TDR200-Based Time-Domain Reflectometry System
15
(p. 19)
). Acceptable values are 0 (no filtering) to 10 (maximum
filtering).
For the
TDRLaa
parameter, option 2 is best for noisy environments or when the
length of probes is short (Section
Algorithms for Calculation of TDR
(p. 19)
).
Additional descriptions of parameters are provided in in the
CRBasic Help
.
7.2.2 PortsConfig CRBasic Instruction
If using a CR800, CR850, or CR1000, the
PortsConfig()
CRBasic instruction
may be needed at the end of the program. When the TDR200 is connected to
the data logger for SDM control and SDM8X50 multiplexers are also
connected, the
PortsConfig()
instruction is required to properly configure the
control ports. If this is not done,
PC-TDR
will not control the multiplexers
because the control ports present a low impedance to the SDM lines and this
will load the signal issued by TDR200 when
PC-TDR
is used to control
multiplexers.
PortsConfig
(&B00000111,&B00000000)
8. Operation
This section discusses TDR principles, conditions that effect water content
measurements, filtering and averaging used to improve measurements in noisy
environments, and the algorithms used to calculate the measurements.
8.1 TDR Principles
The travel time for a pulsed electromagnetic signal along a waveguide is
dependent on the velocity of the signal and the length of the waveguide. The
velocity is dependent on the dielectric constant of the material surrounding the
waveguide. This relationship can be expressed by
∆
t
L K
c
a
=
2
[1]
Where
K
a
is the apparent dielectric constant,
c
is the velocity of
electromagnetic signals in free space,
∆
t
is the travel time, and
L
is the
waveguide length. The dielectric constant of water relative to other soil
constituents is high. Consequently, changes in volumetric water content can be
directly related to the change in the dielectric constant of bulk soil material.
Equation [1] can be simplified to express the apparent dielectric constant as the
ratio of the apparent probe length (
L
a
= c
∆
t/2
) to the real probe length.
K
L
L
a
a
=
[2]
The relationship between dielectric constant and volumetric water content has
been described by, among others, Topp et al. (1980) and Ledieu et al. (1986) in
an empirical fashion using both polynomial and linear forms. These
expressions are presented here since numerous research efforts show these