Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 23)

M ajor, G ., 1992: Estim ation of the error caused by the circ um solar radiation when m easuring global
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radiation as a sum of direct and diffuse radiation. Solar Energy, 48. (S ee T able 4.1 for comm on
combinations of pyrheliometers and pyranometers.)
Bush, B.C ., F.P.J. Valero and A.S. Sim posn, 2000: C haracterization of therm al effects in pyranometers: A
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data correction algorithm for improved m easurem ent of surface insolation. Jour. Atm os. O cean. Tech., 17,
165 - 175.
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A solar tracker with an accuracy of ±0.10° or better, is needed to accom m odate the
pyrheliom eter, the ACR and, during calibrations, a second ACR. It is recom m ended that the
tracker pointing be m onitored using a four-quadrant sensor because pointing accuracy is
im portant in determ ining the quality of the direct beam m easurem ent. The sam pling rate should
be the sam e as that of the instrum ents attached to the tracker. A variety of high-quality trackers
are now available that use four-quadrant sensors for actively positioning the tracker on the solar
beam during periods of high irradiance and a solar position algorithm during low intensity
conditions.
The param eters to be m onitored are: output of the pyrheliom eter therm opile; outputs of the ACR
(U, I or therm opile signals for a passive instrum ent); body tem peratures of the pyrheliom eter and
ACR; output of the four-quadrant sensor. O nly the values associated with the calculated
irradiance (m ean, m axim um , m inim um and standard deviation) are required by the BSR N archive.
All other raw data should be archived at the centre responsible for the m easurem ents.
2.2.1.2 Diffuse Radiation
The original instrum ent configuration associated with the BSRN target uncertainty of 4% (5 W m ) -2
was a ventilated pyranom eter with a sensor to m onitor the instrum ent therm opile tem perature (to
be used to correct for tem perature-related changes in therm opile responsivity and therm al
offset). Shading of the instrum ent from the direct sun was to be by a shading disk. The shade
geom etry of the com bination of the sensor and the disk was to replicate the geom etry of the direct
beam sensor when pointing toward the zenith (5° full-angle from the centre of the detector) . The
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instrum ent’s sensor and dom e m ust be com pletely shaded. Incorrect geom etry alone can lead to
errors of up to 5 W m depending on instrum entation and atm ospheric conditions.
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A relationship has been shown between pyranom eter therm al offsets and diffuse irradiance that
can significantly affect the quality of the m easurem ent . A possible solution to overcom e the offset
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problem is to use a ‘black and white’ type pyranom eter (B&W ) for the m easurem ent of diffuse
radiation. This type of instrum ent does not exhibit the therm al offset of ‘black’ therm opile
instrum ents because both hot and cold junctions are exposed to the sam e therm al regim e. B&W
instrum ents, currently, do not have the sam e optical qualities (spectral and directional) as the
black therm opile instrum ents recom m ended for use at BSRN stations and cannot be used for the
m easurem ent of global radiation. Using a B&W instrum ent for the m easurem ent of diffuse
irradiance would therefore m ean a second type of instrum ent would have to be used for the global
irradiance m easurem ent, which m ay increase the overall uncertainty of the three-com ponent
m easurem ents.
Research is presently ongoing to determ ine whether a correction factor can be applied to the
‘black’ pyranom eter therm al offset. In Section 9.2.2 experim ental m ethods of correcting this offset
are presented. Further research continues into the design of a ‘black’ pyranom eter that does not
exhibit therm al offset. Several instrum ents that use other technologies (e.g., PRT) are available
that do not exhibit offset problem s associated with radiative cooling.
The BSRN has yet to recom m end a standard m ethod of correcting for therm al offset or selecting
a particular type of instrum ent for m easuring diffuse irradiance. A careful uncertainty analysis of
any diffuse system will determ ine the quality of the m easurem ent if each com ponent is properly
addressed.
The param eters to be acquired are: output of pyranom eter therm opile; pyranom eter body
tem perature. Only the values associated with the calculated irradiance (m ean, m axim um ,
m inim um and standard deviation) are required by BSRN archive. All other raw data should be
archived at the centre responsible for the m easurem ents.