Kipp&Zonen CMA 6 Albedometers - Manual (Page 37)

37
Instruction Manual - CMP/CMA series
Appendix A. Pyranometer physical properties
A.1 Spectral range
The spectrum of the solar radiation reaching the Earth’s surface is in the wavelength range between 280 nm and 4000 nm,
extending from ultraviolet (UV) to the far infrared (FIR). Due to the excellent physical properties of the glass dome(s) and black
absorber paint, Kipp & Zonen CMP and CMA series radiometers are equally sensitive in a wide spectral range. 97 - 98% of the
total energy will be absorbed by the thermal detector.
A.2 Sensitivity
The radiometer thermopile sensitivity is mainly determined by the physical properties of the detector itself. The thermoelectric
power, thermal conductivity of the junctions and the overall dimensions of the sensing element are related to its sensitivity. The
sensitivity is determined under standard conditions, and compared with a reference, that are stated on the calibration certificate.
A.3 Response time
Any measuring device requires a certain time to react to a change in the parameter being measured. The radiometer requires time
to respond to changes in the incident radiation. The response time is normally quoted as the time for the output to reach 95%
(sometimes 1/e, 63 %) of the final value following a step-change in irradiance. It is determined by the physical properties of the
thermopile and the radiometer construction.
A.4 Impedance
The radiometer impedance is defined as the total electrical impedance at the radiometer output connector fitted to the housing. It
arises from the electrical resistance in the thermal junctions, wires, connections, and passive electronics within the radiometer.
A.5 Non-linearity
The non-linearity of a pyranometer is defined in ISO 9060:1990 as the percentage deviation in the sensitivity over an irradiance
range from 100 to 1000 W/m² compared to the sensitivity at the calibration irradiance of 500 W/m². The non-linear effect is largely
due to convective and radiative heat losses at the black absorber surface which make the conditional thermal equilibrium of the
radiometer non-linear.
.
A.6 Tempearture dependence
The sensitivity change of a radiometer with ambient temperature change is related to the thermo-dynamics of the radiometer
construction. The is given as percentage deviation with respect to the calibrated sensitivity at +20 °C.
CMP 3, CMP 6 and CMA 6 do not have any built-in temperature compensation.
CMP10, CMP 11 and CMA 11 have a passive temperature compensation circuit with a standardised linear function.
CMP 21 and CMP 22 have individually optimised linear temperature compensation. As the temperature dependence is not linear,
the compensation cannot be exact. Therefore these instruments are supplied with the individual data for the (small) residual
temperature dependence. The internal temperature can be monitored with the integrated sensor and the irradiance data can
then be post-processed to minimise any remaining errors.
A.7 Tilt error
This is the deviation from the sensitivity at 0 ° tilt (exactly horizontal) over the range from 0° to 90° tilt (vertical) under 1000 W/m²
of normal incidence irradiance. The tilt response is proportional to the incident radiation. The error can be corrected in applications
where it is necessary to install the pyranometer on an inclined surface, but is usually insignificant.
.
A.8 Zero offset type A
By physical laws any object having a certain temperature will exchange radiation with its surroundings. The domes of upward
facing radiometers will exchange radiation primarily with the relatively cold atmosphere. In general, the atmosphere will be
cooler than the ambient temperature at the Earth’s surface. For example, a clear sky can have an effective temperature up to
50 °C cooler, whereas an overcast sky will have roughly the same temperature as the Earth’s surface.
Due to this, the pyranometer domes will ‘lose’ energy to the colder atmosphere by means of radiative transfer. This causes the
dome to become cooler than the rest of the instrument. This temperature difference between the detector ‘view’ and the instrument
housing will generate a small negative output signal which is commonly called Zero Offset Type A. This effect is reduced by using
an inner dome. This inner dome acts as a ‘radiation buffer’. Also see section 7.2.
This offset can be minimized by applying appropriate ventilation of the instrument. No ventilation unit is available for the CMP 3,
CMA 6 or CMA 11. The CVF4 ventilation unit can be used with the CMP 6, CMP10, CMP 11, CMP 21 and CMP 22.
Response [arbitrar
y units]
300 200 100 0
0
0.5
1.0
400 500 1000 2000 3000 4000
Solar radiation spectrum at sea level
Spectral response of CMP 3
Spectral response of CMP 6, CMP10, CMP 11, CMA 6 and CMA 11
Spectral response of CMP 22 Wavelength [nm]
.
A.9 Zero offset type B
Proportionally to the ambient temperature the instrument temperature varies and causes thermal currents inside the instrument.
This will cause an offset commonly called Zero Offset Type B. It is quantified in ISO 9060:1990 as the response in W/m² to a 5 K/hr
change in ambient temperature.
A.10 Operating temperature
The operating temperature range of the radiometer is determined by the physical properties of the individual parts. Within the specified
temperature range Kipp & Zonen radiometers can be operated safely. Outside this temperature range special precautions should be
taken to prevent any physical damage or performance loss of the radiometer. Please contact your Kipp & Zonen representative for
further information regarding operation in extreme temperature conditions.
A.11 Field of view
The field of view is defined as the unobstructed open viewing angle of a radiometer. ISO and WMO require that a pyranometer
for the measurement of global solar radiation has a field of view of 180 ° in all directions (i.e. a hemisphere). The inherent field
of view of the instrument should not be confused with the clear field of view of the installation location.
A.12 Directional response
Radiation incident on a flat horizontal surface originating from a point source with a defined zenith position (such as the sun)
will have an intensity value proportional to the cosine of the zenith angle of incidence. This is sometimes called the ‘cosine-law’
or ‘cosine-response’ and is illustrated below. α is the zenith angle, where 0 ° is vertical and 90 ° is horizontal.
Ideally a pyranometer has a directional response which is exactly the same as the cosine-law. However, in a pyranometer the
directional response is influenced by the detector and by the quality, dimensions and construction of the dome(s). The maximum
deviation from the ideal cosine-response of the pyranometer is given up to 80 ° angle of incidence with respect to 1000 W/m²
irradiance at normal incidence (0 ° zenith angle).
A.13 Maximum irradiance
The maximum irradiance is defined as the total irradiance level beyond which physical damage may occur to the instrument.
A.14 Non-stability
This is the percentage change in sensitivity over a period of one year. This effect is mostly due to degradation by UV radiation
of the black absorber coating on the thermopile surface.
Kipp & Zonen recommends recalibration every two years. However, for quality assurance purposes some institutes, companies or
networks may require more or less frequent recalibration. Please read the chapter on the calibration procedure for pyranometers
for more information.
.
A.15 Spectral selectivity
Spectral selectivity is the variation of the dome transmittance and absorption coefficient of the black detector coating with
wavelength and is commonly specified as % of the mean value.
A.16 Environmental
The CMP and CMA series are intended for outdoor use under all expected weather conditions. The radiometers comply with IP 67
and their solid mechanical construction is suitable to be used under all environmental conditions within the specified ranges.
For use in harsh marine environments, such as offshore; the CMP 6, CMP10, CMP 11, CMP 21 and CMP 22 are available to special
order in stainless steel construction - at extra cost, and subject to a minimum order quantity.
A.17 Uncertainty
The measurement uncertainty of a pyranometer can be described as the maximum expected hourly or daily uncertainty with
respect to the ‘absolute truth’. The confidence level is 95%, which means that 95% of the data-points lie within the given
uncertainty interval representing the absolute value. Kipp & Zonen empirically determines uncertainty figures based on many
years of field measurements for typical operating conditions.
When a pyranometer is in operation, the performance is correlated to a number of parameters, such as; temperature, level of
irradiance, angle of incidence, etc. If the operating conditions differ significantly from the calibration conditions, uncertainty in
the calculated irradiances must be expected.
For a ‘High Quality’ pyranometer the WMO expects maximum uncertainty in the hourly radiation totals of 3 %. In the daily total
an uncertainty of 2 % is expected; because some response variations cancel each other out if the integration period is long. See
the WMO ‘Guide to Meteorological Instruments and Methods of Observation’ Seventh Edition, 2008. ISO 9060:1990 does not
refer to hourly or daily uncertainties.
Many years of experience has shown that pyranometer performance can be improved concerning zero offset type A by using a
well-designed ventilation system. The Kipp & Zonen CVF4 ventilation unit is recommended for the CMP 6, CMP10, CMP 11,
CMP 21 and CMP 22 to minimise this small remaining error.