Why should you recalibrate a pyranometer? Well, like all measuring devices the calibration can drift with time in operation, according to the experts at Kipp & Zonen. This is particularly true for pyranometers that may be working in environments ranging from the Antarctic to hot deserts to tropical conditions.
The guideline IEC 61724-1 March 2017 Photovoltaic system performance – Part 1: Monitoring specifies recalibration intervals for sensors monitoring Global Horizontal Irradiance (GHI) and Plane Of Array (POA) irradiance in PV plants.
For Class C (Basic Accuracy) monitoring, it is “as per manufacturer’s requirements”,
for Class B (Medium Accuracy) it is every 2 years,
for Class A (High Accuracy) it is every year.
The standard also specifies that pyranometers must be calibrated to ISO 9846:1993 or ISO 9847:1992. ISO 9846 is outdoor calibration using a reference pyrheliometer. ISO 9847 is for calibration against a reference pyranometer and there are both outdoor and indoor methods. An outdoor calibration to either standard requires 2-3 days of clear skies. So, field pyranometers are normally calibrated indoors. Calibrations must be fully traceable to the World Radiometric Reference (WRR), which is located at the World Radiation Centre (WRC) in Davos, Switzerland.
Kipp & Zonen’s higher performance pyranometers can measure the daily total of solar irradiance with an uncertainty of around 2%, but yes, they could drift in sensitivity by a maximum of 0.5% per year and after 2 years this could have a significant impact on the measurement accuracy. This is why Kipp & Zonen recommends recalibrating pyranometers every two years.
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— Solar Builder magazine