Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 146)

134
p h
Since the irradiance on the surface by a pyrheliom eter adjusted for the horizontal surface F is
the following relation m ust be kept,
where the subscript d stands for the variable for the diffusom eter.
This last equation shows two im portant aspects concerning the geom etry of the diffusom eter.
d d d d
(1) In principle it is im portant to keep R/L=R /L and r/R=r /R ,
(2) Since it is not practical to change geom etrical dim ensions of the involved parts, it is a practically
better solution to adjust geom etry for representative solar zenith angle. The earth's equivalent
d d
solar zenith angle of 53º, can be regarded as a working angle. Therefore, R /L which is slightly
d d
larger than R/L, or r /R which is slightly larger than r/R suits the best for the present requirem ent.
Since the latter possibility is difficult to fulfil du to a relatively large difference in radii between
d d
the sensor and the outer glass dom e, the best solution can be obtained by adjusting R /L ,
d
by m aking L sm aller than the designated length which is calculated based a perfect sym m etry
requirem ent.
C 4.6 Conclusion
Most diffusom eters adopted by the BSRN stations are capable of keeping the errors induced by the
shadow effect within 2 to 3 W m . The best recom m ended diffusom eter is to use Eppley B/W pyranom eter
-2
8-48, with the shading disc radius of 28.0 m m and the arm 's length of 641 m m .
C 4.7 References
Major, G., 1994: Circum solar correction for pyrheliom eters and diffusom eters. W MO/TD-No.635, 42
pp.
Major, G., Nagy, Z., and Putsay, M., 2000: The effect of diffusom eter shading geom etry. Report for
the BSRN Meeting, May 1-5, 2000, Melbourne, 8 pp.