What is the ISO 9060:2018? And what is the AA-class?
The ISO 9060 standard classifies pyrheliometers and pyranometers. In its 2018 version a new class, the AA-class, of pyrheliometers has been introduced. These classes are defined, using several criteria such as long term stability, temperature dependence, spectral error, etc. For the AA-class all criteria allow a maximal deviation of 100 ppm per criteria. These criteria ask for a whole new level of precision. “Classic cavity radiometers” such as the Hickey-Frieden radiometer (HF/AHF) from Eppley or the PMO6 from PMOD/WRC do not fall into this class. See also the article on diffraction effects.
Does the PMO8 Basic system fulfil the specifications of the ISO 9060:2018 AA-class?
The currently available PMO8 Basic system does not comply with the AA-class. While the PMO8 Basic system is already setting a high standard with its specifications and performance, it is not designed to match the AA-class criteria. It is intended to replace the well established “classic cavity radiometers” like the PMO6, the HF/AHF, etc. and to serve as a reference instrument for solar radiation measurements.
Since all pyrheliometers suffer from wavelength dependent diffraction effects, the lack of AA-class does not lead to a significantly higher uncertainty when calibrating field instruments with a PMO8 Basic system, compared to a hypothetical AA-class system.
Is Davos Instruments offering an ISO 9060:2018 AA-class radiometer?
In a next step Davos Instruments is introducing the PMO8 Advanced, an “AA-ready” system, and in a second step it is planned to offer a complete AA-class system, that fully complies with all AA-class criteria. While the AA-class system will be able to automatically compensate for wavelength dependent diffraction effects, the “AA-ready” system requires the user to apply the wavelength dependent diffraction corrections based on a spectrum determined in situ. Please contact us for further information, if you are in need of an AA-class radiometer.
Does the PMO8 have a native SI scale?
Typically measurements of solar irradiance are traceable to the World Radiometric Reference (WRR), which is a SI standard for solar irradiance. As the WRR standard is an artefact based standard, there is a small difference between WRR and the SI base units. It has been estimated in several publications to be around 0.3%. An absolute radiometer is a radiometer that has its own native scale, traceable to the SI base unit, a scale that is independent from WRR. If this native scale is precise enough it can be used to establish SI scales independent from WRR. In order to establish a precise native scale with low uncertainties, several characterisation experiments are to be performed with the individual instrument. Davos Instruments is planning to offer a precise native scale upon request.