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4.0 Installation of Radiation Instruments
4.1 General
The installation of pyranometers, pyrheliometers and pyrgeometers is relatively simple (Annex B provides
information on some of the instruments that may be suitable for use at BSRN stations), but nevertheless
requires care and attention to detail.
Originally, the BSRN recommended that the manufacturer and type of instrument used for the
measurem ent of global radiation should also be used for the measurem ent of diffuse radiation. This
was to reduce uncertainties associated with the temperature and angular responsivities of the
instrumentation. In consideration of the uncertainty associated with the thermal offset associated with
‘black’ thermopile instruments, high quality B&W instruments, that have low thermal offsets, have identical,
or a similar dome spectral characteristics (particularly in the shorter wavelength range), and have good
directional responsivity may be substituted. A careful analysis should be performed to determine that
the means of measuring both diffuse and global irradiance minimizes the overall uncertainty of the
measurement.
It was also recommended originally that the pyrheliometer used be of the same manufacturer as the
pyranometers, primarily to ensure that the spectral response of the two instruments be identical.
Subsequently, it was discovered that the use of instrumentation constructed by the same manufacturer
did not guarantee similar spectral responses because different materials were used for the pyranometer
domes and for the optical flats of the pyrheliometers. Rayleigh scattering alters the spectral distribution
of energy at the surface so that the proportion of near infrared (NIR) radiation in the diffuse irradiance
signal is less than that measured in the direct beam and in the global radiation spectra. To capture
this radiation signal, which can be significant in a dry atmosphere, the suggestion has been brought
forward to use quartz, sapphire or calcium fluoride optical flats on pyrheliometers. The manufacture
of domes using the latter two materials is uncommon, expensive and unnecessary for the measurement
of diffuse irradiance. Therefore, the selection of the types of instruments to be used for the measurement
of the various components must be based on a careful uncertainty analysis of the instrumentation and
the atmosphere under which the measurements are to be made. Included in this analysis must be
the recognition that the three measurements are to be used in the quality assurance of the data.
A number of documents, including manuals provided by manufacturers, have been published that include
information on the installation of these instruments. Documents published by technical agencies include:
Radiation Measurement. International Field Year for the Great Lakes, Technical Manual Series No.
2, National Research Council of Canada, 1972.
Revised Instruction Manual on Radiation Instruments and Measurements. World Climate Research
Programme, WCRP Publication Series No. 7, WMO/TD No. 149, 1986.
Meteorological measurements concerning questions of air pollution, Global radiation, direct solar radiation
and net total radiation. VDI-Richtlinien, VDI 3786, Part 5, 1986.
Solar Energy - Field Pyranometers - Recommended practice for use. International Standards Organization
Technical Report TR9901, 1990.
4.2 Installation of pyranometers and pyrgeometers
4.2.1 Pre-installation Checks and Service
Before installing any pyranometer the instrument should be carefully inspected.
(1) If not provided by the manufacturer, the instrument should be calibrated so that the following
information is available:
(i) the responsivity of the instrument to radiation
(ii) the spectral range of the instrument
(iii) the linearity of the instrument between 0 and 1500 Wm
-2
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