Campbell CR23X, Руководство оператора

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SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-9
The offset is determined after the pressure
transducer is installed in the stilling well. The
sensor is installed 65 cm below the water level
at the time of installation. The depth of water at
this time is determined to be 72.6 cm relative to
the desired reference. When programmed with
the multiplier determined above and an offset of
0, a reading of 65.12 is obtained. The offset for
the actual measurements is thus determined to
be 72.6 - 65.12 = 7.48 cm.
The lead length is approximately 10 feet, so
there is no appreciable error due to lead wire
resistance. Example 7.13 shows a means of
compensating for long lead lengths.
CR23X
FIGURE 7.11-1. Wiring Diagram for Full
Bridge Pressure Transducer
PROGRAM
1: Full Bridge (P6)
1: 1 Reps
2: 22 50 mV, 60 Hz Reject, Slow
Range
3: 1 DIFF Channel
4: 1 Excite all reps w/Exchan 1
5: 5000 mV Excitation
6: 1 Loc [ WATER_cm ]
7: 50.334 Mult
8: 7.48 Offset
7.12 LYSIMETER - 6 WIRE FULL
BRIDGE
When a long cable is required between a load
cell and the CR23X, 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 CR23X 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 (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).
FIGURE 7.12-1. Lysimeter Weighing
Mechanism
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 CR23X 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 CR23X
multiplied by the ratio of the load cell resistance,
R
s
, to the total resistance, R
T
, of the circuit. If
Instruction 6 were used to measure the load