Campbell CR510, Operator'S Manual

Page: 87 / 264

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-9
CR510
FIGURE 7.9-1. Wiring Diagram for Full Bridge Pressure Transducer
FIGURE 7.10-1. Lysimeter Weighing Mechanism
7.10 LYSIMETER - 6 WIRE FULL BRIDGE
When a long cable is required between a load cell
and the CR510, 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 CR510 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
or
31,416 cm
2
, 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
water) in the lysimeter before the counterbalance
would have to be readjusted.
There is 1000 feet of 22 AWG cable between the
CR510 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