Accuracy and errors
Overall accuracy limits are given in the specifications, which give the
expected performance in normal use. There are some specific
circumstances which may show unexpected results.
Spectral response
Because the SPN1 spectral response runs from 400nm upwards, it misses
out some of the blue end of the solar spectrum, and this may show an
under-reading of the diffuse component under very clear blue skies, or at
high altitudes.
Cosine response
In relative terms, the cosine response error increases when the sun is
close to the horizon. This may appear as an overall sensitivity error in
clear-sky conditions when the sun is very low. This is true for all cosine-
corrected sensors.
Errors due to inaccurate levelling also show up in these conditions – while
a 0.5° levelling error has very little effect when the sun is high in the sky,
or under overcast conditions, it can give a 5% output error when the sun is
bright and 10° above the horizon.
Offsets
Most thermopile pyranometers show a negative output during the night,
due to radiative cooling of the earth into space. The construction of the
SPN1 includes three separating elements between the atmosphere and
the thermopiles, so this effect is minimal. The electronics within the SPN1
will only measure and output positive signals, so the output should never
go below zero. In general, there will be a small positive output (<3W.m
-2
)
in dark conditions, due to the effects of noise in the system.
The SPN1 is sensitive to fast changes in temperature, and these will
create a positive error on cooling, or a negative error on warming. This
may be visible if you move the sensor from a warm room into a cold
atmosphere outside, until the sensor reaches ambient temperature.
Thermopile matching
The SPN1 outputs are based on 7 individual sensor readings. The
sensors are closely matched at calibration, but will never be exactly
identical in all respects. These small variations will sometimes show up as
small steps in the output time series, as the shadowmask shades or
exposes different sensors.
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z
Accuracy and errors
SPN1 User Manual v1.0
