Campbell Scientific CR7 Measurement and Control System - Manual (Page 80)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-12
ohms at the maximum temperature, then, at the
minimum temperature, the resistance is:
(1-25x0.004)33 ohms = 29.7 ohms
The actual excitation voltage at the load cell is:
V1 = 350/(350+29.7) Vx = .92 Vx
The excitation voltage has increased by 1%,
relative to the voltage applied at the CR7. In the
case where we were recording a 91 mm change
in water content, there would be a 1 mm diurnal
change in the recorded water content that would
actually be due to the change in temperature.
Instruction 9 solves this problem by actually
measuring the voltage drop across the load cell
bridge. The drawbacks to using Instruction 9
are that it requires an extra differential channel
and the added expense of a 6 wire cable. In
this case the benefits are worth the expense.
The load cell has a nominal full scale output of
3 millivolts per volt excitation. If the excitation is
5 volts, the full scale output is 15 millivolts; thus
the ±15 millivolt range is selected. The
calibrated output of the load cell is 3.106 mV/V1
at a load of 250 pounds. Output is desired in
millimeters of water, with respect to a fixed
point. The calibration in mV/V1/mm is:
3.106mV/V1/250lb x 2.2lb/kg x
3.1416kg/mm/4 =
0.02147mV/V1/mm
The reciprocal of this gives the multiplier to
convert mV/V1 into millimeters (the result of
Instruction 9 is the ratio of the output voltage to
the actual excitation voltage multiplied by 1000,
which is mV/V1):
1/0.02147mV/V1/mm = 46.583 mm/mV/V1
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 diagrammed in Figure 7.15-1,
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 = 375 mm). The experiment is
started at the beginning of what is expected to
be a period during which evapotranspiration
exceeds precipitation. Instruction 9 is
programmed with the correct multiplier and no
offset. After hooking everything up, the
counterbalance is adjusted so that the load cell
is near the top of its range, this will allow 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 266. However,
it is decided to add this offset in a separate
program 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 millimeters
is output to Final Storage (not shown in Table)
every hour. The average is used, instead of a
sample, in order to cancel out the effects of
wind loading on the lysimeter.
FIGURE 7.15-2. 6 Wire Full Bridge
Connection for Load Cell
PROGRAM
01: P9 Full BR w/Compensation
01: 1 Rep
02: 8 5000 mV slow EX Range
03: 3 15 mV slow BR Range
04: 1 IN Card
05: 1 IN Chan
06: 1 EX Card
07: 1 EX Chan
08: 1 Meas/EX
09: 5000 mV Excitation
10: 1 Loc [:mm RAW ]
11: 46.583 Mult
12: 0 Offset