Campbell CR510, Руководство оператора

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SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-10
of the bridge in the load cell is 350 ohms. The
voltage drop across the load cell is equal to the
voltage at the CR510 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 cell, the excitation voltage actually applied
to the load cell, V
1
, would be:
V
1
= V
x
R
s
/R
T
= V
x
350/(350+33) = 0.91 V
x
Where V
x
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
copper changes 0.4% per degree C change in
temperature. Assume that the cable between the
load cell and the CR510 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:
V
1
= 350/(350+29.7) V
x
= .92 V
x
The excitation voltage has increased by 1%,
relative to the voltage applied at the CR510. 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/V
1
at a load of
250 pounds. Output is desired in millimeters of
water with respect to a fixed point. The "4" found
in equation 7.12-1 is due to the mechanical
advantage. The calibration in mV/V
1
/mm is:
3.106 mV/V
1
/250 lb x 2.2 lb/kg x
3.1416 kg/mm/4 = 0.02147 mV/V
1
/mm
The reciprocal of this gives the multiplier to
convert mV/V
1
into millimeters. (The result of
Instruction 9 is the ratio of the output voltage to
the actual excitation voltage multiplied by 1000,
which is mV/V
1
):
1/0.02147 mV/V
1
/mm = 46.583 mm/mV/V
1
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 7.10-1, the
output voltage would be positive when the load
cell was under tension.)
CR510
FIGURE 7.10-2. 6 Wire Full Bridge Connection for Load Cell