Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 16)

CMP/CMA series manual
Page 15
3.2. Detector
The thermopile sensing element is made up of a large number of thermocouple junction pairs
connected electrically in series. The absorption of thermal radiation by one of the thermocouple
junctions, called the active (or ‘hot’) junction, increases its temperature. The differential temperature
between the active junction and a reference (‘cold’) junction kept at a fixed temperature produces an
electromotive force directly proportional to the differential temperature created. This is a thermoelectric
effect. The sensitivity of a pyranometer depends on the individual physical properties of the thermopile
and construction. The sensitivity of each thermopile is unique and therefore each radiometer has
unique calibration factor, even with the same radiometer model.
On the top surface of the sensor a black paint is deposited which has a very rough structure containing
many micro-cavities that effectively ‘’trap’’ more than 97% of the incident radiation in a broad spectral
range. Furthermore, the spectral selectivity is less than 2%. This means that within the spectral range
of the pyranometer, the absorption for each wavelength is equal to within 2%. The black painted
sensing element forms the detector. Considering the long-term stability of the instrument, the black
paint is one of the most crucial and delicate parts of the pyranometer. Kipp & Zonen black paint gives
the best possible stability over a long period of time under all meteorological circumstances.
3.3. Housing
The radiometer housing accommodates all fundamental pyranometer parts. The anodized Aluminium
parts are light weight and give a high mechanical and thermal stability to the instrument. Due to its fine
mechanical construction all pyranometers are virtually sealed and comply to the international standard
IP 67. Each pyranometer model can be leveled by using the bubble level and two leveling feet. For
ease of maintenance the bubble level is situated next to the dome of the instrument and due to the
special shape of the sun shield it is visible from above. The sun shield acts to protects all the external
parts from radiation and to reduce solar heating of the housing.
3.4. Drying cartridge
In case moisture enters the radiometer body the silica-gel desiccant regulates the humidity level inside
the pyranometer. Initially the desiccant will have an orange color. After some time it becomes saturated
with moisture and the colour will change to become clear (transparent). At this time the contents of the
drying cartridge should be replaced with fresh, unsaturated orange colored desiccant as soon as
possible. Replacement desiccant is available through Kipp & Zonen distributors.
3.5. Cable and connector
For ease of installation and replacement during recalibration of the radiometer, the CMP/CMA series
are provided with a weather proof signal cable connector.
Kipp & Zonen radiometers use a custom-made cable that is selected as a low noise type particularly
suited to handle the low voltage output of the thermopile or of a temperature sensor.
The shield of the cable is connected to the metal body of the connector and preferably should be
connected to ground at the readout equipment. Cables come pre-wired to the connector plug in a
range of lengths.