Lysimeter - 6 Wire Full Bridge; Wiring Diagram For Full Bridge Pressure Transducer; Lysimeter Weighing Mechanism - Campbell Measurement and Control Module CR10 Operator's Manual

FIGURE 7.12-1. Wiring Diagram for Full Bridge Pressure Transducer

7.13 LYSIMETER - 6 WIRE FULL BRIDGE

When a long cable is required between a load cell
and the CR10, 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 CR10 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 counter-
balance, 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 (i.e., 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 m
2
31,416 cm , 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 (28 cm of
SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
FIGURE 7.13-1. Lysimeter Weighing Mechanism
2
or
water) in the lysimeter before the counterbalance
would have to be readjusted.
There is 1000 feet of 22 AWG cable between the
CR10 and the load cell. The output of the load cell
is directly proportional to the excitation voltage.
When Instruction 6 (4 wire half 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 CR10 multiplied by the ratio of the
load cell resistance, R
s
of the circuit. If Instruction 6 were used to measure
the load cell, the excitation voltage actually applied
to the load cell, V , would be:
1
V = V R /R = V 350/(350+33) = 0.91 V
1 x s T x
Where V is the excitation voltage. This means
x
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 evapotranspira-
tion 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
, to the total resistance, R
x
7-11
,
T