Kipp&Zonen CMA 6 Albedometers - Manual (Page 28)

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Instruction Manual - CMP/CMA series
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CMP pyranometers and CMA albedometers are simple to maintain and do not require any special tools or training. There are no
service items requiring scheduled replacement.
5.1 Daily maintenance
On clear windless nights the outer dome temperature of horizontally placed radiometers will decrease, even to the dew point
temperature of the air, due to infrared radiation exchange with the cold sky. The effective sky temperature can be 30 °C lower
than the ground temperature.
Depending upon the weather conditions; dew, glazed frost or hoar frost can be precipitated on the top of the dome and can stay
there for several hours in the morning. An ice cap on the dome is a strong diffuser and decreases the pyranometer signal drastically,
up to 50% in the first hours after sunrise. Snow may completely cover the dome.
The frequency of cleaning is highly dependent upon the local weather and environmental conditions, such as dust, airborne
pollutants, or salt spray in marine locations. Ideally, the dome of the pyranometer should be cleaned every morning before
sunrise. The frequency of cleaning can be reduced by the use of a ventilation unit (not available for the CMP 3, CMA 6 and
CMA 11), with the heaters switched on when necessary.
Note Clean the dome using pure alcohol or distilled water and a lint-free cloth. Ensure that no smears or deposits are
left on the dome.
5.2 Monthly maintenance
Check the desiccant in the drying cartridge. This is a non-toxic self-indicating silica-gel. When it requires replacement the
colour changes from orange to clear (transparent).
To replace the desiccant unscrew the cartridge from the radiometer housing, if it is tight a 16 mm or 5/8" open-ended
wrench/spanner can be used to loosen it. Remove the cap from the end of the cartridge and safely dispose of the used silica-gel.
Refill with fresh desiccant, and refit the end cap to the cartridge. Make sure that the o-ring seal and its seat in the housing are
clean and grease the seal with Vaseline if it is dry.
Note Screw in the drying cartridge hand-tight only, to avoid distorting the o-ring seal.
Desiccant refill packs are available from Kipp & Zonen. One pack is sufficient for one complete refill.
Check that the pyranometer is level and adjust if necessary.
Check that the sun shield is firmly clipped on.
5.3 Yearly maintenance
Check all the electrical connections. Unscrew the plugs, clean if necessary and then reconnect.
Check cables for damage caused by accident or by rodents.
Check the instrument mountings and any base supports are secure.
5.4 Calibration
An ideal radiometer gives an output that is proportional to the absolute irradiance level. This relationship can be expressed as a
constant ratio called ‘sensitivity’. CMP and CMA series radiometers are very stable instruments, but they do change very slightly
with time. This is largely due to exposure of the black detector coating to UV solar radiation. Re-calibration is recommended every
two years. Normally this is carried out at the Kipp & Zonen factory or at an authorised calibration facility.
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5.4.1 Calibration principle
At the Kipp & Zonen factory pyranometers are calibrated, or re-calibrated, in our laboratory according to ISO 9847:1992 ‘Solar
energy - Calibration of field pyranometers by comparison to a reference pyranometer’, Annex A ‘Calibration devices using
artificial sources’. The specific method is given in Annex A.3.1 and is described in the standard as the ‘Kipp & Zonen (calibration)
device and procedure’.
This is based on a side-by-side comparison of the test pyranometer with a reference pyranometer of the same type under a stable
artificial sun. Kipp & Zonen uses a Metal-Halide high-pressure gas discharge lamp with precise voltage stabilisation. The irradiance
at the radiometers is approximately 500 W/m².
The reference pyranometers are regularly calibrated outdoors at the World Radiation Centre (WRC) in Davos, Switzerland.
The spectral content of the laboratory calibration lamp differs from the outdoor solar spectrum at the World Radiation Centre.
However, this has no consequences for the transfer of calibration, because the reference and test radiometers have the same
characteristics.
To minimise stray light from the walls and the operator, the light is restricted to a small cone around the two radiometers. The
test radiometer and the reference radiometer are placed side by side on a small rotating table. The lamp is centred on the axis
of this table. The table is used to interchange the positions of the pyranometers to allow for inhomogeneity of the light field.
The pyranometers are illuminated and after time for the output to stabilise the readings of both radiometers are integrated over
a measurement period. The lamp housing and beam restrictors heat up and emit long-wave infrared radiation which warms up
the pyranometer dome(s) slightly. This causes a small offset that is embodied in the pyranometer response under illumination.
To determine this offset both radiometers are shaded, and after time to stabilise, the signals of both radiometers are integrated
over a period.
The radiometer positions are interchanged by rotating the table and the whole procedure is repeated.
The sensitivity of the test pyranometer is calculated by comparison to the reference pyranometer readings and the calibration
certificate is produced. At Kipp & Zonen the complete process is automated under computer control.
Kipp & Zonen produces the CFR Calibration Facility for Radiometers for customers to make their own pyranometer calibrations
to ISO 9847, Annex A.3.1. CMA albedometers are calibrated twice, once for the upper pyranometer and once for the lower. An
adapter is available to mount a CMA albedometer on the CFR turntable.
5.4.2 Calibration traceability to the WRR
Our reference pyranometers are calibrated at the World Radiation Centre (WRC) in Davos, Switzerland by comparison to the World
Radiometric Reference (WRR). They are also fully characterized for linearity, temperature dependence and directional response to
enable transfer of the sensitivity under the measurement conditions in Davos to our calibration laboratory conditions.
Kipp & Zonen keeps at least two reference instruments for each pyranometer model. These reference instruments are sent
alternate years to the WRC for calibration, so that production and calibration in Delft can continue without interruption.
Kipp & Zonen calibration certificates include an overview of the calibration method, details of the reference pyranometer used,
traceability to the WRR, and the uncertainty in the calibration chain from the WRR to the pyranometer being calibrated.