2216 TestMethodforLaboratoryDeterminationofWater
2 applicationiseitherthroughmanualinsertionintothesoilorby
(Moisture) Content of Soil and Rock by Mass
burying a probe in the subsurface to acquire moisture content
D 4643 Test Method for Determination of Water (Moisture)
2 data at a specific location. In addition, core samples may be
Content of Soil by the Microwave Oven Method
2 tested withTDR at a drill site to acquire real-time soil moisture
D 4700 Guide for Soil Sampling from the Vadose Zone
data.
D 4944 Test Method for Field Determination of Water
(Moisture) Content of Soil by the Calcium Carbide Gas
6. Interferences
6.1 TDRmeasurementsinconductivesoilsarehamperedby
the conductivity of the soil and the resulting signal attenuation.
ThistestmethodisunderthejurisdictionofASTMCommitteeD18onSoiland
Rock and is the direct responsibility of Subcommittee D18.21.02 on Vadose Zone
Typically, the amplitude of the voltage pulse reflected back to
Monitoring.
the TDR instrument is diminished in proportion to the soil’s
Current edition approved June 10, 2000. Published September 2000.
Annual Book of ASTM Standards, Vol 04.08.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 6565
electrical conductivity. When the soil’s electrical conductivity effect can be seen when the top of the probe is not seated
is high enough, there is insufficient signal strength for theTDR properly (see Fig. 1(b)).
instrument to detect. TDR probes employing a balancing balun 6.7 This test method is not appropriate for measuring the
transformerareparticularlysusceptibletothiseffect.Thebalun moisture content of frozen soils.
transformer compounds the problems in analyzing the signals 6.8 Metals of natural (for example, ores) or manmade (for
from probes with rod lengths of 15 cm or less (1). example, barrels) origin may affect measurements if they are
6.2 Clay soils also attenuate a TDR probe signal. Conduc- present in sufficient quantity and are within the volume of soil
tive soils which have a significant amount of clay attenuate the tested by the device.
signal the most.Apartial solution to signal loss is to reduce the 6.9 As the moisture content decreases below 5 % by vol-
length of the probe (2). However, as the probe length shrinks, ume, the difference in dielectric between the soil and water
the precision of the moisture content estimates worsens. diminishes and the ability of the TDR technique to quantify
6.3 Asolution to the problem is to use a probe rod length of moisture content with any degree of certainty is compromised.
15 cm and to electrically insulate the probe (3). This can be
7. Apparatus
accomplished by spraying the probe rods with a clear resin
coating or applying a very thin layer of marine epoxy resin.
7.1 The basic TDR system consists of a Tektronix 1502B
The marine epoxy resin is a hard, non-conductive, and non-
cable tester (or comparable unit) and a cable/probe assembly,
absorbing coating which adheres well to the metallic rods. The
as shown in Fig. 2. The cable tester generates a fast rise time
rods should be slightly abraded to enhance resin adhesion. The
pulse which propagates along the coaxial transmission line
coating should have a minimal effect upon the accuracies
until it reaches an impedance change.At this point, a portion of
observed if applied in an even thickness about the rods. The
the signal is reflected back to the cable tester and is displayed
coatings should be inspected on a regular basis for wear.
as a change in amplitude. If the reflection point is lower in
6.4 Temperature effects have been observed when using
impedance than the cable, then the reflection will be displayed
TDR in the field. Temperature effects are particularly trouble- as a drop in amplitude. If it is higher in impedance, then it will
some for systems where the user has predefined a probe
be displayed as a rise in amplitude. The cable tester measures
beginning point within the software and employs long TDR the time for a pulse to travel the distance between the
probe cable lengths (;30 m or more). The cable shrinks and
beginning and end points of the probe, as displayed on the
contracts as a function of temperature. Naturally the maximum screen, and converts this time to a distance. Fig. 3 shows a
and minimum cable lengths occur during the warmest, and
typical TDR trace with the probe connected to the instrument
coolest times of the day, respectively. The solution is to avoid and inserted into a wet soil sample. It should be noted that the
defining a beginning point of the cable tester trace within the
impedance of the probe assembly in the wet soil is lower than
users software. Also, thermal effects can be minimized by the cable, hence the amplitude of the return signal is lower.At
burying the cable or otherwise protecting the cable from
the end of the probe assembly the impedance again changes
exposure. In addition, the dielectric of the soil changes as a (impedance increases) and is reflected in Fig. 3 as a gradual
function of temperature.
rise in amplitude.
6.5 A static charge on the coaxial cable may cause damage
7.2 TDR probes are typically divided into two categories:
to the TDR cable tester unit. To avoid possible damage to the
Two rod probes employing a balancing balun transformer, and
electronics, always dead-short the TDR probe leads to each
multi-rod probes which do not require a balancing balun
other. This will discharge the static charge in the cable prior to
transformer. Fig. 4 is an example of a multi-rod TDR probe
connecting the cable assembly to the TDR cable tester unit.
while Fig. 5 is a typical two rod/balun TDR probe. Typically
6.6 Voids in the path of, or adjacent to, the probe can cause
rod lengths, materials, diameters, and rod spacings vary from
the soil moisture to appear lower than it actually is. This same
The Tektronix 1502B is the instrument around which the TDR probe technol-
ogy has been developed. With rare exception, commercial companies selling a TDR
The boldface numbers given in parentheses refer to a list of references at the
probe system employ this instrument in their systems.
end of the text.
Fig. 1(a) Properly installed probe Fig. 1(b) Improperly installed probe
FIG. 1 Properly and Improperly Installed TDR Probes
D 6565
7.2.2 The balun transformer used in two rod probes has
typically been a source of signal loss.Anew type of balun has
been developed (4) which alleviates the problems encountered
with the typical balun.
7.2.3 Multi-rod, or multi-waveguide, probes employ, at a
minimum, three conductive rods arranged in a symmetric
pattern. The diameter of the rods, length, rod material, and
spacing may vary.
7.2.4 Some commercially available systems offer the user
multi-probe configurations, direct digital read out, data storage
FIG. 2 Basic TDR System
capabilities, probe lengths up to 120 cm long, time tagging of
stored data, and real time data acquisition and control by
computer through an RS232 serial interface.
7.3 With the use of a computer, and a series of multiplexers,
a large number of TDR probes may be queried in a sequential
fashion, and the data stored for later retrieval and analysis.
8. Preparation of Apparatus
8.1 Determining the proper propagation velocity (V)tobe
p
usedinconjunctionwiththeTDRprobeisanimportantitemin
FIG. 3 Typical TDR Trace When the TDR Probe is Inserted Into
setting up a TDR probe system. The first step is to accurately
Moist Soil
measure the length of the cable and probe assembly. The
propagation velocity may then be determined by performing
the following procedure.
8.1.1 After attaching the TDR probe cable to the TDR cable
tester, adjust the distance/division control to the appropriate
setting. For example, if the cable/probe assembly is 1 m, adjust
the distance/division control to 1 m/div.
8.1.2 Turn the position adjustment until the distance reading
is the same as the cable/probe assembly length.
8.1.3 Turn the propagation control (V ) until the cursor is
p
FIG. 4 Typical 3-Rod TDR Probe Configuration
resting on the first rising portion of the reflected pulse, that is,
the end of the probe. Shorting the ends of the probe together
will aid in determining the end of the probe as reflected in the
TDR trace.The V controls of the instrument are now set to the
p
V of the cable/probe assembly.
p
9. Calibration and Standardization
9.1 For best results, the TDR system should be calibrated to
the soil to be tested. This can be accomplished by acquiring a
sufficient quantity of the soil to be tested, so as to provide a
FIG. 5 Typical 2-Rod TDR Probe With Balun Transformer
minimumofsevensoilsamples,mixedtoauniformvolumetric
water content, as shown in Table 1.
probe to probe. These parameters are chosen as a function of 9.2 Use an oven-drying or microwave-drying technique to
the soil to be tested, the longevity of the test, and the remove any residual water from the soil samples. Periodic
sensitivities required from the probe. weighing of the sample is required to establish when the soil is
7.2.1 The two-rod balun probe makes use of a signal dry. Two successive measurements of equal weight should be
balancing balun. A balun (also known as a balancing trans- sufficient.
former) allows the user to connect two wires of dissimilar 9.3 Usingthedrysoil,preparesevensoilsampleshavingthe
impedances. A typical example of impedance mismatch appli-
volumetric water content as outlined in Table 1. Place the
cable to TDR probes is the 50 ohm coaxial cable connected to calibration samples into a plastic container. Do not use a metal
twin lead 185 ohm antenna wire. The balun transformer is
container for the calibration samples. The calibration container
inserted at this junction so as to balance the impedance
mismatch.
TABLE 1 Example of Volumetric Moisture Content of Calibration
Sam
A typical balun used here is the Type T.P. 103 Balun as sold by the 5% 10% 15% 20% 25% 30% 35%
Adams-Russel Co., of Burlington, MA.
D 6565
should be of a sufficient size such that the radius of influence the TDR instrument. It is very important to be able to properly
of the TDR probe is not affected by the container or surround- determine the correct beginning and end points of the probe
ing air. when inserted into a soil sample.The end point, as observed on
9.4 Insert the probe into the calibration sample and, using the TDR instrument, will vary with water content. Improperly
the procedure outlined in Section 11, and calculate the volu-
determining the trace end point will introduce gross errors in
metric water content. Repeat this procedure until all samples the predicted water content. Traditionally, the end points are
have been sampled at least three times.
determined graphically by the user. More recently, various
9.5 Use an oven-drying or microwave-drying technique to algorithms have been developed to automate the processing of
quantifytheactualwatercontentofthesoilsamplesusedinthe
the data to determine the endpoints through mathematical
TDR calibration (as outlined in Test Methods D 2216 or analyses. The application of these automated analysis tech-
D 4643).
niques is beyond the scope of this standard. Using the
9.6 If inaccuracies are noted then conduct a regression
equationsoutlinedinSection11,calculatethevolumetricwater
analysis. Inaccuracies are defined as repeated errors greater
...
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