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
General Information
Standards Content (Sample)
INTERNATIONAL
Is0
STANDARD
9846
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)
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IS0 9846:1993(E)
Contents
Page
1
1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Normative references . . . . . . . . .~. 1
. . . . . . .~.~.,,.~. 1
3 Definitions
2
4 Selection of methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
5 Alternating sun-and-shade method
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*. 5
6 Continuous sun-and-shade method
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7 Certificate of calibration
8
8 Uncertainty . . . . . . . . . . . . . . . . . . . . . . . .*.
Annexes
9
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
17
method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
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
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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
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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.
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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.
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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
2
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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
effect on the measurements varies monotonically at
e) The shade disc must be easy to remove and re-
a small rate (see 5.7.1). Specular reflection by ob-
place, so that the change from the shade phase to
structions should be avoided. In the case of inclined
the hemispheric solar irradiance phase, or vice
pyranometers, signal contributions from the radiation
versa, takes less than 5 % of the phase duration.
reflected by the foreground should vary in the sense
mentioned above.
The five types of shade disc devices briefly described
in annex A are the designs of different institutions;
Space must be provided around the pyranometers for
only one of them is commercially available at present.
the movement of the shade disc. The distance to
other instruments should be large enough so that
possible interference can be neglected.
5.2.5 Data acquisition system.
The distance between the pyrheliometer and the
To acquire the values, in millivolts, of the radiometer
pyranometer should be less than 30 m, otherwise
readings, the system should be equipped with a pre-
both radiometers may not be similarly affected by the
cise digital voltmeter with a resolution of 1 PV and an
same atmospheric events (for example, structured
uncertainty of 0,l % of the pyranometer’s calculated
turbidity elements).
output at 1 100 W m-*. High temperature stability is
required for outdoor operation. The data sampled
from all radiometers should be recorded within about 5.5 Installation
1 s. A time resolution for calculating the correspond-
ing solar elevation angle with an uncertainty of less The installation of the pyrheliometer and the solar
than 0,l O is required. For documenting the variation tracker as well as the pyranometer with the shade
of the measured values during the calibration period, disc device shall be carried out as described in the
the data should be appropriately recorded. appropriate operation and manufacturer’s manuals,
and considering 5.4.
Pyranometers which are used in combination with a
5.3 Measurement conditions
ventilation device should also be ventilated during the
calibration procedure.
Clear sky conditions are essential for reduced variance
In the case of inclined pyranometers, the cable outlets
in the results. However, clouds are tolerable if they
should point downwards to avoid interference from
are at a large angular distance from the sun (> 45”)
rain or direct solar radiation (see ISO/TR 9901).
and have a low angular velocity, to guarantee stable
values of diffuse solar radiation within the cycle time
of the measurement procedure (see 5.7.1); i.e. the
5.6 Calibration procedure
change in the diffuse solar irradiance must be negli-
gible. In the case of tilted pyranometers, clouds which
5.6.1 Preparatory phase
are outside the field of view have minimal influence
on the measurement procedure.
Start the preparatory phase about 30 min before the
measurement phase to allow for:
In principle, the other environmental conditions during
calibration should be similar to the typical conditions
- acclimatization of the radiometers, electronics and
during normal use of the pyranometer. The most im-
data acquisition system;
portant parameter is the range of solar elevation, fol-
lowed by the ambient air temperature, level of
- adjustment of radiometers, solar tracker and shade
hemispherical solar irradiance and tilt angle.
disc device;
During calibration, wind conditions are also important,
since pyrheliometers operating with open tubes are
- checking of the electrical connections, test volt-
disturbed by strong wind speeds, especially gusts ages and zeroing tests;
coming from the sun’s azimuthal direction. It is rec-
ommended that pyrheliometers are operated with
- final cleaning of the optica windows.
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IS0 9846:1993(E)
5.6.2 Measurement phase (single series) where to is that time interval or response time in
which the pyranometer signal achieves a final value
deviating less than 0,3 % from the theoretical final
The measurement phase consists of (212 + 1) intervals.
vajue.
During (n + 1) intervals the pyranometer is shaded;
during rt intervals, which alternate with the former, the
NOTE 3 The setting of the same time interval to for the
pyranometer is exposed to hemispheric solar radi-
shading and hemispherical solar radiation phase is based on
ation.
the assumption that the response time of the pyranometer
during increasing and decreasing signals is approximately
During the measurement phase
the same.
The choice of to should consider the setting conditions dur-
a) the pyranometer, tilted at an angle pI indicates:
ing fine weather and cloudless conditions.
- if shaded, the signal for diffuse solar irradiance
Generally, for commercially available pyranometers to lies
Eo B (including reflected solar irradiance if
, between 1 and 4 min; to may be reduced by wind or the
j # 0), (n + 1) values. Read at the end of each
artificial ventilation of the instrument.
shading interval to (see graph below)
The time interval between readings of the radiometer
- if exposed to hemispheric solar radiation, the
may be up to about 20 % longer than the shortest
signal for hemispherical solar irradiance Ec 8,
estimate to if the system is unstable (owing to gusts
yt values. Read at the end of each exposure ‘to
or dust clusters, for instance). In any case, the time
hemispherical solar radiation interval to (see
of measurement must be recorded carefully for cal-
graph below);
culation of the solar incidence angles (see 5.2.5).
Restrict the number of intervals ~1 of each series so
b) the pyrheliometer indicates the signal for direct
normal solar irradiance E,, yt values. Read simul- that the duration (2n+ l)to of a series is not longer
B (see graph below); than 36 min. The mean value of series of such dura-
taneously with Ec
I
tion can then be associated with small ranges of solar
c) the thermometer indicates ambient air tempera- elevation and temperature. The number of intervals yt
shall not be less than 3.
ture or radiometer temperature T, with readings
at least at the start and the end of the series.
5.6.3 Number of series
The time interval of each phase to corresponds to the
Since the results of each series can exhibit scatter,
time interval within which the pyranometer signal
achieves its assumed final value. Set (or remove) the at least 10 series should be completed, from which
shade disc immediately (see 5.2.4) after the readings the final result can be determined. Complete the se-
-
of Ec,B (or E,,J have been taken. ries over three or more days to obtain enough series
at mean solar incidence angles which deviate less
The measurement sequence is illustrated in the fol- than + 5” from that angle representing the normal
lowing scheme: operating conditions.
Shade disc set set
- . set ‘----,jPf
Time of reading 0 t, 2t0 3 to 4 to st() zn*t(J (Z/7+ I) et,
I I I I
1 ----I
-m--p,
Reading of E,, /1 * *
* *
Reading of EG, p
t--* *-----*
Reading of E,
I* *-----*
* -----
1 Reading of T *
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IS0 9846:1993(E)
total number of reading inter-
5.7 Determination of the calibration factor
vals (2n + 1).
Identify and reject those Rs(i) which deviate by more
5.7.1 Evaluation of a single series
than 1 % from Es. If more than n/2 are rejected,
eliminate the series from further calculations.
Determine the responsivity Rs(i> and the mean
responsivity i?s, expressed as mrcrovolts per watt per
If there are sufficient Rs(i), calculate a corrected value
square metre or as millivolts per milliwatt per square
R,:
centimetre, from the appropriate group of measure-
ments according to:
Rs = Es(i,i = 1 ,n for i #j) . . .
(3)
.
=
R,(l)
where j are those measurements i which were
identified as deviating by 1 % from Es.
{VG, &2i) - 0,5[v,, /@ - 1) + VD, ,(2i + I)]}
=
{ v,(2i)=FPm cos[?J(2i)]}
5.7.2 Evaluation of the final results
. . .
(1)
-
If p calibration series are carried out at the desired
R, =
parameter ranges, the final responsivity R is calculated
n
as the mean of all responsivities R,:
{VG &2i) - 0,5[v, &2i - 1) + VD &2i + I)]}
, I I
c
i=l
=
n
v,(2i)q cos[?J(2i)]
c
If a reduction formulaf(T,T,) is available, and there are
some series in which the temperature deviates sig-
. . .
(2)
nificantly from the desired value Tn, then apply f(T,T,)
to each R, according to:
where
indicates the measurement
(5)
within the series;
indicates the series;
S
NOTE 4 For some types of pyranometers, temperature
is the hemispherical solar ir-
coefficients dc are specified so that simply
vG, &2i)
radiance signal measured at
flT,T,) = [I - a(T - Tn)] is applicable.
position 2i within the series, in
millivolts, for instance;
Present the final result also in the form of a calibration
factor F, expressed in watt square metres per micro-
vD, ,(2i - 1) 01
volt:
is the diffuse solar irradiance
vD, &2i + 1 >
F+
.
(6)
signal measured at position
(2i- 1) or (2i+ 1) within the
and the responsivity R.
series, in millivolts, for in-
stance;
V,(2i)mF, is the direct solar irradiance 6 Continuous sun-and-shade method
calculated from the product of
the pyrheliometer signal Vl(2i)
6.1 Principle
and its calibration factor Fp;
The pyranometer is compared with two reference
is the angle between the direc-
VW)
radiometers, namely a pyrheliometer and a well-
tion of the solar beam and the
calibrated pyranometer measuring diffuse solar radi-
perpendicular to the receiver
The hemispherical solar irradiance is
ation.
plane of the pyranometer, at
determined by the sum of the direct solar irradiance
the time corresponding to set
and the diffuse solar irradiance. The direct solar ir-
position 2i. The angle of inci-
radiance is derived from the pyrheliometer signal. The
dence 7 is calculated (see an-
diffuse solar irradiance is measured continuously by
nex B) from the inclined
the second pyranometer with a shade disc device.
position of the pyranometer
The method can deliver continuous results if continu-
and the solar position;
ous pyrheliometer readings can be taken. The refer-
is the number of readings of ence pyranometer is traceable to a pyranometer
& B and E, to be used from the calibrated with the sun-and-shade method.
I
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IS0 9846:1993(E)
6.2 Apparatus 6.5 Installation
See 5.5, but the two pyranometers shall be installed
6.2.1 Pyranometer.
in the same inclined position.
In principle this method can be applied to any type of
pyranometer. Secondary standard or first class instru-
ments which have to be calibrated regularly are rec-
6.6 Calibration procedure
ommended as reference
pyranometers (see
IS0 9060). Pyranometers recommended for measur-
ing diffuse solar irradiance in clear sky conditions are
6.6.1 Preparatory phase
those which have low directional errors, the smallest
possible decrease in spectral responsivity in the
5.6.1.
As desc ribed in Take great care in determining
ultraviolet region > 0,3 pm and a small diameter of
zero offsets of the radiometer
the signals
the effective receiver surface compared with the di-
ameter of the glass domes.
6.6.2 Measurement phase (single series)
6.2.2 Pyrheliometer.
The measurement phase of each series consists of
See 5.2.3, but well-calibrated pyrheliometers provid-
between 10 and 20 sets of readings (each measure-
ing continuous signals are preferred.
ment being completed within 1 s) of:
6.2.3 Solar tracker.
- hemispherical solar irradiance signal VG B from the
test pyranometer at the tilt angle ~3; ’
See 5.2.3.
- diffuse solar irradiance signal VD B from the refer-
6.2.4 Shade disc device.
ence pyranometer at the tilt angl’e /?;
See 5.2.4, but without item e). Use of an automatic
- direct solar irradiance signal V, from the reference
shade disc is recommended.
pyrheliometer.
Measure the temperature of the ambien t air or of a
6.2.5 Data acquisition system.
radiometer body, as well as the irradiance zero, at the
See 5.2.5, but in the case of continuous data
beginning and at the end of each series, and of each
measurement, consideration should be given to re-
set.
duce data acquisition to periods of up to 10 min.
Take the readings periodically if this is required by the
operational mode of the pyrheliometer; if not, limit the
6.3 Measurement conditions
timing of measurements to the most stable periods.
However, take sets from all parts of the series so that
In general, conditions as described in 5.3. Since the
the result is representative of the total period. Care-
data can be acquired continuously, data obtained un-
fully record the time of each set for a subsequent
der cloudy conditions are also acceptable, as long as
calculation of the solar incidence angle.
- the angular distance of the clouds from the sun is
Choose the sampling frequency depending on the
greater than 15” and
operational mode of the pyrheliometer to be between
2 per minute and 0,5 per minute. The number of sets
- the angular velocity of clouds within the field of
per series should be between 10 and 20, with the
view of the pyranometer is small enough so that
duration of a series between 10 min and 30 min.
the diffuse solar irradiance varies less than 1 % in
about 10 s. NOTE 5 If the pyrheliometer is capable of measuring the
direct solar irradiance continuously, the use of integrated
values is possible. The integration interval should be no
6.4 Measurement site
longer than 6 min or 2 min, in the case of clear or cloudy
sky, respectively. Non-negligible uncertainties may be intro-
In general, the measurement site should be as speci- duced in the calculation of R, [equation (7)] by using the
mean solar incidence angle q over the integration interval.
fied in 5.4. The test pyranometer shall be sufficiently
separated from the vicinity of the shading disc device
of the reference pyranometer to ensure that it is not
influenced by the disc device. Additionally, any ob- 6.6.3 Number of series
structions within the fields of view of the reference
and test pyranometers must be nearly identical and, Measure at least 10 series spread over at least three
for inclined installations, the foregrounds must be days. If the data are taken over less than three days,
nearly identical. justification/documentation shall be provided.
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IS0 9846:1993(E)
ceiver plan e of the tes
6.7 Determination of the calibration factor
pyranomete r at t he tim eo
the reading of set i (see an-
6.7.1 Evaluation of a single series
nex B);
.
a) Eliminate from the calculation all sets which devi-
is the calibration factor of
ate from the corresponding series mean by more
the reference pyrhelio-
than
...
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