ASTM D6565-00(2005)
(Test Method)Standard Test Method for Determination of Water (Moisture) Content of Soil by the Time-Domain Reflectometry (TDR) Method (Withdrawn 2014)
Standard Test Method for Determination of Water (Moisture) Content of Soil by the Time-Domain Reflectometry (TDR) Method (Withdrawn 2014)
SIGNIFICANCE AND USE
The determination of the water-content, or moisture content, of soil is one of the fundamental needs in the soil physics and hydrology disciplines. The need arises from requirements for defining the optimal time for irrigation, the infiltration rate, the soil-moisture flux, contaminant transport rates, and evaluating the potential for leakage from a waste site or a surface or subsurface barrier.
The TDR application covered in this test method is that used for point measurements of moisture content in soil. The application is either through manual insertion into the soil or by burying a probe in the subsurface to acquire moisture content data at a specific location. In addition, core samples may be tested with TDR at a drill site to acquire real-time soil moisture data.
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.
WITHDRAWN RATIONALE
This test method covered the determination of water content (or moisture content) in soil by the use of the electromagnetic technique called Time-Domain Reflectometry (TDR).
Formerly under the jurisdiction of Committee D18 on Soil and Rock, this test method was withdrawn in July 2014 in accordance with section 10.6.3 of the Regulations Governing ASTM Technical Committees, which requires that standards shall be updated by the end of the eighth year since the last approval date.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:D6565 −00(Reapproved 2005)
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 D6565; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope (Moisture) Content of Soil and Rock by Mass
D4643 Test Method for Determination of Water (Moisture)
1.1 This test method covers the determination of water
Content of Soil by Microwave Oven Heating
content (or moisture content) in soil by the use of the
D4700 Guide for Soil Sampling from the Vadose Zone
electromagnetic technique called Time-Domain Reflectometry
D4944 TestMethodforFieldDeterminationofWater(Mois-
(TDR).
ture)ContentofSoilbytheCalciumCarbideGasPressure
1.2 This test method was written to detail the procedure for
Tester
conventional TDR measurements of soil. Other TDR applica-
D5220 Test Method for Water Mass per Unit Volume of Soil
tions exist for the purpose of quantifying water content in soil
and Rock In-Place by the Neutron Depth Probe Method
and are not covered here, such as flat probe technologies and
wetting front advance methods.
3. Terminology
1.3 Commercial TDR applications exist which automate the 3.1 Definitions:
TDR methodology and are not detailed in this test method. It is
3.1.1 time domain reflectometry (TDR)—an electromagnetic
likely that overlap exists in the automated commercial systems method used in the determination of water content of soil.
versus this applied method, and the user is encouraged to
3.1.2 Definitions of other terminology used in this guide
adhere to this test method when applicable.
may be found in Terminology D653.
1.4 This test method is one of a series on vadose zone
4. Summary of Test Method
characterization methods. Other standards have been prepared
on vadose zone characterization techniques.
4.1 Aspeciallyconstructed,multi-wave-guideTDRprobeis
inserted into the soil. The electronic cable tester (or automated
1.5 This standard does not purport to address all of the
commercial TDR electronics) is used to send a pulsed wave-
safety concerns, if any, associated with its use. It is the
form to the probe.The cable tester then receives a return signal
responsibility of the user of this standard to establish appro-
which was influenced by the dielectric constant of the soil,
priate safety and health practices and determine the applica-
which in turn is a function of water content.An analysis of the
bility of regulatory limitations prior to use.
waveform trace supplies the necessary information to calculate
2. Referenced Documents the water content of the soil.
2.1 ASTM Standards:
5. Significance and Use
D653 Terminology Relating to Soil, Rock, and Contained
5.1 The determination of the water-content, or moisture
Fluids
content, of soil is one of the fundamental needs in the soil
D1452 Practice for Soil Exploration and Sampling byAuger
physics and hydrology disciplines. The need arises from
Borings
requirements for defining the optimal time for irrigation, the
D2216 Test Methods for Laboratory Determination of Water
infiltration rate, the soil-moisture flux, contaminant transport
rates, and evaluating the potential for leakage from a waste site
or a surface or subsurface barrier.
This test method is under the jurisdiction ofASTM Committee D18 on Soil and
Rock and is the direct responsibility of Subcommittee D18.21.02 on Vadose Zone
5.2 The TDR application covered in this test method is that
Monitoring.
used for point measurements of moisture content in soil. The
Current edition approved Nov. 1, 2005. Published December 2005. Originally
applicationiseitherthroughmanualinsertionintothesoilorby
approved in 2000. Last previous edition approved in 2000 as D6565 – 00. DOI:
10.1520/D6565-00R05.
burying a probe in the subsurface to acquire moisture content
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
data at a specific location. In addition, core samples may be
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
tested withTDR at a drill site to acquire real-time soil moisture
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. data.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6565−00 (2005)
Fig. 1(a ) Properly installed probe Fig. 1(b) Improperly installed probe
FIG. 1Properly and Improperly Installed TDR Probes
6. Interferences 6.5 A static charge on the coaxial cable may cause damage
to the TDR cable tester unit. To avoid possible damage to the
6.1 TDRmeasurementsinconductivesoilsarehamperedby
electronics, always dead-short the TDR probe leads to each
the conductivity of the soil and the resulting signal attenuation.
other. This will discharge the static charge in the cable prior to
Typically, the amplitude of the voltage pulse reflected back to
connecting the cable assembly to the TDR cable tester unit.
the TDR instrument is diminished in proportion to the soil’s
electrical conductivity. When the soil’s electrical conductivity 6.6 Voids in the path of, or adjacent to, the probe can cause
is high enough, there is insufficient signal strength for theTDR the soil moisture to appear lower than it actually is. This same
instrument to detect. TDR probes employing a balancing balun effect can be seen when the top of the probe is not seated
transformerareparticularlysusceptibletothiseffect.Thebalun properly (see Fig. 1(b)).
transformer compounds the problems in analyzing the signals
6.7 This test method is not appropriate for measuring the
from probes with rod lengths of 15 cm or less (1).
moisture content of frozen soils.
6.2 Clay soils also attenuate a TDR probe signal. Conduc-
6.8 Metals of natural (for example, ores) or manmade (for
tive soils which have a significant amount of clay attenuate the
example, barrels) origin may affect measurements if they are
signal the most.Apartial solution to signal loss is to reduce the
present in sufficient quantity and are within the volume of soil
length of the probe (2). However, as the probe length shrinks,
tested by the device.
the precision of the moisture content estimates worsens.
6.9 As the moisture content decreases below 5 % by vol-
6.3 Asolution to the problem is to use a probe rod length of
ume, the difference in dielectric between the soil and water
15 cm and to electrically insulate the probe (3). This can be
diminishes and the ability of the TDR technique to quantify
accomplished by spraying the probe rods with a clear resin
moisture content with any degree of certainty is compromised.
coating or applying a very thin layer of marine epoxy resin.
The marine epoxy resin is a hard, non-conductive, and non-
7. Apparatus
absorbing coating which adheres well to the metallic rods. The
7.1 The basic TDR system consists of a Tektronix 1502B
rods should be slightly abraded to enhance resin adhesion. The
cable tester (or comparable unit) and a cable/probe assembly,
coating should have a minimal effect upon the accuracies
as shown in Fig. 2. The cable tester generates a fast rise time
observed if applied in an even thickness about the rods. The
pulse which propagates along the coaxial transmission line
coatings should be inspected on a regular basis for wear.
until it reaches an impedance change.At this point, a portion of
6.4 Temperature effects have been observed when using
the signal is reflected back to the cable tester and is displayed
TDR in the field. Temperature effects are particularly trouble-
as a change in amplitude. If the reflection point is lower in
some for systems where the user has predefined a probe
impedance than the cable, then the reflection will be displayed
beginning point within the software and employs long TDR
as a drop in amplitude. If it is higher in impedance, then it will
probe cable lengths (;30 m or more). The cable shrinks and
be displayed as a rise in amplitude. The cable tester measures
contracts as a function of temperature. Naturally the maximum
the time for a pulse to travel the distance between the
and minimum cable lengths occur during the warmest, and
beginning and end points of the probe, as displayed on the
coolest times of the day, respectively. The solution is to avoid
screen, and converts this time to a distance. Fig. 3 shows a
defining a beginning point of the cable tester trace within the
typical TDR trace with the probe connected to the instrument
users software. Also, thermal effects can be minimized by
and inserted into a wet soil sample. It should be noted that the
burying the cable or otherwise protecting the cable from
impedance of the probe assembly in the wet soil is lower than
exposure. In addition, the dielectric of the soil changes as a
the cable, hence the amplitude of the return signal is lower.At
function of temperature.
The Tektronix 1502B is the instrument around which the TDR probe technol-
The boldface numbers given in parentheses refer to a list of references at the ogy has been developed. With rare exception, commercial companies selling a TDR
end of the text. probe system employ this instrument in their systems.
D6565−00 (2005)
FIG. 4Typical 3-Rod TDR Probe Configuration
FIG. 2Basic TDR System
FIG. 5Typical 2-Rod TDR Probe With Balun Transformer
FIG. 3Typical TDR Trace When the TDR Probe is Inserted Into
stored data, and real time data acquisition and control by
Moist Soil
computer through an RS232 serial interface.
7.3 With the use of a computer, and a series of multiplexers,
the end of the probe assembly the impedance again changes
a large number of TDR probes may be queried in a sequential
(impedance increases) and is reflected in Fig. 3 as a gradual
fashion, and the data stored for later retrieval and analysis.
rise in amplitude.
8. Preparation of Apparatus
7.2 TDR probes are typically divided into two categories:
Two rod probes employing a balancing balun transformer, and
8.1 Determining the proper propagation velocity (V)tobe
p
multi-rod probes which do not require a balancing balun usedinconjunctionwiththeTDRprobeisanimportantitemin
transformer. Fig. 4 is an example of a multi-rod TDR probe
setting up a TDR probe system. The first step is to accurately
while Fig. 5 is a typical two rod/balun TDR probe. Typically measure the length of the cable and probe assembly. The
rod lengths, materials, diameters, and rod spacings vary from
propagation velocity may then be determined by performing
probe to probe. These parameters are chosen as a function of the following procedure.
the soil to be tested, the longevity of the test, and the
8.1.1 After attaching the TDR probe cable to the TDR cable
sensitivities required from the probe.
tester, adjust the distance/division control to the appropriate
7.2.1 The two-rod balun probe makes use of a signal
setting. For example, if the cable/probe assembly is 1 m, adjust
balancing balun. A balun (also known as a balancing trans-
the distance/division control to 1 m/div.
former) allows the user to connect two wires of dissimilar
8.1.2 Turn the position adjustment until the distance reading
impedances. A typical example of impedance mismatch appli-
is the same as the cable/probe assembly length.
cable to TDR probes is the 50 ohm coaxial cable connected to
8.1.3 Turn the propagation control (V ) until the cursor is
p
twin lead 185 ohm antenna wire. The balun transformer is
resting on the first rising portion of the reflected pulse, that is,
inserted at this junction so as to balance the impedance
the end of the probe. Shorting the ends of the probe together
mismatch.
will aid in determining the end of the probe as reflected in the
7.2.2 The balun transformer used in two rod probes has
TDR trace.The V controls of the instrument are now set to the
p
typically been a source of signal loss.Anew type of balun has
V of the cable/probe assembly.
p
been developed (4) which alleviates the problems encountered
9. Calibration and Standardization
with the typical balun.
7.2.3 Multi-rod, or multi-waveguide, probes employ, at a
9.1 For best results, the TDR system should be calibrated to
minimum, three conductive rods arranged in a symmetric
the soil to be tested. This can be accomplished by acquiring a
pattern. The diameter of the rods, length, rod material, and
sufficient quantity of the soil to be tested, so as to provide a
spacing may vary.
minimumofsevensoilsamples,mixedtoauniformvolumetric
7.2.4 Some commercially available systems offer the user
water content, as shown in Table 1.
multi-probe configurations, direct digital read out, data storage
9.2 Use an oven-drying or microwave-drying technique to
capabilities, probe lengths up to 120 cm long, time tagging of
remove any residual water from the soil samples. Periodic
weighing of the sample is required to establish when the soil is
dry. Two successive measurements of equal weight should be
A typical balun used here is the Type T.P. 103 Balun as sold by the
Adams-Russel Co., of Burlington, MA. sufficient.
D6565−00 (2005)
TABLE 1 Example of Volumetric Moisture Content of Calibration
10.5 The beginning point should be the same as noted with
Sam
the probe in air. If the beginning point is not the same, remove
5% 10% 15% 20% 25% 30% 35%
the probe from the calibration sample, and check for damage.
If the probe is found to be in proper working order, reinsert the
probe into the calibration sample, and again note the beginning
and end points.
9.3 Usingthedrysoil,preparesevensoilsampleshavingthe
volumetric water content as outlined in Table 1. Place the
10.6 If the beginning point is the same as that found in air
calibration samples into a plastic container. Do not use a metal
then continue with the sampling; if not, cease sampling, and
container for the calibration samples. The calibration container
check the system for damage or improper instrument settings.
should be of a sufficient size such that the radius of influence
Repair or readjust as required.
of the TDR probe is not affected by the container or surround-
10.7 Insert the probe into the sample following instructions
ing air.
10.3-10.6. Determine the apparent trace length, l , as shown on
a
9.4 Insert the probe into the calibration sample and, using
the TDR instrument. It is very important to be able to properly
the procedure outlined in Section 11, and calculate the volu- determine the correct beginning and end points of t
...
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