Campbell Scientific TDR100 Time Domain Reflectometry Systems - Manual (Page 10)

TDR100
port 1 is the default setting and can be changed in PCTDR menu
Settings/Communications. The baud rate is factory set to 57600.
3. Connect 12 volt power to TDR100
12 volt power to the TDR100 is connected using terminals +12V and
GROUND on the panel 5-terminal connector. An external power supply
or the 12V terminals of a datalogger can be used for power. The C1, C2
and C3 terminals are for SDM (synchronous device for measurement
communication protocol) communications. The C1, C2 and C3 terminals
are not used for single probe monitoring with a computer using PCTDR.
4. Connect a TDR probe to the BNC connector of the TDR100
5. Start PCTDR by selecting PCTDR under Programs of the Windows
Start Menu or double-clicking the PCTDR icon.
6. View a waveform using Get Waveform
In the Waveform section of PCTDR, set Start to 0 or 1 m and Length to
the apparent length (see note below) of the attached probe cable plus 5
meters. Press Get Waveform.
3.1 Discussion of Distances and Propagation Velocity (V
p
)
when using TDR100
A TDR system is typically comprised of components with different signal
propagation properties. The V
p
for a particular component depends on
transmission line characteristics such as the dielectric constant of inter-
conductor insulating material. Setting V
p
= 1.0 and using apparent distances
simplifies system setup. The displayed position of a waveform is apparent
distance. The value chosen for V
p
does not affect water content or electrical
conductivity measurement. The selected V
p
value does affect waveform
display.
The relationship between real and apparent distance is given as
apparent distance = (actual distance) x (selected V
p
/actual V
p
).
For example, if the actual length of a cable having a V
p
of 0.78 is 5 meters and
the selected V
p
is 1.0, the apparent distance to the end of the cable is 5 x
(1.0/0.78) = 6.41 meters. Typical cable V
p
’s range from 0.67 to 0.9. Campbell
Scientific TDR probes use RG-58 with a Vp of 0.67 and RG-8 with a Vp of
0.78.
An example is presented in Figure 3-1. Displayed is the waveform for a
CS610 in water. The actual cable length is about 5 m. The apparent cable
length is about 6 m.
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