Campbell Scientific LP02 Hukseflux Pyranometer - Manual (Page 22)

LP02 Pyranometer
8. Maintenance
Inspect and clean the outer dome at regular intervals, for example, every week
or so. Clean any accumulated dust, etc. off the dome and pyranometer body
using a soft cloth dampened with water or alcohol. Check that there is no
condensation within the dome.
It is also important to check the data returned from the sensor as it will show
the first indication of a fault. Be aware of several expected phenomena that can
cause strange measurements. In particular, on clear, windless nights the outer
dome temperature of horizontally placed pyranometers can fall as low as the
dewpoint temperature of the air, due to infrared radiation exchange with the
cold sky. (The effective sky temperature can be 30°C lower than the ground
temperature, which results in an infrared emission of –150 W m –2 ). If this
happens, dew, glazed frost or hoar frost can be precipitated on the top of the
outer dome and can stay there for several hours in the morning. An ice cap on
the dome is a strong diffuser and can decrease the pyranometer signal by up to
50% in the first hours after sunrise.
The calibration of the LP02 may drift with time and exposure to radiation.
Recalibration every two years is recommended. The sensor should be returned
to Campbell Scientific, the manufacturer, or a calibration lab with facilities to
calibrate radiation sensors.
9. Troubleshooting
Symptom: NAN, –9999, or radiation values around 0
1. Check that the sensor is wired to the differential channel specified by the
measurement instruction.
2. Verify that the range code is correct for the datalogger type.
3. Measure the impedance across the sensor wires. This should be around
100 Ω plus the cable resistance (typically 0.1 Ω m –1 ). If the resistance is
very low, there may be a short circuit (check the wiring). Resistances
somewhat lower than expected could be due to water ingress into the
sensor or enclosure connectors. If the resistance is infinite, there is a
broken connection (check the wiring).
4. Disconnect the sensor cable and check the voltage between pins 1 and 3
on the sensor. With the sensor located 8 inches below a 60 W
incandescent light bulb the voltage should be approximately 2.5 mV. No
voltage indicates a problem with the sensor.
Symptom: sensor signal is unrealistically high or low
1. Check that the right calibration factor has been properly entered into the
datalogger program. Please note that each sensor has its own individual
calibration factor.
2. Check the condition of the sensor cable.
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