TDR X FDR

[Wenceslau Geraldes Teixeira]

Dear sowacs users,

in the framework of a project where we are monitoring the water dinamics
we have used the Campbell CS616 sensors.

In the manual of this sensor it is called Water Content Reflectometer and
the words TDR are not mentioned. In the SOWACS home page it is classified as
TDR ? is it correct or CS616 should be classified as FDR ?

I will appreciate verymuch if someone can explain better how the CS616
should be classified and described. We also observe some variation in the
data during the day without rainfall. Does someone have experienced this
with CS616?

Regards from the Amazon

Wenceslau Geraldes Teixeira
Embrapa Amazônia Ocidental - Manaus - Brazil

[Raimundo Cosme de Oliveira Junior]

Dear Wenceslau,
I have instaleed 38 FDR (CS615 and CS616) all are
fdr. Look at lba home page and look for by my poster about cs615 calibration.
The variation during the day is normal because
the water move down and the temperature make
evaporation happen. About this I have 12 CS616
installed in pasture and soybean field and soil moisture decrease slowly.
A big hug and i hope see you to talk about a new project in Amazon region.

Cosme

Raimundo Cosme de Oliveira Junior
Pesquisador II Embrapa Amazônia Oriental
Fone: 55-93-522.2203
55-93-5234138
55-91-99919275
email: co...@cpatu.embrapa.br
co...@lbaeco.com.br

[Sally Logsdon]

The CS616 is called a Water Content Reflectometer. It is NOT any sort of frequency-probe, but rather is a new generation sytem based on time domain.

There really are no field-soil moisture probes that are "frequency-domain." Rather they give results (voltage, impedance, etc.) at a given frequency or the resonance frequency is the result.

Sally Logsdon
USDA-ARS, NSTL

>>> l...@cpaa.embrapa.br 05/08/06 07:40AM >>>

[Marty Grogan]

Wenceslau:

Time and Frequency variations depend on identical electromagnetic phenomena--dielectric properties of conducting media--which control transit times.  Timing variations typically consider more than a single frequency and are indicated in units of time such as repetition rate.  Sometimes, as with the CS616, square wave output signals are designated by a  fundamental frequency.  A frequency counter can interpret squarewave and sine wave outputs equally well by indicating the time interval between zero voltage points.

Based on the CS616 manual, the output signal would be most accurately characterized as square-wave FDM.  Using a square wave vs a sine wave simplifies the circuitry required to process the analog part of the signals.  Some secondary calibration value may be derived from the extended bandwidth of the signal as well.

Lab quality transmission line TDR systems, potentially costing thousands of dollars,  precisely indicate impedance (dielectric) variations using wave reflections and typically display results on a CRT.  Intelligent processing of resulting waveforms can be employed to provide specific impedance values.  See http://cpliterature.product.agilent.com/litweb/pdf/5989-0271EN.pdf for technical details of a modern unit.  These units generally use a very narrow pulse, < 1ns, to electronically stimulate the transmission line--not a square wave.

TDR systems used for moisture measurement require elimination of most complexity and calibration for specific media to provide values. 

If the two rods of the CS616 are viewed as two poles of a capacitor, the distinction between TDR and FDM becomes mostly moot.  Use of higher frequencies and square waves enables more convenient and less costly circuitry to be used to obtain adequate precision for measuring moisture content.

Marty

[Jim Bilskie]

Greetings:

There are some very good points being made in this discussion with regard to time
domain and frequency domain reflectometry. I would like to clarify a couple of issues
based on my experience with the CS616.

As Marty points out, the output of the probe is a square wave and the reason a
square wave was chosen is because it is easy to determine the frequency of a
square wave using a datalogger. In the case of the CS616, in which the output
alternates above and below ground, the point of zero crossing in one direction is
detected and the time between these crossing used to determine period or
frequency. A sine wave could provide similar results. The probe output is actually
the output of a scaler which divides the actual frequency of travel time of the applied
signal along the probe rods.

The CS616 has a circuit which generates a voltage transistion with rise time of about
2 nanoseconds. This signal is similar to that of the Tek 1502 or TDR100 except
about one order of magnitude slower. The signal is connected to the 2 parallel rods.
It travels the length of the rods, is reflected from the end and returns to the probe
head to be detected by a comparator circuit. When the reflection is detected, the
next signal is applied. The time between detected reflections is directly related to
propagation time along the rods. This can be observed using a high speed Oscope
connected to the circuit board, and I have done this during my testing and
development. It takes 2048 reflections for one cycle of square wave output because
the CS616 is bipolar, i.e. a positive transition is followed by a negative transition. If
one considers the propagation time for a signal along 2 times the the length of the 30
cm rods and adds a little more time for propagation through the circuit components
and along the trace within the probe head, the numbers work out exactly. Since the
CS616 response is dependent on propagation time of an EM wave along a
transmission, it would be considered a time domain device.

I am happy to answer questions about the CS616. When I wrote the manual I chose
to not make distinctions about time or frequency domain as I feel this is not important
when it comes to using the probe to measure water content and it brings some
confusion into things. Having said that, the manual section on principles of operation
describes a time domain device.

My regards,
Jim

Jim Bilskie, Ph.D.
Soil Physicist
Campbell Scientific, Inc.

[Marty Grogan]

Jim:

I must respectfully disagree with your final assessment that the CS616
operates as a TDR device. Substitution of a single sine wave oscillator
followed a phase locking circuit would produce an identical result into
your dividing circuit--characteristics of a frequency variable device.
In fact, you might consider using such an implementation to produce a dc
output for logging purposes. That's probably already available however.

Anyway, distinction between TDR and FM devices really is only one of
semantics and marketing unless different combinations of spectra produce
different results. That's a study I would like to see. Anyone ever
researched moist soil using a network analyzer and sweep generator?
Perhaps an optimum spectrum exists. How about running a box of dirt
through an NMR or MRI? Portability might be a problem for field use.

Marty

[Jim Bilskie]

Thank you, Marty. The dividing circuit is only on the digital level, and I've observed
no phase shifts. We could easily convert the output to dc using a FtoV converter but
can get better resolution with frequency output. In the interim, it is semantics, but in
the end it's good to discuss these things as it can lead to better understanding of
instrument behavior and ultimately better field measurements.

My regards,
Jim

[Arie Nadler]

If someone wants to define his instrument as similar/equal/identical to XDR
(X=T, F, Z), that's fine.
But before, it will be nice to get from him a set of measurements comparing
both techniques (his and TDR) in an applicable spectrum of salinities,
temperatures, textures, water contents and layering.
Isn't it basic?
While following the contributed statements and ideas, I was wondering: Why
did nobody offer Mr. Wenceslau to read the recent literature.
arie

Arie Nadler
Dept. of Soil Physics
Institute of Soil, Water, and the Environment
A.R.O., Volcani Center, Min. of Agriculture
POB 6 Bet Dagan, Israel, 50250
Tel: 972-3-9683865
Fax: 972-3-9604017
Home: 972-8-9404161
Email: vw...@volcani.agri.gov.il
http://www.agri.gov.il/People/ArieNadler.html

My regards,
Jim

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