Campbell CMP11-L, Instruction Manual

Page: 23 / 42

CMP6-L, CMP11-L, and CMP21-L Pyranometers
7.3.1.1 Input Range
The output voltage is usually between 5 and 20 mV per 1000 W • m
–2
. When
estimating the maximum likely value of sensor output a maximum value of
solar radiation of 1100 W
• m
–2
can be used for field measurements on a
horizontal surface. Plane of array irradiances can exceed 1500 W
•
m
–2
.
Select the input range as follows:
1. Estimate the maximum expected input voltage by multiplying the
maximum expected irradiance (in W • m
–2)
by the calibration factor (in µV /
W • m
–2
). Divide the answer by 1000 to give the maximum in millivolt
units.
2. Select the smallest input range which is greater than the maximum
expected input voltage. Normally the 50 mV range for the CR3000,
CR5000, CR9000(X), CR7, and CR23X and the 25 mV or 250 mV range
for the CR800, CR850, CR1000, CR510, and CR10(X) will be suitable.
The exact range will depend on the sensitivity of your individual sensor
and the maximum expected reading. With some dataloggers an autorange
option can be used if measurement time is not critical.
The parameter code for the input range also specifies the measurement
integration time. The slow or 60 Hz rejection integration gives a more noise-
free reading. The 250 µs (CRBasic) or a fast (Edlog) integration takes less
power and allows for faster throughput.
7.3.1.2 Multiplier
The multiplier converts the millivolt reading to engineering units. The
sensitivity value supplied by the manufacturer gives the output of the sensor as
µ V (micro-volts) / W • m
–2
. As the datalogger voltage measurement
instructions give a default output in mV, the following equation should be used
to calculate the multiplier to give the readings in W
• m
–2
:
m = 1000/c
Where,
m = multiplier
c = sensor output in µV / W
• m
–2
Other units can be used by adjusting the multiplier as shown in TABLE 7-5.
TABLE 7-5. Multipliers Required for Flux Density and Total Fluxes
Units Multiplier Output Processing
W • m
–2
M Average
MJ • m
–2
M * t * 0.000001 Totalize
kJ • m
–2
M *
t
* 0.001
Totalize
cal • cm
–2
M * t * 0.0239 * 0.001 Totalize
cal • cm
–2
• min
–1
M * 1.434 * 0.001 Average
W • hr • m
–2
t / 3600 Totalize
M = calibration factor with units of W • m
–2
/ mV
t = datalogger program execution interval in seconds
15