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CMP6-L, CMP10-L, CMP11-L, and CMP21-L Pyranometers

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 infra-red

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 increase the

pyranometer signal by up to 50% in the first hours after sunrise.

8.4 Recalibration

The calibration of the pyranometer may drift with time and exposure to

radiation. Recalibration every two years is recommended. The sensor should

be returned to Campbell Scientific for recalibration. A Returned Materials

Authorization (RMA) is required (refer to the Assistance page for more

information).

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 red and blue sensor wires. This should

be around 100 ohms plus the cable resistance (typically 0.1 ohm•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 output from the sensor.

With the sensor located 8” 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.

Symptom: sensor signal shows unexpected variations

1. Check for the presence of strong sources of electromagnetic radiation

(radar, radio, etc.).

2. Check the condition and the connection of the sensor shield wire.

3. Check the condition of the sensor cable.

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