Easily adapted to dataloggers
1. Select a cable length.
Easily calibrated
2. Select a measurement circuit and null it for
Easily installed
the selected cable length.
Ruggedness
3. Find Cair for each probe/cable combination
Resistance to environmental parameters
using water as the reference material.
such as temperature and humidity.
4. Check Cair for each probe/cable assembly
using methanol as the checkpoint material.
2. This capacitance method for the measure-
5. Install the probes at the field site and at the
ment of soil moistures can and should be used for
same time get an average soil density.
both laboratory and field measurement programs,
6. Calibrate a typical probe/cable combination
subject to the following limitations:
using a test jig that provides volume moisture
contents using field site soils that approximate
Each box or measurement circuit should
the dry density that was observed for the test site.
be individually calibrated to the specific
This calibration could be either directly in capaci-
soil in which it will be used.
tance or in dielectric constant vs. volumetric mois-
For each application, select a specific cable
ture content.
length and use it for all measurements in
7. Derive the calibration equations for the volu-
that particular application.
metric soil moisture contents and use them to
Work only with nulled circuits.
solve for the soil moisture at the field site.
If there is any chance that the salinity of
8. Use the dry density data from the field site
the soil will cause errors, use an insulated
to find percent dry weight if required.
probe and analyze the data using the
"series capacitance" correction technique
Note that the calibration to actual soils (steps 6
using water as the reference material.
and 7) could be eliminated if a correction ratio for
the 10-MHz test frequency were to be determined
3. Start a program to put a 10-MHz circuit in a
through a laboratory test program. The equations
probe so that cable length is no longer a problem.
of either Topp et al. or Roth et al. could then be
4. Start a program to find a better probe insu-
used as "universal" calibration equations. Note
lation.
also that the procedures recommended here are
5. Interface these circuits directly to dataloggers
general in nature and could be applied equally
since the accuracy of these circuits is worse than
well to probes of different size as well as for
the accuracy of dataloggers themselves. No fur-
different test frequencies. The only restrictions
ther testing is needed in this area.
would be that the probe electrodes must be close
6. Find a second "checkpoint" material besides
enough to have a capacitance of a few picofarads
methanol. This second material should have a
in air but at the same time be large enough to
dielectric constant of approximately 16.
sample a representative amount of soil. For ex-
7. For small specific applications, a probe and
ample, a typical TDR probe could be used.
circuit can be calibrated for a specific soil and
interfaced to an inexpensive single- or two-
channel datalogger (such as the OmniData Data-
LITERATURE CITED
pods).
8. Start a developmental program that explores
Campbell, J.E. (1988) Dielectric properties of moist
the possibility of using a simpler, much less ex-
soils at RF and microwave frequencies. Dartmouth
pensive circuit (such as the bridged-tee circuit,
College, Hanover, New Hampshire.
for instance) and reverse the present measure-
Roth, K., R. Schulin, H. Fluhler, and W. Attinger
ment philosophy so that the limits of the soils for
(1990) Calibration of time domain reflectometry
which this circuit will work properly determine
for water content measurement using a compos-
the new circuit and its probe's suitability for the
ite dielectric approach. Water Resources Research,
soil moisture measurements under consideration.
26(10): 22672273.
Topp, G.C., J.L. Davis, and A.P. Annan (1980)
This method can and will be improved. In the
Electromagnetic determination of soil water con-
meantime, the following procedure yields accept-
tent: Measurements in coaxial transmission lines.
able soil moisture measurements using presently
Water Resources Research, 16(3): 574582.
available equipment and the procedures described
in this report:
22
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