Campbell TDR100 Instruction Manual - page 28
TDR100
18
Option: The Option parameter determines the output of the instruction.
Code
Description
0
Measure L
a
/L (ratio of apparent to physical probe rod length).
Dividing L
a
by the real rod length, L, gives the square root of
dielectric constant. The L
a
/L value is empirically related to
volumetric water content using calibration functions of the form
θ
v
= f(L
a
/L). See Section
(p. 19)
, for commonly
used calibration functions.
1
Collect Waveform values – Outputs reflection waveform values as an
array of floating point numbers with a range of –1 to 1. The
waveform values are prefaced by a header containing values of key
parameters for this instruction (averaging, propagation velocity,
points, cable length, window length, probe length, probe offset,
multiplier, offset)
2
Collect Waveform plus First Derivative – Returns (2*n–5)+9 values
where n is the number of waveform reflection values specified by the
Points parameter.
3
Measure Electrical Conductivity – Outputs a value that when
multiplied by the Multiplier parameter determines soil bulk electrical
conductivity in S/m.
Mux/ProbeSelect: The Mux/Probe Select parameter is used to define the setup
of any multiplexers and attached probes in the system. The addressing scheme
used is ABCR, where A = level 1 multiplexer channel, B = level 2 multiplexer
channel, C = level 3 multiplexer channel, and R = the number of consecutive
probes to be read, starting with the channel specified by the ABC value
(maximum of 8). 0 is entered for any level not used.
WaveAvg: The WaveAvg parameter is used to define the number of waveform
reflections averaged by the TDR100 to give a single result. A waveform
averaging value of 4 provides good signal-to-noise ratio under typical
applications. Under high noise conditions averaging can be increased. The
maximum averaging possible is 128.
Vp: The Vp parameter allows you to enter the propagation velocity of a cable
when using the instruction to test for cable lengths or faults. Vp adjustment is
not necessary for soil water content or electrical conductivity measurement and
should be set to 1.0 for output Option 1, 2, or 3.
Points: The Points parameter is used to define the number of values in the
displayed or collected waveform (20 to 2048). An entry of 251 is
recommended for soil water measurements. The waveform consists of the
number of Points equally spaced over the WindowLength.
CableLength: The CableLength parameter is used to specify the cable length,
in meters, of the TDR probes. If a 0 is entered for the Option parameter, cable
length is used by the analysis algorithm to begin searching for the TDR probe.
If a 1 or 2 is entered for the Option parameter, cable length is the distance to
the start of the collected waveform.
The value used for CableLength is best determined using PC-TDR100 with the
Vp = 1.0. Adjust the CableLength and WindowLength values in PC-TDR100