TDR SOIL MOISTURE MEASUREMENTS
7
1990, and calculates 1/Vp; the multiplier and
offset (parameters 7 and 8) may be used to
obtain the result directly in volumetric water
content (Section 5).
"Raw data", Code 1: 256 values are output for
each probe measured; the 251 values from the
waveform followed by the distance to the cursor
in meters, the distance between waveform
points, the gain, the offset, and the sample
number.
"Manual Step Through", Code 98: When the
program is compiled and run, the 1502B will be
left under manual control and the datalogger will
switch to the first channel and pause. To switch
to the next channel, set flag 8 (*6 Mode or with
TERM); the datalogger will switch to the next
channel and set flag 8 low. Repeatedly setting
flag 8 allows the user to step through all
programmed channels.
"Signature", Code 99: The signature of the
PROM in the SDM1502 is calculated and stored
in the location specified in parameter 6.
Parameter 3, Probe Length: The length of the
soil probe in meters. This length is used to
calculate the water content result (apparent
length divided by actual length, or 1/Vp), and to
determine the time/length resolution of the
waveform captured for the water content
calculation or raw output.
The probe length parameter is used to set the
resolution of the raw waveform for applications
where there is no actual "probe length". Table
4-1 lists the distance per division used at
different probe lengths.
TABLE 4-1. Waveform Resolution
Determined by Probe Length
Probe Length
Distance/Division
0
to
0.02
0.025
0.03 to
0.06
0.05
0.07 to
0.13
0.1
0.14 to
0.2
0.25
0.3
to
0.4
0.5
0.5
to
0.9
1.0
1.0
to
2.0
2.5
2.1
to
4.3
5.0
4.4
to
9.2
10.0
9.3
to
9.9
25.0
>
10.0
50.0
Parameter 4 Cable length: The "length" in
meters to the probe or 1502B cursor. Enter "0"
and the datalogger will automatically search for
the soil probe.
This "length" is not the actual length as
measured with a meter stick; it is the distance
the 1502B measures using a propagating
velocity of 0.99 the speed of light (Vp = 0.99).
When capturing the waveform for raw output or
calculating water content, the cursor (start of
probe) is placed 1 division into the 1502B
display (10 divisions total).
If an automatic search for the soil probe fails,
and it is desired to enter the length to the cable,
it can be determined by manually stepping
through the multiplexer and finding the probe
with the 1502B display.
Parameter 5, Multiplexer channel/Reps:
Determines the multiplexer, first measurement
channel, and number of probes to measure.
The entry required is: channel number on level
1 multiplexer, channel on level 2 multiplexer,
channel on level 3 multiplexer, and number of
probes to measure. For example, 1008 would
indicate that there is 1 multiplexer; start on
channel 1 and step through 8 probes. The entry
2318 tells the datalogger to set the level one
multiplexer to channel 2, the level two
multiplexers to channel 3, and measure 8
probes that are connected to channels 1-8 on
the level 3 multiplexer.
Parameter 6, Input location: The Input
location in which to start storing data. When
outputting water content, one location is used
for each probe measured. When outputting raw
data, 256 locations are required for each
repetition.
5. CALIBRATION FOR WATER
CONTENT
When the water content output option is
selected, the datalogger calculates the ratio of
the apparent length of the probe (the 1502B
results are in terms of length) to the actual
length as entered in parameter 3. The
datalogger corrects the length to what it would
be if the propagation velocity was the speed of
light. Thus, the ratio of lengths is the same as
the ratio of the speed of light divided by the
velocity of the signal down the probe.
