Kipp&Zonen CMA 6 Albedometers - Manual (Page 32)

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Instruction Manual - CMP/CMA series
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Note There are no user-serviceable parts within the CMP pyranometers and CMA albedometers. They must not be opened
without the agreement and instruction of Kipp & Zonen.
7.1 Output signal not present or incorrect
The following contains a procedure for checking the instrument in case it appears that it does not function correctly:
1. Check the radiometer cable wires are properly connected to the readout equipment.
2. Check the instrument location. Are there any obstructions that cast a shadow on the instrument by blocking the direct sun
during some part of the day?
3. Check the dome, it should be clear and clean. If condensation is deposited on the inside, please change the desiccant. If too
much water is deposited internally the drying cartridge should be removed and the instrument warmed to dry it and then
replace the cartridge with new desiccant. It may take several days for the sensitivity to fully recover to the original value.
4. Check the data logger or readout offset by connecting a dummy load (100 Ohm resistor). This should give a ‘zero’ reading.
5. Check levelling. The bubble should be at least half inside the marked ring of the level.
6. If water, frost or ice is deposited on the dome, clean it. Usually, water droplets will evaporate in less than one hour under sunlight.
Any malfunction or visible damage should be reported to your Kipp & Zonen representative, who will suggest the appropriate action.
7.2 Frequently asked questions
The most frequently asked questions are listed below. For an update or further information refer to our website at
www.kippzonen.com .
Q: Negative output during night-time measurements?
A: This effect is related to Zero Offset Type A. Normally this zero offset is present when the (inner) dome has a different temperature
from the cold junctions of the sensor (the instrument housing). In practice, this is always the case when there is a clear sky. Because
of the low effective sky temperature (< 0°C) the Earth’s surface emits roughly 100 W/m² of long-wave infrared radiation upwards. The
(outer) glass dome of a pyranometer also has this emission and is cooling down several degrees below air temperature (the emissivity
of glass for the particular wavelength region is nearly 1).
Heat is emitted from the body by conduction in the domes, by wind, and from the domes through infrared radiation. The heat flow
is opposite to the heat flow when absorbing solar radiation and causes the well-known zero depression at night. This negative zero
offset is also present in day-time with a clear sky but is hidden within the solar radiation signal.
Zero Offset Type A can be checked by placing a light and infrared reflecting cap over the pyranometer. The response to solar
radiation will decay with the response time of the instrument, but the dome temperature will go to equilibrium with a time
constant of several minutes. So after about half a minute the remaining signal is mainly Zero Offset Type A.
Good ventilation of the domes and housing minimises zero offsets and increases stability. Using the Kipp & Zonen CVF4 ventilation
unit can reduce Zero Offset Type A by about 50%.
Q: Maximum and minimum irradiation quantities?
A: Due to reflection from certain types of clouds the global irradiance at sea level can rise above the extra-terrestrial direct irradiance
(the Solar Constant) of 1367 W/m² at the top of the atmosphere (WMO 1982). Values up to 1500 W/m² have been reported.
Because the clouds move, this irradiance value mostly appears as short events of a few minutes duration.
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Q: What is the primary entry point for humidity?
A: The CMP 3 is fully sealed, but this also means that it is not serviceable. The construction of the other CMP and CMA radiometers
allows servicing, such as dome replacement. However, this means that there are seals in the construction that are waterproof, but
not gas-tight. Therefore, water vapour can slowly enter due to temperature and pressure changes. The CMP10 has a special
sealing that makes the internal desiccant last for 10 years.
Q: Is the pyranometer calibration affected by the length of the signal cable?
A: With longer cable lengths the impedance increases, however it does not affect the radiometer sensitivity for the following reason.
The maximum output impedance of a radiometer with 100 m of Kipp & Zonen cable (R = 75Ω/km) is about 200Ω. If the input
impedance of the voltage measurement (readout) device is at least 1MΩ (as recommended) this represents only 0.02%. Therefore,
the current through the signal cable, and any signal reduction, is a similar percentage and is negligible.
The loading will slightly affect the passive temperature compensation circuit of the CMP10, CMP 11, CMP 21 and CMP 22, but
this is also negligible.
When the body temperature of the CMP 21 and CMP 22 is measured with the standard 10 kΩ thermistor temperature sensor, the
nominal resistance at +50 °C is 3893 Ω and 100 m of Kipp & Zonen cable adds 15 Ω (2 x 7.5 Ω). Therefore, the error is 0.4 %
compared to the nominal temperature sensor uncertainty of 0.1%. The optional Pt-100 temperature sensor used in 4-wire mode
compensates for the cable resistance and no additional errors occur.