126
C 3.2.2 The pyrheliometers and pyranometers
For several pyrheliometers the basic geometric data can be found in (Major 1995). From these, three
instruments have been selected that:
(1) can operate continuously at radiation stations,
(2) cover a relatively wide range of slope angles.
The Hickey-Frieden pyrheliometer (H-F) is a cavity type with a slope angle of 0.78 deg.
The Kipp and Zonen CH-1 has a flat thermopile sensor. Its slope angle is 1 deg.
The Eppley Inc. NIP is a wide angle (its slope angle is 1.78 deg) thermopile pyrheliometer. The geometry
of the diffusometers will be fitted to these pyrheliometers.
It is seen from Table C 3.2 that these pyrheliometers receive different scattered radiation from the
same 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 smallest slope angle
of the involved instruments 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. Circumsolar irradiances (W m ) on the pyrheliometric 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 pyranometers are used norm ally.
To avoid the problem of zero deflection (sometimes known as nighttime offset), B&W instruments
are being considered for the measurement of diffuse irradiance. The sensing surface is circular for
each type. Table C 3.3 gives the sensor radius and diameter of the outer glass dome for each of the
instruments considered in this study. Thanks are due to Mr. John Hickey (EPPLAB) and Mr. Leo van
Wely (Kipp and Zonen) for providing detailed descriptions of their pyranometers.
C 3.2.3 Existing diffusometers
For investigating the circumsolar effect, some BSRN station scientists provided the diameter of the
shading disk/sphere and the arm length (distance between the pyranometer and the shading device)
of their diffusometers. Each station has its individual preference, however most of the full opening angles
are 5°, exceptions being the North American devices that match 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. Pyranometer characteristics.
As a first step, the arm length values of the diffusometers were varied. Equation (5) gives a DE value
for each sky function and for each pyrheliometer-diffusometer system. The optimal arm length of a
system is that for which the mean of the DE values derived for the 10 sky functions is zero, this way
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Comentários a estes Manuais