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considered in these cases. The first is the normal range of the instrument, for example a pyranometer
range may be -0.1 to 12 mV, while the second is the absolute range such as 0 - 5 V for a pressure
transducer. In setting bounds checks on the former, one can simply be observing an unusual phenomenon,
while on the latter, if the limit is exceeded, an instrument problem has occurred.
9.3.1.4 Conversion to solar tim e
While visually inspecting the data while it is being archived in local standard or UTC is useful, converting
the data, either in real time or post-processing, provides an excellent means of determining accuracy
of the system time. For systems recording data with a frequency of one-minute or greater, the symmetry
around solar noon (clear or partly clear days) can provide a means of independently checking the system
time. Corrections to the time can be made by adjusting the time stamp on the data to restore the curve’s
symmetry. Care must be taken in correcting data in this manner and a timing flag should be set to
allow future users to know a time shift has occurred. Once noted, correction of the problem should
be undertaken as rapidly as possible.
9.3.1.5 Scanning Minimum, Maximum, Standard Deviations
While difficult to accomplish in real time, post-processing scans or plots of the min, max, and standard
deviations of the signals should be done both on individual channels and on multiple common channels
to check for any short term uncertainties that have not been noticed using only the mean values. Minimum
values dipping below zero or maximums exceeding reasonable values, particularly in comparison with
other similar signals, provide a rapid way of focussing on potential problems. A simple example would
be the cleaning of the dome of an instrument during cloudy bright conditions. Although the mean value
of one minute may not be significantly altered, the minimum value and the standard deviation could
be altered profoundly. In this manner, single data points could be flagged for times of increased uncertainty.
During any period, if several peculiar events occur, the frequency and periodicity of the events should
be tested. Such periodic problem s could indicate potential electronic failures, buffering problem s in
the transfer of data or difficulties associated with the DAS.
9.3.2 Procedures for specific fluxes
9.3.2.1 Direct, diffuse and global
Testing the direct and diffuse against the global radiation is a simple and straightforward test, with
the exception of time near sunrise and sunset, and to a lesser extent during times of rapidly changing
irradiance levels (because of different instrument response times). This test should be done on all
irradiance data before submitting the data to the archive. Simply,
GLOBAL = DIFFUSE + DIRECT cos(2)
where the zenith angle (2) must be calculated according to the station location, date and tim e
(Annex I provides an algorithm)
During clear sky or stable conditions, the difference between the global and the summation should
be within the uncertainty levels given to the instruments. In the case of a cavity radiometer and two
well-calibrated pyranometers the differences should be less than 2% or 15 W m , whichever is less,
-2
at solar elevations greater than 10°. At lower solar elevations and during changing conditions, the
differences should be less then 3.5% or 20 W m , whichever is less. For larger differences further
-2
tests should be done to determine the cause of the discrepancy.
NOTE: If the direct and shaded radiometers are on the same platform care must be taken when using
this procedure. During times when the solar tracking is slightly misaligned, the errors in the direct beam
and the diffuse can be offsetting within the range of the uncertainty values given above.
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