Dome(S); Detector; Housing; Drying Cartridge - Kipp & Zonen SMP series Instruction Manual

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5.2.1 Dome(s)

The material of the radiometer dome(s) defines the spectral measurement range of the instrument. In general 97 to 98 % of the
solar radiation spectrum will be transmitted through the domes and will be absorbed by the detector. The solar irradiance can
come from any direction within the hemisphere above the radiometer and therefore the domes are designed to minimize errors
in measurement at all incident angles (the directional response).
SMP3 pyranometers have a single 4 mm thick optical quality glass dome. The the other SMP's have one inner dome and one outer
dome. Each is 2 mm thick and of higher quality glass, with a broader spectral range and finer finishing and tolerances, than the SMP3.
The SMP22 has two high quality 4 mm quartz domes to minimize o sets and widen the spectral range.

5.2.2 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 di erential temperature between the active junction and a reference ('cold') junction kept at a fixed temper-
ature produces an electromotive force directly proportional to the di erential temperature created.
This is a thermoelectric e ect. The sensitivity of a pyranometer depends on the individual physical properties of the thermopile
and its construction. The sensitivity of each thermopile is unique and therefore each radiometer has an individual calibration
factor. This sensitivity is stored in the SMP pyranometer configuration memory.
The unique black coating on the top surface of the thermopile has a rough structure that e ectively 'traps' more than 97 % of the
incident radiation and heats up the hot junctions. The black-coated thermopile forms the detector, which has a spectral selectivity
of 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 absorptive coating is one of the most crucial and delicate parts of the pyranometer, Kipp & Zonen's provides
the best possible stability over a long period of time under all meteorological circumstances.

5.2.3 Housing

The radiometer housing accommodates all the key parts of a SMP pyranometer. The anodized aluminium parts are lightweight
and give high mechanical and thermal stability to the instrument. The stainless steel fixings are isolated where necessary to
prevent electrolytic corrosion.
Due to fine mechanical construction SMP pyranometers are virtually sealed and comply with international standard IP 67. SMP's can
be levelled with two adjustable feet using the bubble level, situated next to the dome of the instrument. For ease of maintenance
the bubble level of the SMP6 to SMP22 is visible from above without removing the snap-on white sun shield. The sun shield acts to
protect all the external parts and to reduce solar heating of the housing.

5.2.4 Drying cartridge

To keep the detector and electronics dry and to prevent condensation forming inside the domes with temperature changes a
desiccant is used to absorb humidity within the pyranometer. For the SMP's (except SMP11) this desiccant is internal and lasts for
10 year. The desiccant will be exchanged when these instruments comes back to a Kipp & Zonen service location for re-calibration.
The SMP11 has an external drying cartridge, when fresh, the desiccant has an orange colour. After some time absorbing moisture
the colour will change to clear (transparent). At this time the silica gel is not fully saturated, but should be replaced with new
orange desiccant as soon as possible. Replacement desiccant is available through your Kipp & Zonen representative.
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