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Section 7. Measurement Programming Examples
7-17
The output from the load cell is connected so that the voltage increases as the
mass of the lysimeter increases. (If the actual mechanical linkage was as shown in
Figure 7-11, the output voltage would be positive when the load cell was under
tension.)
When the experiment is started, the water content of the soil in the lysimeter is
approximately 25% on a volume basis. It is decided to use this as the reference
(i.e. 0.25 x 1500mm = 375mm). The experiment is started at the beginning of
what is expected to be a period during which evapotranspiration exceeds precipi-
tation. Instruction 9 is programmed with the correct multiplier and no offset. After
connecting everything up, the counterbalance is adjusted so that the load cell is
near the top of its range; this allows a longer period before readjustment is
necessary. The result of Instruction 9 (monitored with the *6 Mode) is 109. The
offset needed to give the desired initial value of 375mm is therefore 266.
However, it is decided to add this offset in a separate instruction so that the result
of Instruction 9 can be used as a ready reminder of the strain on the load cell
(range = ±140mm). When the strain on the load cell nears its rated limits, the
counterbalance is readjusted and the offset recalculated to provide a continuous
record of the water budget.
The program table has an execution interval of 10 seconds. The average value in
millimetres is output to Final Storage every hour (the instructions to do this are
not shown). The average is used, instead of a sample, in order to cancel out the
effects of wind loading on the lysimeter.
Program
01: Full Bridge w/mv Excit (P9)
01:
1
Rep
02:
35
2500mV 50Hz rejection EX Range
03:
32
7.5mV 50Hz rejection BR Range
04:
1
IN Chan
05:
1
Excite all reps w/EXchan 1
06:
2500
mV Excitation
07:
1
Loc [:RAW mm ]
08:
46.583
Mult
09:
0
Offset
02: Z=X+F (P34)
01:
1
X Loc RAW mm
02:
266
F
03:
2
Z Loc [:mm H2O ]
Connections
Figure 7-12 6-Wire Full Bridge Connection for Load Cell
CR10X