To evaluate the quality of the instantaneous measurements, for two representative days (one with clear and one with clouded sky) the instantaneous values of the sensors when G(device) >10 W/m2 were statistically analysed for their deviation to the CM21. The guaranteed accuracy of the instantaneous measurements of the reference pyranometer CM21 is ± 0,5 %. For the analysis, mean value, standard error, the maximum upper and the maximum lower deviation were calculated for the absolute and relative deviation between sensor and pyranometer. The relative deviation was calculated by dividing the difference between sensor value and pyranometer value through the pyranometer value. This approach allows the comparison of the sensors independent from the level of irradiance.

For estimating the accuracy of long-term measurements, the sums of total irradiance measured by the sensors over a period of 8 days with changing weather were compared to the energy measured by the CM21. As a potential source of error for long-term measurements, the overnight off-set of the sensors was observed for its effect.

The measurements were carried out in Vienna in October 2003. The outdoor test stand was installed with a declination of 45 degrees, an azimuth of 0 degrees and an inclined surface with a length of 116 cm and a horizontal width of 160 cm, which is dyed black for simulating the roofage. The 10 tested irradiance sensors were placed approximately in middle height of the stand. Simultaneously to the irradiance, the ambient air temperature was measured by 2 calibrated Pt100 sensors, one mounted directly on the test stand and irradiation sheltered by an isolated and ventilated pipe-in-pipe system, the other one mounted closely beside the test stand sheltered by a standard irradiation protection casing. Over a period of 8 days the measured values were recorded partly in one minute and two minute intervals. Figure 1 and 2 exemplify the recorded trends of all measured devices during a day with clear sky and one with clouded sky. It can be seen, that with one exception the irradiance sensors tend to measure lower instantaneous irradiances than the reference-pyranometer.

Figure 2: Recorded Trends during a sector of October 23rd 2003

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