Specifications

Section 7. Measurement Instructions
Parameter
& Data Type
Enter
Option
The Option parameter determines the output of the instruction.
Constant or
Code Description
Variable 0 Measure La/L (ratio of apparent to physical probe rod length)
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
Constant
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
Constant
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
Constant or
Variable
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
Constant or
Variable
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 PCTDR100 with
the Vp = 1.0. Adjust the CableLength and WindowLength values in
PCTDR100 until the probe reflection can be viewed. Subtract about 0.5
meters from the distance associated with the beginning of the probe
reflection.
Note that the specified CableLength applies to all probes read by this
instruction; therefore, all probes must have the same cable lengths.
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