Kippzonen BSRN Scientific Solar Monitoring System Instrukcja Użytkownika Pobierz pdf (Strona 146)

134

Since the irradiance on the surface by a pyrheliometer adjusted for the horizontal surface F is

the following relation must be kept,

where the subscript d stands for the variable for the diffusometer.

This last equation shows two important aspects concerning the geometry of the diffusometer.

dd d d

(1) In principle it is important to keep R/L=R /L and r/R=r /R ,

(2) Since it is not practical to change geometrical dimensions of the involved parts, it is a practically

better solution to adjust geometry for representative solar zenith angle. The earth's equivalent

solar zenith angle of 53º, can be regarded as a working angle. Therefore, R /L which is slightly

larger than R/L, or r /R which is slightly larger than r/R suits the best for the present requirement.

Since the latter possibility is difficult to fulfil du to a relatively large difference in radii between

the sensor and the outer glass dome, the best solution can be obtained by adjusting R /L ,

by making L smaller than the designated length which is calculated based a perfect symmetry

requirem ent.

C 4.6 Conclusion

Most diffusometers adopted by the BSRN stations are capable of keeping the errors induced by the

shadow effect within 2 to 3 W m . The best recommended diffusometer is to use Eppley B/W pyranometer

-2

8-48, with the shading disc radius of 28.0 mm and the arm's length of 641 mm.

C 4.7 References

Major, G., 1994: Circumsolar correction for pyrheliometers and diffusometers. WMO/TD-No.635, 42

pp.

Major, G., Nagy, Z., and Putsay, M., 2000: The effect of diffusometer shading geometry. Report for

the BSRN Meeting, May 1-5, 2000, Melbourne, 8 pp.

1 2 ... 141 142 143 144 145 146 147 148 149 150 151 ... 187 188

Brak uwag

Kippzonen BSRN Scientific Solar Monitoring System Instrukcja Użytkownika Strona 146