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Time domain methods (reflectometry--TDR, transmissometry--TDT) measure the
time a fast pulse edge takes to travel along a section of transmission
line. In each case, the transmission line is driven at one end with the
transmit signal--the simplest being a fast rise step. For TDT, the pulse is
observed at the other end of the transmission line from the transmitter,
and the time measured is a one-way propagation time. For TDR, the pulse is
observed, after reflection from some impedance discontinuity in the
transmission line and superimposed on the transmitted signal, at the same
end of the transmission line as the transmitter, and the time measured is a
two-way or round-trip propagation time.
The essential measurement in each case is a time interval. The propagation
time of an electromagnetic wave along a given length of transmission line
is proportional to the square root of the permittivity of the medium which
the transmission line is immersed in. If the medium is soil/water/air the
permittivity of the water dominates the mixture of permittivities and the
measurement can then be used to determine the water content of the mixture.
The reason you would use TDR or TDT in a given situation is a matter of
trading off various practical considerations. For example, TDR allows all
the equipment to be at one end of the transmission line, so the far end
which may be underground in a moisture sensor need not have any other
connection to it. TDT needs electrical connection to both ends of the
transmission line, but the signal received is simpler to analyze, allowing
for less expensive equipment.
ESI sells both TDR & TDT systems. So, if anyone would like further
information on which system would best suit a specific application, please
contact me.
Mike Sly
Business Development Manager
E.S.I. Environmental Sensors Inc.,
100 - 4243 Glanford Avenue,
Victoria. B.C.
V8Z 4B9
Phone: (250) 479-6588
Fax: (250) 479-1412
E-mail: msly@esica.com.
Web: http://www.esica.com
-----Original Message-----
Sent: Tuesday, November 17, 1998 10:44 AM
Subject: Re: choose a sensor
I'm not clear on this explanation for the "Gro·Point" system:
"Gro·Point utilises a single-point, instrument designed for soil moisture
monitoring and irrigation control. The Gro·Point sensor is designed to be
buried in soil and the system uses Time Domain Transmissometry (TDT) to
measure the volumetric moisture content in soils, peat and other porous
materials."
Is this, or is it not Time Domain Reflectometry (TDR)? As far as I
understand the various electronic technologies, there are only resistivity,
capacitive ("Frequency" domain) and TDR (effective dielectric constant of
soils of various moisture content predictably affects velocity of
propagation of an RF signal along a balanced transmission line).
Theoretically, there could be an inductive sensor on the other side of the
Smith Chart to correspond to the capacitive scheme; it doesn't seem likely
to be worth the trouble--but it would probably be patentable, saving the
mumbo-jumbo surrounding the proprietary devices!
I'd like to hear from users (satisfied AND unsatisfied) and the manufacturer
on this.
Dave B.
Dave Barnett
KB1CWT
Technical Specialist I
UMass CEE Dept
Marston Hall 139
Amherst MA 01003
Fax: 413 545-2840