LP02 Pyranometer
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. A fast integration takes less power and allows for faster
throughput.
7.2.2 Multiplier
The multiplier converts the millivolt reading to engineering units. The
calibration supplied by the manufacturer gives the output of the sensor (c) as
microvolts (V x 10
–6
)
per 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 (m) to give the readings in W m
–2
:
m = 1000/c
Other units can be used by adjusting the multiplier as shown in TABLE 7-3.
TABLE 7-3. Multipliers Required for Flux Density and Total Fluxes
Units
Multipliers
Output Processing
W m
–2
m
Average
MJ m
–2
M
•
t
•
0.000001
Total
kJm
–2
M
•
t
•
0.001
Total
cal cm
–2
M
•
t
•
0.0239*0.001
Total
cal cm
–2
min
–1
M
•
1.434
•
0.001
Average
m = calibration factor in W m
-2
/mV
t = datalogger program execution interval in seconds
7.2.3 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
several factors (for example, rate of change of sensor temperature), so it cannot
be removed with a fixed offset. Some users may wish to remove small
negative readings by including code after the measurement instructions that
sets negative readings to zero.
7.2.4 Example Programs
The following programs measure the LP02 every 10 seconds and convert the
mV output to W m
–2
and MJ m
–2
. A sensor calibration of 15.02 µV per W m
–2
is used for the example programs. Both programs output an hourly average
flux (W m
–2
), and a daily total flux density (MJ m
–2
).
12
