SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

FIGURE 7.13-2. 6 Wire Full Bridge Connection for Load Cell

copper changes 0.4% per degree C change in temperature. Assume that the cable between the load cell and the CR10 lays on the soil surface and undergoes a 25°C diurnal temperature fluctuation. If the resistance is 33 ohms at the maximum temperature, then at the minimum temperature, the resistance is:

(1-25x0.004)33 ohms = 29.7 ohms The actual excitation voltage at the load cell is:

V1 = 350/(350+29.7) Vx = .92 Vx

The excitation voltage has increased by 1%, relative to the voltage applied at the CR10. In the case where we were recording a 91 mm change in water content, there would be a 1 mm diurnal change in the recorded water content that would actually be due to the change in temperature. Instruction 9 solves this problem by actually measuring the voltage drop across the load cell bridge. The drawbacks to using Instruction 9 are that it requires an extra differential channel and the added expense of a 6 wire cable. In this case, the benefits are worth the expense.

The load cell has a nominal full scale output of 3 millivolts per volt excitation. If the excitation is 2.5 volts, the full scale output is 7.5 millivolts; thus, the

±7.5 millivolt range is selected. The calibrated output of the load cell is 3.106 mV/V1 at a load of 250 pounds. Output is desired in millimeters of water with respect to a fixed point. The "4" found in equation 7.13-1 is due to the mechanical advantage. The calibration in mV/V1/mm is:

3.106 mV/V1/250 lb x 2.2 lb/kg x

3.1416 kg/mm/4 = 0.02147 mV/V1/mm

The reciprocal of this gives the multiplier to convert mV/V1 into millimeters. (The result of

7-12

Instruction 9 is the ratio of the output voltage to the actual excitation voltage multiplied by 1000, which is mV/V1):

1/0.02147 mV/V1/mm = 46.583 mm/mV/V1

The output from the load cell is connected so that the voltage increases as the mass of the lysimeter increases. (If the actual mechanical linkage was as shown in Figure 7.13-1, the output voltage would be positive when the load cell was under tension.)

When the experiment is started, the water content of the soil in the lysimeter is approximately 25% on a volume basis. It is decided to use this as the reference (i.e., 0.25 x 1500 mm = 375 mm). The experiment is started at the beginning of what is expected to be a period during which evapotranspiration exceeds precipitation. Instruction 9 is programmed with the correct multiplier and no offset. After hooking everything up, the counterbalance is adjusted so that the load cell is near the top of its range; this will allow a longer period before readjustment is necessary. The result of Instruction 9 (monitored with the *6 Mode) is 109. The offset needed to give the desired initial value of 375 mm is 266. However, it is decided to add this offset in a separate instruction so the result of Instruction 9 can be used as a ready reminder of the strain on the load cell (range = ±140 mm). When the strain on the load cell nears its rated limits, the counterbalance is readjusted and the offset recalculated to provide a continuous record of the water budget.

The program table has an execution interval of 10 seconds. The average value in millimeters is output to Final Storage (not shown in Table) every hour.