ISO 9847:2023
(Main)Solar energy — Calibration of pyranometers by comparison to a reference pyranometer
Solar energy — Calibration of pyranometers by comparison to a reference pyranometer
This document specifies two preferred methods for the calibration of pyranometers using reference pyranometers; indoor (Type A) and outdoor (Type B). Indoor or type A calibration, is performed against a lamp source, while the outdoor method B, employs natural solar radiation as the source. Indoor calibration is performed either at normal incidence (type A1), the receiver surface perpendicular to the beam of the lamp or under exposure to a uniform diffuse lamp source using an integrating sphere (type A2). Outdoor calibration is performed using the sun as a source, with the pyranometer in a horizontal position (type B1), in a tilted position (type B2), or at normal incidence (type B3). Calibrations according to the specified methods will be traceable to SI, through the world radiometric reference (WRR), provided that traceable reference instruments are used. This document is applicable to most types of pyranometers regardless of the type technology employed. The methods have been validated for pyranometers that comply with the requirements for classes A, B and C of ISO 9060. In general, all pyranometers may be calibrated by using the described methods, provided that a proper uncertainty evaluation is performed. Unlike spectrally flat pyranometers, non-spectrally flat pyranometers might have a spectral response that varies strongly with the wavelength even within the spectral range from 300 to 1 500 nm, and therefore the calibration result may possibly be valid under a more limited range of conditions. The result of a calibration is an instrument sensitivity accompanied by an uncertainty. This document offers suggestions for uncertainty evaluation in the annexes.
Énergie solaire — Étalonnage des pyranomètres par comparaison à un pyranomètre de référence
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 9847
Second edition
2023-01
Solar energy — Calibration of
pyranometers by comparison to a
reference pyranometer
Énergie solaire — Étalonnage des pyranomètres par comparaison à
un pyranomètre de référence
Reference number
ISO 9847:2023(E)
© ISO 2023
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ISO 9847:2023(E)
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ISO 9847:2023(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Pyranometer calibration . 4
4.1 General . 4
4.2 Pyranometer sensitivity, measurement equation, measurand . 4
4.3 Indoor and outdoor calibration compared . 6
4.4 Method validation . 6
4.5 Calibration uncertainty . 6
5 Measuring equipment . 7
5.1 Data acquisition and recording. 7
5.2 Instrument platforms . 8
5.3 Pyranometers . 8
6 Indoor calibration (Type A) . 8
6.1 Introductory remarks on indoor calibration . 8
6.2 Reference pyranometers for indoor calibration . 8
6.3 Indoor calibration systems . 9
6.3.1 System with a direct beam source (type A1) . 9
6.3.2 Systems with an integrating sphere source (type A2) . 9
6.4 Indoor calibration procedures . 9
6.4.1 Calibration procedure requirements (types A1 and A2). 9
6.4.2 Indoor calibration procedures (types A1 and A2) . 9
6.4.3 Calculation of the sensitivity . 10
6.4.4 Calibration conditions and optional correction of reference operating
conditions . 11
6.4.5 Uncertainty evaluation . 11
7 Outdoor calibration (Type B) .12
7.1 Introductory remarks on outdoor calibration .12
7.2 Reference pyranometers for outdoor calibration .12
7.3 Outdoor calibration systems . 12
7.3.1 Site selection for outdoor calibration .12
7.3.2 Tracking for normal incidence calibration (type B3) .13
7.4 Outdoor calibration procedures . 13
7.4.1 Calibration procedure requirements (B1, B2, B3) .13
7.4.2 Outdoor horizontal calibration procedure (type B1) .13
7.4.3 Outdoor tilted calibration procedure (type B2) . 14
7.4.4 Outdoor normal incidence calibration procedure (type B3) .15
7.4.5 Calculation of the sensitivity . 15
7.4.6 Calibration conditions and optional correction of reference operating
conditions . 16
7.4.7 Uncertainty evaluation . 16
8 Calibration certificate .17
Annex A (informative) Examples of calibration systems using artificial sources .18
Annex B (informative) Calculation of daily average zenith angle .22
Annex C (informative) Introduction of a new pyranometer sensitivity .24
Annex D (informative) Data quality review for outdoor calibration .26
Annex E (informative) Uncertainty evaluation for outdoor calibration .29
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ISO 9847:2023(E)
Annex F (informative) Uncertainty evaluation for indoor calibration .30
Bibliography .31
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ISO 9847:2023(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
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on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 180, Solar energy, Subcommittee SC 1,
Climate – Measurement and data.
This second edition cancels and replaces the first edition (ISO 9847:1992) which has been technically
revised.
The main changes are as follows:
— focus on current calibration practices;
— adapted recommendations for mathematical treatment of data;
[1]
— adaptation of the terminology to the revised ISO 9060:2018 and ISO Guide 99 ;
[2]
— added comments on uncertainty evaluation of the calibration with reference to ASTM G213 and
ISO/IEC Guide 98-3;
— inclusion of reference to non-spectrally-flat pyranometers, that are now also included in ISO 9060.
Annexes A, B, C, D, E and F are given for information only.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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ISO 9847:2023(E)
Introduction
Pyranometers are instruments used to measure the irradiance (power per unit area) received from the
sun for many purposes.
In recent years the application of hemispherical solar radiation measurement, using pyranometers,
has risen sharply. The main application of pyranometers now is no longer scientific research, but
assessment of the performance of solar power plants.
Accurate measurements of the hemispherical solar radiation are required for
a) the determination of the energy input to solar energy systems such as photovoltaic (PV) -, and solar
thermal systems, as a basis for performance assessment,
b) the testing and assessment of solar technologies,
c) the geographic mapping of solar energy resources, and
d) other applications such as agriculture, building efficiency, material degradation and reliability,
climate, weather, health, etc.
Today’s growing solar energy performance assessment markets demand the lowest possible
measurement uncertainties. To meet this demand, a measurement requires an uncertainty evaluation
[3]
and an accurate time stamp .
Calibration of measuring instruments is an essential part of the uncertainty evaluation and part of
any quality management system. Regular instrument re-calibration according to this standard helps
attaining the required low measurement uncertainties. Calibration usually will show the instrument is
stable and then serves as:
— confirmation that the measurement data collected over the time interval from the previous to the
present calibration are reliable
— the instrument is expected to remain stable, future measurement data are expected to be reliable.
Uncertainties mentioned in this document are expanded uncertainties with a coverage factor k = 2.
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INTERNATIONAL STANDARD ISO 9847:2023(E)
Solar energy — Calibration of pyranometers by
comparison to a reference pyranometer
1 Scope
This document specifies two preferred methods for the calibration of pyranometers using reference
pyranometers; indoor (Type A) and outdoor (Type B).
Indoor or type A calibration, is performed against a lamp source, while the outdoor method B, employs
natural solar radiation as the source.
Indoor calibration is performed either at normal incidence (type A1), the receiver surface perpendicular
to the beam of the lamp or under exposure to a uniform diffuse lamp source using an integrating sphere
(type A2).
Outdoor calibration is performed using the sun as a source, with the pyranometer in a horizontal
position (type B1), in a tilted position (type B2), or at normal incidence (type B3).
Calibrations according to the specified methods will be traceable to SI, through the world radiometric
reference (WRR), provided that traceable reference instruments are used.
This document is applicable to most types of pyranometers regardless of the type technology employed.
The methods have been validated for pyranometers that comply with the requirements for classes A,
B and C of ISO 9060. In general, all pyranometers may be calibrated by using the described methods,
provided that a proper uncertainty evaluation is performed.
Unlike spectrally flat pyranometers, non-spectrally flat pyranometers might have a spectral response
that varies strongly with the wavelength even within the spectral range from 300 to 1 500 nm, and
therefore the calibration result may possibly be valid under a more limited range of conditions.
The result of a calibration is an instrument sensitivity accompanied by an uncertainty. This document
offers suggestions for uncertainty evaluation in the annexes.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 9060, Solar energy — Specification and classification of instruments for measuring hemispherical solar
and direct solar radiation
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
me a s ur ement (GUM: 1995)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
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ISO 9847:2023(E)
3.1
pyranometer
radiometer designed for measuring the irradiance on a plane receiver surface which results from the
radiant fluxes incident from the hemisphere above within the wavelength range 0,3 µm to 3 µm.
[SOURCE: ISO 9060:2018, 3.5, modified — Tolerances have been changed and the Note 1 to entry was
deleted.]
3.2
hemispherical solar radiation
solar radiation received by a plane surface from a solid angle of 2π sr
[SOURCE: ISO 9060:2018, 3.1, modified — Note 1 to entry was deleted.]
3.3
global horizontal solar irradiance
GHI
G
hemispherical solar radiation received by a horizontal plane surface
[SOURCE: ISO 9060:2018, 3.2, modified — Symbol G and abbreviation GHI were added and Note 1 to
entry was deleted.]
3.4
sensitivity
quotient of the change in an indication of a measuring system and the corresponding change in a value
of a quantity being measured
Note 1 to entry: See Reference [1].
3.5
calibration of a pyranometer
determination of the relationship between the pyranometer (3.1) output and the irradiance, with
associated measurement uncertainties, under well-defined operating conditions
Note 1 to entry: For most pyranometers, the output varies linearly with the irradiance and the calibration result
is expressed as a single sensitivity.
Note 2 to entry: See References [1] and [4].
3.6
reference pyranometer
pyranometer (3.1) used as reference standard, i.e. an instrument used for calibration of other
pyranometers in a given organization
3.7
test pyranometer
pyranometer (3.1) being calibrated
Note 1 to entry: Called field pyranometer in the previous version of this document.
3.8
calibration conditions
conditions, ambient- or instrument, during the calibration process
3.9
reference-operating condition
operating condition prescribed for evaluating the performance of a measuring instrument or measuring
system or for comparison of measurement results
Note 1 to entry: For practical purposes these typically are the conditions specified for the reported sensitivity.
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ISO 9847:2023(E)
Note 2 to entry: For measurement results, see Reference [1].
3.10
world radiometric reference
WRR
measurement standard representing the SI unit of irradiance with an uncertainty of less than ±0,3 %
Note 1 to entry: The reference was adopted by the World Meteorological Organization (WMO) and has been in
effect since 1 July 1980. The WRR is maintained by the WMO World Radiation Centre at Davos. The distinguishing
feature of traceability to WRR is that reference-operating conditions include the spectrum of natural direct solar
radiation.
3.11
sample
data acquired from a sensor or measuring device
Note 1 to entry: See Reference [5].
3.12
sampling interval
time between samples (3.11)
Note 1 to entry: See Reference [5].
3.13
record
data recorded and stored in data log, based on acquired samples (3.11)
Note 1 to entry: See Reference [5].
3.14
data series
set of selected records (3.13)
3.15
correction
value added algebraically to the uncorrected result of a measurement to compensate for systematic
error
[SOURCE: ISO Guide 98-3:2008, B.2.23]
Note 1 to entry: The correction for offsets is equal to the negative of the estimated systematic error. Since the
systematic error cannot be known perfectly, the compensation cannot be complete.
3.16
correction factor
numerical factor by which the uncorrected result of a measurement is multiplied to compensate for
systematic error
Note 1 to entry: Since the systematic error cannot be known perfectly, the compensation cannot be complete.
[SOURCE: ISO Guide 98-3:2008, B.2.24]
3.17
solar tracker
mechanical device capable of rotation around 2 axes, e.g. zenith and azimuth, following the path of the
sun
3.18
integrating sphere
sphere or hemisphere, equipped with one or more lamps, internally coated with a spectrally flat white
paint providing uniform illumination
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ISO 9847:2023(E)
3.19
tilt angle
angle between the horizontal plane and the plane of the pyranometer (3.1) sensor surface
3.20
angle of incidence
angle of radiation relative to the sensor measured from normal incidence (varies from 0° to 90°)
3.21
zenith angle
angle of incidence (3.20) of radiation, relative to zenith (angle between the earth’s surface normal and
the line to the sun)
Note 1 to entry: It equals the angle of incidence (3.20) for horizontally mounted instruments.
3.22
solar azimuth angle
angle between a reference direction (north or south) and the projection of beam radiation on the
horizontal plane
[6]
Note 1 to entry: Duffie and Beckman define the reference direction (zero solar azimuth angle) as south for both
the northern and southern hemisphere. In the Duffie and Beckman definition the azimuth angle ranges from
−180° to +180°, where angles east of south are negative and west of south positive. Other references and models
use north as reference direction.
4 Pyranometer calibration
4.1 General
Calibration of a pyranometer involves a test to determine the relationship between pyranometer output
and irradiance. The result is usually expressed as a single sensitivity, with associated uncertainty,
under well-defined operating conditions. Pyranometer calibration may be carried out according to
[4]
ISO 9846 , outdoors against a pyrheliometer, or according to this document, indoors or outdoors
against a reference pyranometer. Both documents describe how to transfer the sensitivity of the
reference instrument to the test instrument.
The recommended calibration interval for pyranometers differs from one manufacturer to the other.
[5]
IEC 61724-1 recommends instrument recalibration once every 2 years or more frequently according
to manufacturer recommendations for Class A monitoring systems, and according to manufacturer
recommendations for Class B systems. For Class A monitoring systems for global horizontal solar
irradiance and plane of array irradiance measurement, this document requires a calibration
uncertainty of less or equal than 2 %. Under typical but not all conditions, this uncertainty is attainable
[3]
with pyranometers having calibration uncertainties in the order of 1,5 % or better .
4.2 Pyranometer sensitivity, measurement equation, measurand
The relationship between pyranometer sensitivity, output and irradiance or measurement equation for
thermal pyranometers is given by Formula (1):
SV= G (1)
where
2
S is the sensitivity in output units/(W/m );
2
G is the global horizontal solar irradiance in W/m ;
V is the pyranometer output in arbitrary units.
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ISO 9847:2023(E)
Calibration of pyranometers essentially consists of a measurement at or traceable to an irradiance level
in the middle or close to the upper end of the measurement range.
Clear sky conditions are the most common calibration reference condition for calibration, so that during
calibration the measurand formally is global horizontal solar irradiance under a clear sky.
NOTE Calibration reference conditions can differ from the operation conditions in several ways, not only
spectrally, but can also differ in terms of e.g. temperature, wind, solar position, atmospheric conditions (cloud
cover, aerosols) and instrument tilt. Even the measurand can change (calibrated for global irradiance, used
for diffuse or reflected irradiance measurements). If the calibration reference conditions and the operating
conditions are different the user considers this for the uncertainty evaluation of the measurements and considers
proper corrections.
While traditional pyranometers had analogue millivolt output signals, many modern pyranometers
have different, for example digital or current-loop, outputs. These are often standardised outputs.
The calibration process of these instruments typically includes adjustment by programming a new
sensitivity into the firmware, so that the sensitivity after calibration as perceived by the user is
2 2 2
always the same; for example, 1 (W/m )/(W/m ) for digital instruments or 0 W/m = 4 mA, 1 600 W/
2
m = 20 mA for a pyranometer with a current-loop output. For these instruments the measurement
Formula (1) is adapted accordingly.
2
The calibration of instruments with standardised outputs is nevertheless expressed in V/(W/m )
because this gives a clear indication of the correction applied from one calibration to the next, and of the
stability of the sensor. For instruments with such internal signal conversion, the voltage measurement
2 2
usually is not separately calibrated. In such cases the V/(W/m ) shall be interpreted as V/(W/m ) “as
measured by the on-board analogue to digital conversion”.
In exceptional cases laboratories and users may choose to use alternative measurement equations.
They may use a correction factor acting on in S, for example accounting for temperature dependence.
ISO 9060 defines the measurement error "zero offset A". Corrections for zero offset A can be made
during outdoor calibration. It may lead a higher accuracy of the calibration. However, care should be
taken to ensure that the same correction technique used in calibration is then used for subsequent
measurements. The zero offset A is not constant. It depends on the environmental conditions for example
on cloud condition, sky temperature, wind speed (ventilator application), and thermal coupling of the
instrument to its mounting. Applying such corrections may lead to a lower measurement accuracy.
When applying corrections for offsets, the measurement Formula (1) then gets the form of Formula (2):
SV= − VG (2)
()
O
where
2
S is the sensitivity in output units/(W/m );
2
G is the irradiance in W/m ;
V is the pyranometer output in arbitrary units;
V is an offset on the output in arbitrary units.
0
Working with instruments calibrated with a correction for offsets according to Formula (2), users shall
also adapt their measurement equation from Formulae (1) and (2).
Additional corrections for example for temperature dependence, and outdoor solar- or indoor lamp
spectrum can also be implemented.
Annex C contains informative comment on what to do with calibration results; how to introduce a new
pyranometer sensitivity.
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ISO 9847:2023(E)
4.3 Indoor and outdoor calibration compared
Under this document, there are two options for pyranometer calibration: indoors, in the laboratory
using lamps as a source and outdoors under the Sun. There are the following fundamental differences:
— An indoor calibration is only the transfer of the outdoor calibration of the reference instrument to a
test instrument.
— Indoor calibration is done by comparison of the test pyranometer to a reference pyranometer of the
same model, and thus of the same class. Initial (i.e. before making optional corrections) reference-
operating conditions, the condition for which the calibration of the test instrument is valid, are the
conditions reported as valid for the calibration of the reference pyranometer.
— Outdoor calibration is done by comparison of the test pyranometer to the reference pyranometer,
where the reference pyranometer is not necessarily of the same model, typically of a higher or equal
class. Initial reference operating conditions are the outdoor conditions during this calibration.
— For both indoor and outdoor calibration, the reference-operating conditions may later, in the
calibration report, be adapted to other conditions than those to which the calibration is initially
traceable. This then leads to an adapted sensitivity and reduces the calibration accuracy.
In all cases corrections shall also be accounted for in the calibration uncertainty and be reported on the
calibration certificate.
Calibration laboratories may report multiple sensitivities valid for different reference-operating
conditions, so that users may work with a sensitivity valid for conditions as close as possible to actual
operating conditions (e.g. sensitivities for a non-spectrally flat pyranometer operating under clear and
overcast sky conditions).
The uncertainty evaluation for one instrument may be used for other instruments of the
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