ISO 9846:1993
(Main)Solar energy — Calibration of a pyranometer using a pyrheliometer
Solar energy — Calibration of a pyranometer using a pyrheliometer
Its use is mandatory for the calibration of secondary standard pyranometers according to ISO 9060, and is recommended for the calibration of pyranometers which are used as reference instruments in comparisons. Is intended for use by test institutions or test laboratories equipped with well-maintained pyrheliometers and is applicable to all pyranometers. The object is to promote the uniform application of reliable methods to calibrate pyranometers, since accurate calibration factors are the basis of accurate hemispherical solar radiation data which are needed for solar energy test applications or simulations.
Énergie solaire — Étalonnage d'un pyranomètre utilisant un pyrhéliomètre
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Standards Content (Sample)
INTERNATIONAL
Is0
STANDARD
First edition
3 993-I 2-01
Solar energy - Calibration of a
pyranometer using a pyrheliometer
fkergie solaire - ctalonnage d’un pyranom&tre utilisant un pyrh&liom&tre
Reference number
IS0 9846:1993(E)
IS0 9846:1993(E)
Contents
Page
1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Normative references . . . . . . . . .~. 1
. . . . . . .~.~.,,.~. 1
3 Definitions
4 Selection of methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
5 Alternating sun-and-shade method
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*. 5
6 Continuous sun-and-shade method
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7 Certificate of calibration
8 Uncertainty . . . . . . . . . . . . . . . . . . . . . . . .*.
Annexes
A Shade disc devices . . . . . . . . . . . . . . . . .~.
B Calculation of the angle of incidence q of a solar beam on an inclined
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
plane
C Extended version of the sun-and-shade method . . . . . . . . . . . . . . . . . . . 16
D Multiple-reading version of the alternating sun-and-shade
method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E Extended version of the continuous sun-and-shade method
F Comparison of the alternating sun-and-shade method (ASSM) and the
. . . . . . . . . . . . . . . . . . . . . . . . . 19
continuous sun-and-shade method (CoSSM)
G Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
0 IS0 1993
All rights reserved. No part of this publication may be reproduced or utilized in any form or
by any means, electronic or mechanical, including photocopying and microfilm, without per-
mission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
ii
IS0 9846:1993(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work
of preparing International Standards is normally carried out through IS0
technical committees. Each member body interested in a subject for
which a technical committee has been established has the right to be
represented on that committee. International organizations, governmental
and non-governmental, in liaison with ISO, also take part in the work. IS0
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting
a vote.
International Standard IS0 9846 was prepared by Technical Committee
lSO/TC 180, Solar energy, Sub-Committee SC 1, Climate - Measurement
and data.
Annexes A, B, C, D, E, F and G of this International Standard are for in-
formation only.
. . .
Ill
IS0 9846:1993(E)
Introduction
This International Standa .d is one of a series of standards specifying
-
methods and instruments for the measurement of solar radiation.
From meteorological app ication s of pyranometers, considerable experi-
ence has been gained with a number of calibration methods. These
methods may be divided into two groups specified by the type of refer-
ence radiometer used. Calibration methods using pyranometers as a ref-
erence have been treated in IS0 9847; methods using pyrheliometers are
the subject of this standard.
The latter methods are more complicated than the former, because the
pyranometers, which typically have a field-of-view angle of 271, have to be
compared with pyrheliometers, which are designed to measure direct so-
lar radiation within a relatively small field of view.
On the other hand, due to the relatively high accuracy of pyrheliometers,
the latter methods are more accurate than the former ones. Since the
WMO world radiometric reference (WRR), which represents the SI units
of irradiance, is determined by a group of selected pyrheliometers, the
transfer of the scale to pyranometers has to be accomplished by using
standard pyrheliometers (see IS0 9060). Short descriptions of the cali-
brations are given in [l], [2] and [3].
It should be emphasized that “calibration of a pyranometer” essentially
means the transfer of the WRR scale to the pyranometer under selected
conditions. The determination of the dependence of the calibration factor
(calibration function) on variable parameters is called “characterization”.
The characterization of pyranometers is the subject of the appropriate
International Standard for test methods for pyranometers.
INTERNATIONAL STANDARD IS0 9846:1993(E)
Solar energy - Calibration of a pyranometer using a
pyrheliometer
lSO/TR 9901: 1990, Solar energy - Field pyrano-
1 Scope
meters - Recommended practice for use.
The object of this International Standard is to promote
the uniform application of reliable methods to calibrate
3 Definitions
pyranometers, since accurate calibration factors are
the basis of accurate hemispherical solar radiation
For the purposes of this International Standard, the
data which are needed for solar energy test appli-
definitions given in IS0 9060 and the following defi-
cations or simulations.
nitions apply.
This International Standard is applicable to all pyrano-
meters in horizontal as well as in tilted positions. Its
3.1 calibration of a radiometer: Determination of
use is mandatory for the calibration of secondary
the responsivity (or the calibration factor, as its re-
standard pyranometers according to IS0 9060, and is
ciprocal) of a radiometer under well-defined measure-
recommended for the calibration of pyranometers
ment conditions.
which are used as reference instruments in compari-
sons. For other applications, the method using
3.2 reference pyranometer: Pyranometer (see
pyranometers as references may be used (see
IS0 9060), used as a reference to calibrate other
IS0 9847).
pyranometers (see IS0 98471, which is a well-
maintained and carefully selected instrument of rela-
This International Standard is intended for use by test
tively high stability and which has been calibrated
institutions or test laboratories equipped with well-
using a pyrheliometer.
maintained pyrheliometers.
3.3 field-of-view angle of a pyrheliometer: Full
angle of the cone which is defined by the centre of
the receiver surface (see IS0 9060, 5.1) and the bor-
2 Normative references der of the aperture, if the latter is circular and con-
centric to the receiver surface; if not, effective angles
The following standards contain provisions which, may be calculated [4].
through reference in this text, constitute provisions
of this International Standard. At the time of publi-
3.4 solar tracker; sun tracker: Power-driven or
cation, the editions indicated were valid. All standards
manually operated support which is employed to di-
are subject to revision, and parties to agreements
rect a pyrheliometer to the sun.
based on this International Standard are encouraged
“Equatorial trackers” are sun-following devices which
to investigate the possibility of applying the most re-
have an axis of rotation pointing towards the elevated
cent editions of the standards indicated below.
pole; the axes of motion are the hour angle and the
Members of IEC and IS0 maintain registers of cur-
declination of the sun. “Altazimuth trackers” are sun-
rently valid International Standards.
following devices with the solar elevation angle and
the azimuth angle of the sun as coordinates of
IS0 9060: 1990, Solar energy - Specification and
movement.
classification of instruments for measuring hemi-
spherical solar and direct solar radiation.
3.5 sun-shading disc device; shade disc device:
Device which allows movement of a disc in such a
IS0 9847: 1992, Solar energy - Calibration of field
way that the receiver of the radiometer (for example,
pyranometers by comparison to a reference pyrano-
a pyranometer) is shaded from the sun.
meter.
IS0 9846:1993(E)
For calibration purposes, particularly those described to direct solar irradiance are derived from the differ-
in clause 5, quick removal of the disc is mandatory. ence between the measured values of hemispherical
Further details on shade disc devices used in cali- solar irradiance and the diffuse solar irradiance (see
brating pyranometers are given in 5.2.4. note 1, 3.8). These values are measured periodically
by means of a movable sun shade disc. For the cal-
3.6 direct solar radiation: That part of the culation of the responsivity, the difference in ir-
extraterrestrial solar radiation which as a collimated radiance components is divided by the measured
beam reaches the earth’s surface after selective at- direct solar irradiance normal to the receiver plane of
tenuation by the atmosphere. the pyranometer.
In the following subclauses the basic method is de-
The quantity measured is the direct solar irradiance,
scribed. Modifications of this method, which may im-
expressed in watts per square metre (see also
prove the accuracy of the calibration factors but
IS0 9060).
require more operational experience, are mentioned
in annexes C and D.
3.7 hemispherical solar radiation; global radi-
ation: Combined direct solar radiation and diffuse so-
lar radiation.
5.2 Apparatus
The quantity measured is the hemispheric solar ir-
5.2.1 Pyranometer.
radiance, expressed in watts per square metre (see
also IS0 9060).
In principle, this method can be applied to any type
of pyranometer.
3.8 diffuse solar radiation: That part of solar radi-
ation which reaches the earth as a result of being
5.2.2 Pyrheliometer.
scattered by the air molecules, aerosol particles, cloud
and other particles.
The choice of pyrheliometer used as the reference
should be made according to the required accuracy
The quantity measured is the diffuse solar irradiance,
and the operational conditions. Generally, secondary
expressed in watts per square metre (see also
standard or first class instruments (see classification
IS0 9060).
in IS0 9060) which are regularly compared with pri-
mary standards represent a satisfactory level of accu-
NOTE 1 For meteorological purposes, the solid angle
racy (see also clause 8). The pyrheliometer should
from which the scattered radiative fluxes are measured shall
produce at least one reference value every 2 min.
be the total sky hemisphere, excluding a small solid angle
around the sun’s disc.
52.3 Solar tracker, power driven or manually oper-
ated, employed to direct the reference pyrheliometer
4 Selection of methods
to the sun for the entire test period. A solar tracker
of the altazimuth type should be used for pyrhelio-
Two calibration methods have been selected for
meters whose responsivity over the receiver surface
standardization, because they are widely used and are
is not circular-symmetrical. The required tracking ac-
reliable. Both methods use shade disc devices for
curacy depends on the slope angle (see IS0 9060) of
measuring diffuse solar radiation and are based on the
the pyrheliometer. In the usual case the slope angle
hemispherical solar radiation being equal to the sum
is about 1”.
of direct solar and diffuse solar radiations.
5.2.4 Shade disc device, meeting the following re-
The derived calibration factors are representative of
quirements:
cloudless or scattered cloud conditions (see clause 8
for uncertainties). A modification of the calibration
The shade disc shall be positioned perpendicular
a)
method in clause 5 for application during less stable
to the sun’s ray and at a fixed distance d from the
sky conditions is briefly described in annex C.
centre of the receiver surface of the pyranometer.
Annex D contains a short description of an extended
version of the calibration method in clause 6 to de-
The radius r of the shade disc should be larger
b)
termine the dependence of the calibration factors on than the radius of the outer glass dome of the
incidence angles. pyranometer by a minimum of d tan(0,5’) to allow
for the divergence of the sun beam and small
tracking errors.
5 Alternating sun-and-shade method
The ratio r/d should define an angle at the centre
d
5.1 Principle
of the receiver surface which corresponds to the
field-of-view angle of the pyrheliometer.
The pyranometer under test is compared with a pyr-
heliometer measuring direct solar irradiance. The
NOTE 2 A fixed “shade slope angle”, corresponding
voltage values from the pyranometer that correspond to the slope angle of the pyrheliometer, can only be
IS0 9846:1993(E)
stated for pyranometers which are operated in a position wind screens if wind-induced instability of the
normal to the sun’s ray. For other pyranometers, the
measurements is intolerable.
shade slope angle varies according to the angle of inci-
dence of the ray on the receiver plane.
5.4 Measurement site
d) Those parts of the disc holder which obscure the
The measurement site shall offer rigid supports to in-
field-of-view angle of the pyranometer should be
stall the instruments and be of convenient access.
as small as possible in order to restrict the dis-
turbance of the signal to less than 0,5 %. Similar
In the case of horizontal pyranometers, obstructions
regard to interference with other neighbouring in-
on the horizon are tolerable provided they do not ob-
struments should be considered.
scure the sun during the calibration period and their
effec
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