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Schenk Star pyranometer

Schenk Starpyranometer The Schenk Star pyranometer is a black and white star type pyranometer that has six black and six white segments. The temperature difference between the black and white painted sectors is proportional to the incident solar radiation. Because the measurement is a temperature difference, the measurement should not be affected as much by ambient temperature.

Specifications

Measuring range: 0 to 1500 Wm-2.

Spectral sensitivity: 0.3 to 3 µm.
Output: about 15 µV/Wm-2 or 4 to 20 µA for 0 to 1500 Wm-2.

Impedance: about 35 Ohms.

Ambient temperature: -40 to +60°C.
Resolution: < 1 Wm-2.

Stability: < 1 % per year (temporary operation)

Cosine response: < 3% of the value, zenith angle 0° to 80°.

Azimuth response: < 3% of the value.

Temperature effect: < 1 % of the value between -20 and +40°C.

Linearity: < 0.5% in the range 0.5 to 1330 Wm-2.

Response time: < 25 seconds (95%), < 45 seconds (99%).

Accuracy

An Eppley PSP, which measures the temperature difference between a black disk and the body of the pyranometer, has an offset that results from the black disk radiating to the sky. This effect can be seen by the small negative irradiance values generated during the night. (This offset is on the order of 10 W/m2.)
The Schenk pyranometer measures the temperature difference between the black and white surfaces which both see the same sky temperature. Therefore, this effect is much smaller for star-type pyranometers: one W/m2.
For clear day diffuse measurements that are on the order of 100 W/m2 or less, the offset can cause problems. At the Eugene station, we are measuring the diffuse radiation with a shade disk, with both an Eppley PSP and a Schenk Star pyranometer.
Schenk Star pyranometers exhibit systematic errors when tilted. They also exhibit an azimuthal affect as the responsivity varies slightly, depending on whether the sun is over a black wedge or a white wedge.

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© 2000, UO Solar Radiation Monitoring Laboratory.
Last revised: December 17, 2000.


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