CMP6, CMP10, CMP11, and CMP21 Pyranometers
14
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 that is greater than the maximum expected
input voltage. The exact range will depend on the sensitivity of your
individual sensor and the maximum expected reading. With some data
loggers, an autorange option can be used if measurement time is not
critical.
7.4.1.2 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 data logger 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
TABLE 7-3. 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
M • t / 3600
Totalize
M = calibration factor with units of W
•
m
–2
/ mV
t = data logger program execution interval in seconds
7.4.1.3 Offset
The offset will normally be fixed at zero as the sensor should output no
significant signal in dark conditions. In practice, because of the nature of
thermopile detector sensors, there will be some offset in dark conditions;
sometimes this offset can give negative light readings. This offset varies with
factors, such as rate of change of sensor temperature, so it cannot be removed
with a fixed offset. Some users remove small negative readings by including
code after the measurement instructions that sets negative readings to zero.
