Uncertainty; B.16 Uncertainty - Kipp & Zonen SMP series Instruction Manual

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B.16 Uncertainty

The measurement uncertainty of a pyranometer can be described as the maximum expected hourly or daily uncertainty with
respect to the 'absolute truth'. The confidence level is 95 %, which means that 95 % of the data-points lie within the given
uncertainty interval representing the absolute value. Kipp & Zonen empirically determine uncertainty figures based on many
years of field measurements for typical operating conditions.
When a pyranometer is in operation, the performance of it is correlated to a number of parameters, such as temperature, level of
irradiance, angle of incidence, etc. If the conditions di er significantly from calibration conditions, uncertainty in the calculated
irradiances must be expected.
For a 'High Quality' pyranometer the WMO expects maximum uncertainty in the hourly radiation totals of 3 %. In the daily total
an uncertainty of 2 % is expected, because some response variations cancel each other out if the integration period is long. See
the WMO 'Guide to Meteorological Instruments and Methods of Observation' Seventh Edition, 2008. ISO 9060:1990 does not
refer to hourly or daily uncertainties.
Many years of experience has shown that pyranometer performance can be improved concerning zero o set type A by using
a well-designed ventilation system. The Kipp & Zonen CVF4 ventilation unit is recommended for the SMP's to minimise this
small error.
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