Significant Improvements in Pyranometer Nighttime Offsets Using High-Flow DC Ventilation

Ventilators are used to keep the domes of pyranometers clean and dry, but they affect the nighttime offset as well. This paper examines different ventilation strategies. For the several commercial single-black-detector pyranometers with ventilators examined here, high flow rate (50 CFM and higher), 12 VDC fans lower the offsets, lower the scatter, and improve the predictability of the offsets during the night compared with lower flow rate 35 CFM, 120 VAC fans operated in the same ventilator housings. Black-and-white pyranometers sometimes show improvement with DC ventilation, but in some cases DC ventilation makes the offsets slightly worse. Since the offsets for these black-and-white pyranometers are always small, usually no more than 1 Wm-2, whether AC or DC ventilated, changing their ventilation to higher CFM DC ventilation is not imperative. Future work should include all major manufacturers of pyranometers and unventilated, as well as, ventilated pyranometers. An important outcome of future research will be to clarify under what circumstances nighttime data can be used to predict daytime offsets.