TDR SOIL MOISTURE MEASUREMENTS
8
The propagating velocity, Vp is defined as the
ratio of the actual velocity divided by the speed
of light. The result from the datalogger (with a
multiplier of 1 and an offset of 0) is 1/V
p
.
A multiplier of 0.1138 and an offset of -0.1758
gives volumetric water fraction using the
calibration of Ledieu et. al. 1986 (normalized to
accommodate probes of varying length):
W
v
= 0.1138/V
p
- 0.1758
Topp et. al. 1980, used a third order equation to
equate volumetric water content, W
v
, to
apparent dielectric constant Ka. Ka=(1/V
p
)
2
;
with this substitution, the equation is
W
v
= -0.053+0.0292(1/V
p
)
2
-0.00055(1/V
p
)
4
+0.0000043(1/V
p
)
6
Example 2 uses Topp's calibration.
1
3
5
7
1/Vp
VOLUMETRIC WATER CONTENT
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
LEDIEU TOPP
FIGURE 5-1. Comparison of Calibrations
from Topp and Ledieu
6. PROGRAMMING EXAMPLES
There are several things to keep in mind when
writing a datalogger program to make TDR
measurements:
1. Set the control port high that controls the
PS 1502B 5 seconds prior to executing
Instruction 100. Switch it off after all
measurements are made.
2. TDR measurements require considerably
more time than other measurements
(approximately 1 minute each on the first
pass when finding the probe and 15 to 20
seconds each on subsequent
measurements). The program must be
written so that analog measurements,
output intervals other time based decisions
take this into account.
Switching power to the 1502B is valuable not
only for the conservation of batteries; it also
allows the 1502B to be reset if it should "bomb".
If the 1502B or 1502C does not respond
correctly to the datalogger's commands, -99999
will be loaded into the input locations that would
normally contain the results of the
measurements.
6.1 EXAMPLE 1, MEASURE AND RECORD
EVERY HOUR
The following program is executed once an
hour. A single multiplexer with eight probes is
measured. The 1502B is switched on, the
measurements made with output in water
content using Ledieu's calibration, the 1502B is
switched off, and the time and 8 water content
measurements are output to Final Storage.
*
1
Table 1 Programs
01: 3600
Sec. Execution Interval
01:
P86
Do
01:
44
Set high Port 4
02:
P22
Excitation with Delay
01:
1
EX Chan
02:
0
Delay w/EX (units=.01sec)
03:
500
Delay after EX (units=.01sec)
04:
0
mV Excitation
03:
P100
SDM-1502
01:
11
Address
02:
0
Moisture
03:
.3
Probe length (meters)
04:
0
Cable length (meters)
05: 1008
MMMP--Mux channels & Probes
06:
1
Loc [:WATER #1 ]
07:
0.1138
Mult
08:
-0.1758
Offset
04:
P86
Do
01:
54
Set low Port 4
05:
P86
Do
01:
10
Set high Flag 0 (output)
06:
P77
Real Time
01:
110
Day,Hour-Minute
