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On 3 Feb 99, at 10:13, owner-sowacs@aqua.ccwr.ac.za wrote:
> Luke,
>
> You should be aware that the Campbell Sci CS615's are not true TDR and they
> measure the dielectric constant at a lower frequency range than TDR.
If I respond to this comment, I will probably get a long counter
response from Marty, but let me just make a few comments.
First of all what defines TDR? I would have assumed that TDR is
the measure of progation speed and how that changes with
dielectric constant. Since the CS615 probe outputs a fairly low
frequency that changes with soil moisture, some people have
assumed that it is just a capacitance sensor like many other soil
moisture probes in the past. However, the CS615 does not use the
probes as a variable capacitor element in an oscillator.
The CS615 is measuring the travel time for the signal to reflect off
the end of the probe. This period of oscillation is simply determined
by the travel time for the signal to travel down the rods four times
plus the delays of circuit. That high frequency signal is then divided
down for the low frequency output that the datalogger sees.
In contrast most TDR systems use a graphical analysis to
determine the soil moisture. Since the CS615 is a simplification
that isn't graphical it must be calibrated differently and must be
adjusted for local conditions. However that doesn't mean that the
measurement isn't a TDR measurement in the sense that it is
indeed measuring the travel time of the signal.
Thanks,
--------------------------------------------------
Russell Anderson Campbell Scientific, Inc.
(435) 750-9697 815 W. 1800 N.
(435) 750-9639 FAX Logan, UT 84321
russell@campbellsci.com
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