Campbell Scientific LI200S-L LI-COR Silicon Pyranometer - Manual (Page 4)

LI200S PYRANOMETER
2
EXAMPLE
-Sensor Calibration: Assume the sensor
calibration is 87 microamps kW
-1 m -2 . The
LI200S outputs amperage which is converted to
voltage by 100 ohm shunt resistor in the cable,
as shown in Figure 1. To convert the calibration
from microamps to millivolts, multiply the
calibration by 0.100. The example calibration
changes to 8.7 mV kW
-1 m -2 .
-Maximum Irradiance: A reasonable estimate
of maximum irradiance at the earth’s surface is
1 kW m
-2 .
-Input Range Selection: An estimate of the
maximum input voltage is obtained by
multiplying the calibration by the maximum
expected irradiance. That product is 8.61mV
for this example. Select the smallest input
range which is greater than the maximum
expected input voltage. In this case the 15mV
range for the 21X and CR7, and the 25mV
range for the CR10 are selected.
Measurement integration time is specified in the
input range parameter code. A more noise free
reading is obtained with the slow or 60 Hz
rejection integration. A fast integration takes
less power and allows for faster throughput.
MULTIPLIER
The multiplier converts the millivolt reading to
engineering units. Commonly used units and
how to calculate the multiplier are shown in
Table 1.
Table 1. Multipliers Required for Flux
Density and Total Fluxes
UNITS MULTIPLIERS
kJ m-2 (1/C) * t (Total)
kW m-2 (1/C) (Average)
cal cm-2 (1/C) * t * (0.0239) (Total)
cal cm-2 min-1 (1/C) * (1.434) (Average)
C = (LI-COR calibration)*0.100
t = datalogger program execution
interval in seconds
4. OUTPUT FORMAT CONSIDERATIONS
The largest number that the datalogger can
output is 6999 in low resolution and 99999 in
high resolution (Instruction 78, set resolution). If
the measurement value is totalized, there is
some danger of overranging the output limits,
as shown in the following example.
EXAMPLE
Assume that daily total flux is desired, and the
datalogger scan rate is 1 second. With a
multiplier that converts the readings to units of
kJ m
-2 and an average irradiance of .5 kW m -2 ,
the maximum low resolution output limit will be
exceeded in less than four hours.
Solution #1 - Record average flux density and
later multiply the result by the number of
seconds in the output interval to arrive at total
flux.
Solution #2 - Record total flux using the high
resolution format. The drawback to high
resolution is that it requires 4 bytes of memory
per data point, consuming twice as much
memory as low resolution.
5. CONNECTIONS
Differential and single-ended connections to the
datalogger are shown in Figures 2 and 3,
respectively.
Figure 2. Differential Measurement
Connection
*AG in Figure 2 refers to Analog Ground in the CR10
which is the same as ground for the 21X and CR7.
On a differential measurement, jumper the low
side of the signal to AG to keep the signal in
common mode range, as shown in Figure 2.
Figure 3. Single-ended Measurement Connection
*AG in Figure 3 refers to Analog Ground in the
CR10, which is the same as ground for the 21X
and CR7.