Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 138)

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C 3.2.2 The pyrheliom eters and pyranom eters
For several pyrheliom eters the basic geom etric data can be found in (Major 1995). From these, three
instrum ents have been selected that:
(1) can operate continuously at radiation stations,
(2) cover a relatively wide range of slope angles.
The Hickey-Frieden pyrheliom eter (H-F) is a cavity type with a slope angle of 0.78 deg.
The Kipp and Zonen CH-1 has a flat therm opile sensor. Its slope angle is 1 deg.
The Eppley Inc. NIP is a wide angle (its slope angle is 1.78 deg) therm opile pyrheliom eter. The geom etry
of the diffusom eters will be fitted to these pyrheliom eters.
It is seen from Table C 3.2 that these pyrheliom eters receive different scattered radiation from the
sam e sky. It is also seen that the sky function has a wide range of variation. The part of the sky nearer
to the sun than 0.8 degrees has not been included into the calculations since the sm allest slope angle
of the involved instrum ents is 0.78 deg.
Sky function H-F CH-1 NIP
Mountain 60 1.83 1.92 2.54
45 2.11 2.22 2.93
20 3.18 3.34 4.42
Continental 60 2.67 2.82 3.54
45 2.95 3.12 3.92
20 3.59 3.78 4.75
Desert 60 30.2 31.9 40.2
45 27.2 28.7 36.2
20 9.78 10.3 13.0
Measured 45 8.62 9.13 11.3
Table C 3.2. Circum solar irradiances (W m ) on the pyrheliom etric sensors -2
from the part of the sky, z>0.8 deg.
At the BSRN stations Kipp & Zonen CM-11 / CM-21 or Eppley PSP pyranom eters are used norm ally.
To avoid the problem of zero deflection (som etim es known as nighttim e offset), B&W instrum ents
are being considered for the m easurem ent of diffuse irradiance. The sensing surface is circular for
each type. Table C 3.3 gives the sensor radius and diam eter of the outer glass dom e for each of the
instrum ents considered in this study. Thanks are due to Mr. John Hickey (EPPLAB) and Mr. Leo van
W ely (Kipp and Zonen) for providing detailed descriptions of their pyranom eters.
C 3.2.3 Existing diffusom eters
For investigating the circum solar effect, som e BSRN station scientists provided the diam eter of the
shading disk/sphere and the arm length (distance between the pyranom eter and the shading device)
of their diffusom eters. Each station has its individual preference, however m ost of the full opening angles
are 5°, exceptions being the North Am erican devices that m atch the Eppley NIP; their opening angle
is 5.8°.
Instrument Type Sensor’s radius,
mm
Diameter of glass
dome, mm
EPPLEY PSP Black 5.64 50
KIPP CM-11 and CM21 Black 10 50
EPPLEY 8-48 B&W 16 50
SCHENK Star B&W 16 65
Table C 3.3. Pyranom eter characteristics.
As a first step, the arm length values of the diffusom eters were varied. Equation (5) gives a DE value
for each sky function and for each pyrheliom eter-diffusom eter system . The optim al arm length of a
system is that for which the m ean of the DE values derived for the 10 sky functions is zero, this way