SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

FIGURE 7.12-1. Wiring Diagram for Full Bridge Pressure TransducerFIGURE 7.13-1. Lysimeter Weighing Mechanism
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 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 (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 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, 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 excitation voltage. 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 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

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