B. Pyranometer physical properties
B.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 and black
absorber paint, Kipp & Zonen SMP series radiometers are equally sensitive in a wide spectral range. 97 - 98 % of the total
energy will be absorbed by the thermal detector.
B.2 Sensitivity
For the SMP series pyranometers the physical sensitivities are converted to a digital output that is identical for all sensors. The
SMP-V versions all have an analogue output of 0 to 1 Volt for -200 to 2000 W/m². The SMP-A outputs are 4 to 20 mA for 0 to
1600 W/m².
B.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. SMP series radiometers are set to digitally accelerate the physical response.
B.4 Non-linearity
The non-linearity of a pyranometer is the percentage deviation in the sensitivity over an irradiance range from 0 to 1000 W/m²
compared to the sensitivity calibration irradiance of 500 W/m². The non-linear eect is due to convective and radiative heat
losses at the black absorber surface which make the conditional thermal equilibrium of the radiometer non-linear.
B.5 Tempearture dependence
The sensitivity change of the radiometer with ambient temperature change is related to the thermo-dynamics of the radiometer
construction. The temperature dependence is given as percentage deviation with respect to the calibrated sensitivity at +20°C.
The SMP series pyranometers have an integrated temperature sensor and use a fourth-order polynomial function to actively
correct for temperature errors over a -40°C to +70°C range.
B.6 Tilt error
This is the deviation from the sensitivity at 0 ° tilt (exactly horizontal) over the range from 0 ° to 90 ° tilt under 1000 W/m²
normal incidence irradiance. The tilt response is proportional to the incident radiation. The error could be corrected for, in
applications where it is necessary to install the pyranometer on an inclined surface, but is usually insignificant.
.
B.7 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 eective 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 dome 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 dierence between the detector and the instrument
housing will generate a small negative output signal which is commonly called Zero Oset Type A. This eect is reduced by using
an inner dome. This inner dome acts as a ‘radiation buer’.
This oset can be minimized by applying appropriate ventilation of the instrument. The CVF4 ventilation unit can be used with
the SMP’s, no ventilation unit is available for the SMP3.
B.8 Zero offset type B
Proportionally to the ambient temperature the instrument temperature varies and causes heat currents inside the instrument.
This will cause an oset commonly called Zero Oset Type B. It is quantified as the response in W/m² to a 5 K/hr change in
ambient temperature.
B.9 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 unusually harsh temperature conditions.
B.10 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.
.
B.11 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.
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).
B.12 Maximum irradiance
The maximum irradiance is defined as the total irradiance level beyond which the output is no longer linear and out of specifications.
The analogue output for the SMP’s is set to 2000 W/m², which is sucient under normal atmospheric conditions. For special
applications (environmental test rooms) the SMP’s can be set higher, up to 4000 W/².
B.13 Non-stability
This is the percentage change in sensitivity over a period of one year. This eect 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.
B.14 Spectral selectivity
Spectral selectivity is the variation of the dome transmittance and absorption coecient of the black detector coating with
wavelength and is commonly specified as % of the mean value.
B.15 Environmental
The SMP 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.
.
B.16 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 determine uncertainty figures based on many
years of field measurements for typical operating conditions.
When a pyranometer is in operation, the performance of it is correlated to a number of parameters, such as temperature, level of
irradiance, angle of incidence, etc. If the conditions dier significantly from 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 oset type A by using
a well-designed ventilation system. The Kipp & Zonen CVF4 ventilation unit is recommended for the SMP’s to minimise this
small error.
Thermal exchange between:
A Outer dome and atmosphere
B Inner dome and outer dome
C Inner dome and detector
Cold atmosphere
Solar radiation
A
B
C
54
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