Re TDR100
Sally Logsdon
You wondered if it was wise to use several layers of multiplexers with one TDR100. Instead I would recommend purchasing additional TDR100 and use less multiplexing and cables. Every layer of multiplexer and associated cables attenuates the signal and cuts off the high frequency end.
Logsdon, S.D. 2006. Experimental limitations of time domain reflectometry hardware. Soil Sci. Soc. Am. J. 70:537-540.
Logsdon, S.D. 2005. Time domain reflectometry range of accuracy for high surface area soils. Vadose Zone J. 4:1011-1019.
Logsdon, S.D. 2000. Effect of cable length on TDR calibration for high surface area soils. Soil Sci. Soc. Am. J. 64:54-61.
Sally Logsdon
National Soil Tilth Laboratory
bittelli
I agree with you. Indeed on a soil with high clay content, we could not use
the system at all, because the waveform was so flat that there was no second
reflection.
All the EM energy was dispersed in the cables and in the soil with high
conductivity. The problem was not solved by using coated or shorter probes.
Thanks for your feedback, I'll check your papers,
Marco
-----Messaggio originale-----
Da: sow...@googlegroups.com [mailto:sow...@googlegroups.com] Per conto di
Sally Logsdon
Inviato: Wednesday, March 15, 2006 1:40 PM
A: sow...@googlegroups.com
Oggetto: Re TDR100
Dear Wim Cornelis,
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Jim Bilskie
Dr. Logsdon makes a very important point with regard to the effect of signal
attenuation due to cable loss and multiplexers. There actually is little loss in the
Campbell Scientific multiplexers (attenuation values are presented in the manual
which is available on line) but the loss in cables can be significant.
Cable loss is manifest principally in two ways. Attenuation leads to dispersion of the
band of applied/reflected frequencies which is important given the frequency
dependence of dielectric permittivity. Attenuation also affects the rise and decay
times of the waveform reflection as well as the amplitude of the signal. These issues
are also addressed in the manual and Sally's papers are comprehensive on this
matter.
The rise/decay time effect impacts the apparent length determination by the
algorithm because the algorithm is based on intersection of tangents method and the
tangent is a direct function of rise time.
Reductions in signal amplitude lead to compromise and eventually loss of key
features of the waveform with the result that, with increasing cable length, the
apparent beginning and end of the probe cannot be identified. Nearly complete
attenuation of the signal can occur in conductive soils, including compacted clay
soils, which makes it impossible to identify the end of the probe. This is the best
addressed using TDR probes with shorter rods and thus less surface area which
results in less signal loss. CSI offers probes of rod lengths 30 cm, 15 cm and 7.5
cm.
The effects of cable loss on volumetric water content and bulk electrical conductivity
can be minimized by using a couple of methods.
Much work has been done recently by Dr. Paolo Castiglione on EC measurements
using TDR. Castiglione and Shouse (2003) derived a method that does a great job
of accounting for system losses. The method is essentially a calibration which
involves collecting waveform information with the TDR probes in air and with the
probes in air with the rods shorted. Additionally, measuring conductance via the
TDR100 with the probe immersed in a single solution of known conductivity provides
a cell constant (Kp) for the particular configuration. This method has been adapted
for implementation with the TDR100 system. With the results of the calibration
incorporated in the datalogger program as described in the CSI probe manual, bulk
EC measurements can be obtained with very good accuracy even when cable length
exceeds 50 meters.
The effect of cable loss on the apparent length determination that is used for water
content can be minimized using the method for probe offset described in the TDR
probe manual. This method is similar to the Castiglione and Shouse method in that
data is collected with the entire TDR system configured as it will be used in the field
or lab. Measurement with the probe immersed in DI water of known temperature
and some simple calculations are all that is needed to derive a correction factor
(probe offset) that addresses the effect of bandwidth loss.
I wish to offer a few points of clarification. All of the calculations regarding waveform
evaluation are performed in the TDR100. PCTDR functions essentially the same as
a datalogger in that it only collects this information. It then displays the data
graphically and numerically and allows for modification by choice of adjustments
such as probe constant, probe offset, Kp and transformation to volumetric water
content from apparent length. The TDR100 was developed by Campbell Scientific
with extensive consultation from Dr. Agoston. An agreement was subsequently
reached where CSI manufactured a version of the TDR100 for Hyperlabs. I don't
know if this product is still available from Hyperlabs.
I am happy to the discuss specific TDR100 applications and provide information from
over 12 years of design and application experience with the CSI supported Tektronix
systems and the Campbell Scientific TDR100 systems. The TDR probe manual
referenced here is currently in revision but should be released later this week or next
week. It will be availabe online along with all of our manuals at
www.campbellsci.com
My regards,
Jim
Jim Bilskie, Ph.D.
Soils Physicist
Campbell Scientific, Inc.
435/750-9580
P. Castiglione and P.J. Shouse. 2003. The effect of ohmic cable losses on time-
domain reflectometry measurements of electrical conductivity. Soil Sci Soc Am J
2003 67: 414-424.
Priyantha B Kulasekera
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