There are no absolute pyranometers like pyrheliometers. All pyranometer are calibrated instruments.There are number of methods of calibrating pyranometers using the sun or laboratory sources. Two of them are as follows.
I)Calibration against a reference standard pyrheliometer:
A perfectly clear day should be choosen to conduct this calibration. The test pyranometer should be fitted with a shading disk to temporarily eliminate the direct component. The disk should be large enough to shade the glass dome at all zenith angles. The test should begin by keeping the pyrheliometer pointed directly at the sun.At the same time, the shading disk should alternately shade and expose the pyranometer for about 10 minutes. The pyranometer calibration factor K is obtained as
K= The ratio of (pyranometer reading average over exposed period- average over the shaded period) and In Cos thitaz
where In is the direct normal irradiance calculated from the pyrheliometer reading and thitaz is Zenith angle.
II) Calibration against a reference pyranometer:
In this method, the test and reference pyranometer are both placed in a horizontal position and operated simultaneously in a clear day. The calibration factor is a ratio of the measurement from the test pyranometer to the measurement from the reference pyranometer.
I)Calibration against a reference standard pyrheliometer:
A perfectly clear day should be choosen to conduct this calibration. The test pyranometer should be fitted with a shading disk to temporarily eliminate the direct component. The disk should be large enough to shade the glass dome at all zenith angles. The test should begin by keeping the pyrheliometer pointed directly at the sun.At the same time, the shading disk should alternately shade and expose the pyranometer for about 10 minutes. The pyranometer calibration factor K is obtained as
K= The ratio of (pyranometer reading average over exposed period- average over the shaded period) and In Cos thitaz
where In is the direct normal irradiance calculated from the pyrheliometer reading and thitaz is Zenith angle.
II) Calibration against a reference pyranometer:
In this method, the test and reference pyranometer are both placed in a horizontal position and operated simultaneously in a clear day. The calibration factor is a ratio of the measurement from the test pyranometer to the measurement from the reference pyranometer.
No comments:
Post a Comment