Kipp&Zonen CM 121 B/C Shadow Ring - Manual (Page 6)

1. INTRODUCTION
Instruction manual CM 121 5
1. INTRODUCTION
The combination of a CM 121 shadow ring and a pyranometer offers a simple solution for the
measurement of the diffuse solar radiation.
Specific advantages of this type of shadowing are that it is suitable for installation anywhere on
earth (contrary to designs that require the specification of a particular latitude), that it can be combi-
ned with a ventilation unit, and that it has a special U-profile ring which simplifies the mathematical
correction for intercepted diffuse radiation.
The shadow ring will keep the pyranometer in the shade during the entire day, preventing the direct
solar radiation to reach the sensor. As a result only the diffuse solar radiation is measured. A
regular schedule of maintenance would require manual adjustment of the sliding bars (that are
connected to the actual shadow ring) every two days. This adjustment is necessary because the
elevation of the solar course changes slightly from day to day. For proper setting of the sliding bars,
the table of “correction factors and the sliding bar setting” can be consulted.
The CM 121 can be installed everywhere on earth. Normally the pyranometer will be in the
horizontal position, alternatively it can be mounted in tilted positions along the north-south axis.
The ring itself is equipped with a special U-profile so that it has a nearly constant view angle during
the year. A stable stand is guaranteed even at high winds.
Naturally, the shadow ring intercepts also a proportion of the diffuse sky radiation.
A correction factor for this effect is recommended to be used as a refinement of the measurement.
A table of correction factors is combined with the table for sliding bar setting.
The CM 121 can be used with Kipp & Zonen pyranometers. Typical use is with CM 11, CM 6B or
CM 21. Pyranometer CM 3, although not recommended, can be fitted using the additional levelling
fixture type CLF 1 for mounting on CM 121.
Use with SP-Lite is not recommended because of spectral and cosine errors.
There are two types CM 121:
The CM 121B is for use without a ventilation unit.
The CM 121C is for using in combination with the ventilation unit CV 2. For mounting instructions
see chapter 2.2. With the exception of chapter 2 all other instructions apply to both CM 121B and
CM 121C.