Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 10)

CMP/CMA series manual
Page 9
2.2.4. Installation for measurement of diffuse radiation
For measuring sky radiation, the direct solar radiation is
intercepted by a small disk or sphere. The shadow of the disk
must cover the pyranometer domes completely. However, to
follow the sun's apparent motion, a power-driven tracking device
is necessary as shown in Figure 3.
This can be done using a Kipp & Zonen sun tracker, such as the
model 2 AP, designed to track the sun accurately under all
weather conditions. More information about the combination of
pyranometer and tracker is given in the sun tracker manual.
Alternatively, a static shadow ring can be used to intercept the
direct solar radiation; but it is less accurate and may require
periodic manual adjustment. At times the shadow ring also
intercepts a proportion of the diffuse sky radiation. Therefore,
corrections for this to the recorded data are necessary.
Kipp & Zonen produces a universal shadow ring, model CM 121, which is suitable for use at all
latitudes. In the CM 121 manual, installation instructions and correction factors are given.
2.2.5. Installation for measurement of albedo
An albedometer measures both the global solar radiation and the
reflected radiation form the surface below. It can be configured
from two CMP series pyranometers and a suitable mounting
plate, or by using a CMA series integrated albedometer.
The requirements for installation of the upper and lower
pyranometers are the same as in paragraphs 2.2.1 and 2.2.3 for
global and reflected radiation. A typical arrangement is shown in
Figure 4. According to the WMO the height should be 1 m to 2 m
above a uniform surface covered by short grass.
Installation of the CMA series differs slightly because there are
no levelling screws. The integrated mounting rod is fixed to the
mast. CMA has an integrated lower glare screen to prevent
direct illumination of the domes by the sun at sunrise and sunset.
The mast shown intercepts a fraction D/2
πS of the radiation that
Figure 4 Albedo configuration
is coming from the ground. In the most unfavourable situation
(sun at zenith) the pyranometer shadow decreases the signal by
a factor R
2
/H
2
.
2.2.6. Underwater use
The CMP/CMA radiometers are in principle watertight according to the IP 67 standard. However, the
hemispherical air cavity under the dome(s) acts as a negative lens. The parallel beam of direct solar
radiation becomes divergent after the passage of the outer dome. Consequently the intensity at the
sensor is lower than outside the dome(s). The calibrated sensitivity figure is not valid in this case and
must be derived empirically.
Figure 3 2AP Sun Tracker with
shaded pyranometer