Campbell Scientific CR7 Measurement and Control System - Manual (Page 79)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-11
PROGRAM
01: P6 Full Bridge
01: 1 Rep
02: 4 50 mV slow Range
03: 1 IN Card
04: 1 IN Chan
05: 1 EX Card
06: 1 EX Chan
07: 1 Meas/EX
08: 5000 mV Excitation
09: 13 Loc [:HEIGHT cm]
10: 50.334 Mult
11: 7.48 Offset
7.15 LYSIMETER - 6 WIRE LOAD CELL
When a long cable is required between a load
cell and the CR7, the resistance of the wire can
create a substantial error in the measurement if
the 4 wire full bridge (Instruction 6) is used to
excite and measure the load cell. This error
arises because the excitation voltage is lower at
the load cell than at the CR7 due to voltage
drop in the cable. The 6 wire full bridge
(Instruction 9) avoids this problem by measuring
the excitation voltage at the load cell. This
example shows the errors one would encounter
if the actual excitation voltage was not
measured and shows the use of a 6 wire full
bridge to measure a load cell on a weighing
lysimeter (a container buried in the ground, filled
with plants and soil, used for measuring
evapotranspiration).
The lysimeter is 2 meters in diameter and 1.5
meters deep. The total weight of the lysimeter
with its container is approximately 8000 kg. The
lysimeter has a mechanically adjustable
counterbalance, and changes in weight are
measured with a 250 pound (113.6 kg) capacity
Sensotec Model 41 tension/compression load
cell. The load cell has a 4:1 mechanical
advantage on the lysimeter (ie., a change of 4
kg in the mass of the lysimeter will change the
force on the load cell by 1 kg-force or 980 N).
The surface area of the lysimeter is 3.1416 m2
or 31,416 cm2, so 1 cm of rainfall or
evaporation results in a 31.416 kg change in
mass. The load cell can measure ±113.6 kg, a
227 kg range. This represents a maximum
change of 909 kg, or 28 cm of water in the
lysimeter before the counterbalance would have
to be readjusted.
FIGURE 7.15-1. Diagrammatic
Representation of Lysimeter Weighing
Mechanism
There is 1000 feet of 22 AWG cable between
the CR7 and the load cell. The output of the
load cell is directly proportional to the excitation
voltage. When Instruction 6 (4 wire 1/2 bridge)
is used, the assumption is that the voltage drop
in the connecting cable is negligible. The
average resistance of 22 AWG wire is 16.5
ohms per 1000 feet. Thus, the resistance in the
excitation lead going out to the load cell added
to that in the lead coming back to ground is 33
ohms. The resistance of the bridge in the load
cell is 350 ohms. The voltage drop across the
load cell is equal to the voltage at the CR7
multiplied by the ratio of the load cell resistance
Rs, to the total resistance, RT, of the circuit. If
Instruction 6 were used to measure the load
cell, the excitation voltage actually applied to the
load cell, V1 would be:
V1 = Vx Rs/RT = Vx 350/(350+33) = 0.91 Vx
Where Vx is the voltage applied at the
excitation card. This means that the voltage
output by the load cell would only be 91% of that
expected. If recording of the lysimeter data was
initiated with the load cell output at 0 volts, and
100 mm of evapotranspiration had occurred,
calculation of the change with Instruction 6
would indicate that only 91 mm of water had
been lost. Because the error is a fixed
percentage of the output, the actual magnitude
of the error increases with the force applied to
the load cell. If the resistance of the wire was
constant, one could correct for the voltage drop
with a fixed multiplier. However, the resistance
of copper changes 0.4% per oC change in
temperature. Assume that the cable between
the load cell and the CR7 lays on the soil
surface and undergoes a 25 oC diurnal
temperature fluctuation. If the resistance is 33