Kipp&Zonen SMP3 Pyranometers - Manual (Page 31)

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SMP series pyranometers only require suitable sources of power and radiation (light) to operate and make measurements.
However, it is necessary to connect them to some sort of readout or data storage device in order to save the measurements, there
is no internal data memory.
5.1 Data collection
An optimal setting for the data interval is to sample every second and store one minute averages. For setting up the combination
of pyranometer and data storage please refer to the manual of the data collection device.
Take care when using the analogue output to match the output range of the pyranometer closely to the input range of the data
collection device to maximise the available resolution and minimise noise.
This can be done by determining the maximum expected analogue output of the pyranometer in your application and taking the
minimum input range of your data collection device that can just handle that signal.
5.2 Key parts of SMP series pyranometers
The detectors of the SMP3, SMP10 and SMP11 are based on passive thermal sensing element called a thermopile. Although the
detector construction differs between models, the fundamental working principle is applicable to all SMP radiometers.
The thermopile responds to the total energy absorbed by a unique black surface coating developed by Kipp & Zonen, which is
non-spectrally selective. The thermopile warms up and the heat generated flows through a thermal resistance to a heat-sink, the
pyranometer housing. The temperature difference across the thermal resistance of the detector is converted into a small voltage
as a linear function of the absorbed irradiance.
The rise of temperature in the thermopile is easily affected by wind, rain and thermal radiation losses to the environment (for
example, a ‘cold’ sky) and the delicate black coating must be protected. Therefore the detector of the SMP10 and SMP11 are
shielded by two domes (the entry-level SMP3 has only one dome to reduce size and cost). These domes allow equal transmittance
of the direct solar component for every position of the sun in the hemisphere above the detector.
A drying cartridge in the SMP11 radiometer housing is filled with replaceable silica gel and prevents condensation on the inner
sides of the domes, which can cool down considerably on clear windless nights. The SMP3 has a sealed construction with a
non-replaceable internal drying cartridge. The SMP10 has an internal desiccant that lasts for 10 years.
5. Operation and measurement
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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 SMP10 and SMP11 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.
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 differential temperature between the active junction and a reference (‘cold’) junction kept at a fixed temper-
ature 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 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 effectively ‘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. SMP3,
SMP10 and SMP11 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 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
The entry-level SMP3 has a completely sealed construction with a non-replaceable drying cartridge. However, this also makes it
non-serviceable. For serviceability of the higher performance and price SMP10 and SMP11 the construction cannot be sealed. In
this case water vapour can ‘breathe’ into the housing, mainly due to temperature and pressure changes.
To keep the detector and electronics dry and to prevent condensation forming inside the domes with temperature changes a
self-indicating silica gel desiccant is used to absorb humidity within the pyranometer. 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 fresh orange desiccant as soon as possible. Replacement desiccant is available through
Kipp & Zonen representatives. The CMP10 has an internal desiccant that lasts for 10 years.
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5.2.5 Cable and connector
For ease of installation and replacement during re-calibration of the radiometer, the SMP series are provided with a waterproof
cable socket fitted to the pyranometer housing. The matching waterproof plug is normally supplied pre-wired to a very high
quality yellow cable selected for low noise, very wide temperature range and UV resistance.
Cables come pre-wired to the connector plug in a range of lengths, 10 m is standard. 25 m, 50 m and 100 m lengths are also
available. The connector plug can also be supplied loose for the user to fit to their own cable.
smart interface
drying cartridge
inner glass dome
outer glass dome
housing
detector
sun shield
bubble level
connector
fixed foot adjustable feet
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