CMP6, CMP10, CMP11, and CMP21 Pyranometers
17
8.1.2 Check Sensor Output
Check the data returned from the sensor as it will show the first indication of a
fault. However, several expected phenomena can cause strange measurements.
In particular, on clear, windless nights the outer dome temperature of
horizontally placed pyranometers can fall as low as the dew point 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 form 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
increase the pyranometer signal by up to 50% in the first hours after sunrise.
8.2 Recalibration
The calibration of the pyranometer may drift with time and exposure to
radiation. Campbell Scientific recommends recalibrating every two years. The
sensor should be returned to Campbell Scientific for recalibration. Refer to the
page for information on returning the pyranometer to Campbell
Scientific for recalibration.
8.3 Troubleshooting
Symptom: NAN, –9999, or radiation values around 0
1. Check that the sensor is wired to the differential terminal specified by the
measurement instruction.
2. Verify that the range code is correct for the data logger type.
3. Measure the impedance across the white and black sensor wires. This
should be around 100 ohms plus the cable resistance (typically 0.1
ohm
•
m
–1
). If the resistance is 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 output from 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
data logger program. Please note that each sensor has its own individual
calibration factor.
2. Check the condition of the sensor cable.
Symptom: sensor signal shows unexpected variations
1. Check for the presence of strong sources of electromagnetic radiation,
such as radar or radio interference.
2. Check the condition and the connection of the sensor shield wire.
3. Check the condition of the sensor cable.
