Kipp&Zonen CMA 6 Albedometers - Manual (Page 25)

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Instruction Manual - CMP/CMA series
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CMP series pyranometers and CMA albedometers only require a suitable source of radiation (light) to operate and make
measurements, no power is required. 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.
4.1 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 radiometer and data storage please refer to the manual of the data collection device.
Take care to match the output range of the pyranometer to the input range of the data collection device to maximise the availa-
ble 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. Also refer to section 2.9.4.
4.2 Key parts of CMP and CMA series radiometers
The detectors of the radiometers are based on a passive thermal sensing element called a thermopile. Although the detector
construction differs between models, the fundamental working principle is applicable to all the radiometers.
The thermopile responds to the total energy absorbed by a unique black surface coating developed by Kipp & Zonen, which is
spectrally non-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 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 is shielded by two domes (except
for the entry-level CMP 3, which has only one dome to reduce size and cost). These domes allow equal transmittance of the direct
solar radiation component for every position of the sun in the hemisphere above the detector.
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A drying cartridge in the 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 CMP 3 has a sealed construction with a
non-replaceable internal drying cartridge. The CMP10 has an internal desiccant that lasts for 10 years.
4.2.1 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).
CMP 3 pyranometers have a single 4 mm thick optical quality glass dome. CMP 6, CMP10, CMP 11 and CMP 21 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 CMP 3. CMP 22 has two 4 mm thick domes of very high quality optical quartz.
4.2.2 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 radiometer 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 given in the calibration certificate and is on the serial number label attached to the instrument.
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.
4.2.3 Housing
The instrument housing accommodates all the key parts of a radiometer. The anodized aluminium parts are light weight 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, Kipp & Zonen pyranometers and albedometers are virtually sealed and comply with
international standard IP 67. Pyranometers have one fixed foot and two adjustable feet and can be levelled using the integral
bubble level. For all except the CMP 3, the bubble level is visible from above without removing the snap-on white sun shield
(CMP 3 has the bubble level in the base flange). The sun shield acts to protect all the external parts and to reduce solar heating
of the housing.
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4.2.4 Drying Cartridge
The entry-level CMP 3 has a sealed construction with a non-replaceable internal drying cartridge. However, this also makes it
non-serviceable. For serviceability of the higher performance radiometers the construction cannot be completely sealed. In this
case, over time, water vapour can ‘breathe’ into the housing, mainly due to temperature and pressure changes.
To keep the detector and electrical components 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 radiometer. 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. Two packs of replacement desiccant are
supplied with the radiometer. Further packs are available through Kipp & Zonen representatives. The CMP10 has a special
sealing and internal desiccant that needs replacement after 10 years. This is done with every factory re-calibration.
4.2.5 Cable and Connector
For ease of installation of the radiometer, and replacement during re-calibration, the CMP and CMA series are provided with a
waterproof connector 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 are supplied pre-wired to the connector plug in a range of lengths, 10m is standard. 25m, 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.