Accuracy - Kipp & Zonen CM 11 Instruction Manual

TECHNICAL DATA
2.2

ACCURACY

Unfortunately the sensitivity is cross-correlated to a number of
parameters as temperature, level of irradiance, vector of incidence,
etc. The upper limiting values of the resulting sensitivity variations are
listed in the specifications. It classifies the pyranometer CM 11 as a
'secondary standard' according to the classification of the World
Meteorological Organization.
Normally, the supplied sensitivity figure is used to calculate the
irradiances. If the conditions differ from calibration conditions, errors in
the calculated
irradiances must be expected.
For a secondary standard instrument the WMO expects maximum
errors in the hourly radiation totals of 3%. In the daily total an error of
3% is expected, because some response variations cancel out each
other if the integration period is long.
These errors can be reduced further if the actual sensitivity of the
pyranometer is used by the conversion of voltage to irradiance. The
actual sensitivity can be calculated when it is a well-known function of
simply measured parameters (sometimes called transfer function or
sensitivity function). This is especially convenient in connection with a
programmable data acquisition system.
For the CM 11 the effect of each parameter on the sensitivity can be
shown separately, because the parameters show less interaction.
The non-linearity error, the sensitivity variation with irradiance, is
equal for any given CM 11. See figure 7.
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