Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 42)

30
4.0 Installation of Radiation Instruments
4.1 General
The installation of pyranometers, pyrheliom eters and pyrgeom eters is relatively sim ple (Annex B provides
inform ation on som e of the instrum ents that m ay be suitable for use at BSRN stations), but nevertheless
requires care and attention to detail.
Originally, the BSRN recom m ended that the m anufacturer and type of instrum ent used for the
m easurem ent of global radiation should also be used for the m easurem ent of diffuse radiation. This
was to reduce uncertainties associated with the tem perature and angular responsivities of the
instrum entation. In consideration of the uncertainty associated with the therm al offset associated with
‘black’ therm opile instrum ents, high quality B&W instrum ents, that have low therm al offsets, have identical,
or a sim ilar dom e spectral characteristics (particularly in the shorter wavelength range), and have good
directional responsivity m ay be substituted. A careful analysis should be perform ed to determ ine that
the m eans of m easuring both diffuse and global irradiance m inim izes the overall uncertainty of the
m easurem ent.
It was also recom m ended originally that the pyrheliom eter used be of the sam e m anufacturer as the
pyranom eters, prim arily to ensure that the spectral response of the two instrum ents be identical.
Subsequently, it was discovered that the use of instrum entation constructed by the sam e m anufacturer
did not guarantee sim ilar spectral responses because different m aterials were used for the pyranom eter
dom es and for the optical flats of the pyrheliom eters. 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 m easured in the direct beam and in the global radiation spectra. To capture
this radiation signal, which can be significant in a dry atm osphere, the suggestion has been brought
forward to use quartz, sapphire or calcium fluoride optical flats on pyrheliom eters. The m anufacture
of dom es using the latter two m aterials is uncom m on, expensive and unnecessary for the m easurem ent
of diffuse irradiance. Therefore, the selection of the types of instrum ents to be used for the m easurem ent
of the various com ponents m ust be based on a careful uncertainty analysis of the instrum entation and
the atm osphere under which the m easurem ents are to be m ade. Included in this analysis m ust be
the recognition that the three m easurem ents are to be used in the quality assurance of the data.
A num ber of docum ents, including m anuals provided by m anufacturers, have been published that include
inform ation on the installation of these instrum ents. Docum ents 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. W orld Clim ate Research
Program m e, W CRP Publication Series No. 7, W MO/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 pyranom eter the instrum ent should be carefully inspected.
(1) If not provided by the m anufacturer, the instrum ent should be calibrated so that the following
inform ation is available:
(i) the responsivity of the instrum ent to radiation
(ii) the spectral range of the instrum ent
(iii) the linearity of the instrum ent between 0 and 1500 W m -2