ASTM G167-00

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Designation:G167–00
Standard Test Method for
Calibration of a Pyranometer Using a Pyrheliometer
This standard is issued under the fixed designation G 167; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Accurate and precise measurements of total global (hemispherical) solar irradiance are required in
the assessment of irradiance and radiant exposure in the testing of exposed materials, determination
of the energy available to solar collection devices, and assessment of global and hemispherical solar
radiation for meteorological purposes.
This test method requires calibrations traceable to the World Radiometric Reference (WRR), which
represents the SI units of irradiance. The WRR is determined by a group of selected absolute
pyrheliometers maintained by the World Meteorological Organization (WMO) in Davos, Switzerland.
Realization of the WRR in the United States, and other countries, is accomplished by the
intercomparison of absolute pyrheliometers with the World Radiometric Group (WRG) through a
series of intercomparisons that include the International Pyrheliometric Conferences held every five
years in Davos. The intercomparison of absolute pyrheliometers is covered by procedures adopted by
WMO and is not covered by this test method.
It should be emphasized that “calibration of a pyranometer” essentially means the transfer of the
WRR scale from a pyrheliometer to a pyranometer under specific experimental procedures.
1. Scope equitorial tracking mount in which the axis of the radiometer’s
radiation receptor is aligned with the sun during the shading
1.1 This test method covers an integration of Test Method
disk test.
E 913 dealing with the calibration of pyranometers with axis
1.6 The determination of the dependence of the calibration
vertical and Test Method E 941 on calibration of pyranometers
factor (calibration function) on variable parameters is called
with axis tilted. This amalgamation of the two methods
characterization. The characterization of pyranometers is not
essentially harmonizes the methodology with ISO 9846.
specifically covered by this method.
1.2 This test method is applicable to all pyranometers
1.7 This test method is applicable only to calibration pro-
regardlessoftheradiationreceptoremployed,andisapplicable
cedures using the sun as the light source.
to pyranometers in horizontal as well as tilted positions.
1.8 This standard does not purport to address all of the
1.3 This test method is mandatory for the calibration of all
safety concerns, if any, associated with its use. It is the
secondary standard pyranometers as defined by the World
responsibility of the user of this standard to establish appro-
Meteorological Organization (WMO) and ISO 9060, and for
priate safety and health practices and determine the applica-
any pyranometer used as a reference pyranometer in the
bility of regulatory limitations prior to use.
transfer of calibration using Test Method E 842.
1.4 Two types of calibrations are covered: Type I calibra-
2. Referenced Documents
tions employ a self-calibrating, absolute pyrheliometer, and
2.1 ASTM Standards:
Type II calibrations employ a secondary reference pyrheliom-
E 772 Terminology Relating to Solar Energy Conversion
eter as the reference standard (secondary reference pyrheliom-
E 824 MethodforTransferofCalibrationfromReferenceto
eters are defined by WMO and ISO 9060).
Field Radiometers
1.5 Calibrations of reference pyranometers may be per-
E 913 Test Method for Calibration of Reference Pyranom-
formed by a method that makes use of either an altazimuth or
eters with Axis Vertical by the Shading Method
This test method is under the jurisdiction of ASTM Committee G-3 on
Weathering and Durability and is the direct responsibility of Subcommittee G03.09
on Radiometry. Annual Book of ASTM Standards, Vol 12.02.
Current edition approved Feb. 10, 2000. Published June 2000. Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G167–00
E 941 Test Method for Calibration of Reference Pyranom- which is well-maintained and carefully selected to possess
eters with Axis Tilted by the Shading Method relatively high stability and has been calibrated using a
2.2 WMO Document: pyrheliometer.
World Meteorological Organization (WMO), “Measure-
3.2.12 pyrheliometer—see Terminology E 772 and ISO
ment of Radiation” Guide to Meteorological Instruments
9060.
and Methods of Observation, fifth ed., WMO-No. 8,
3.2.13 pyrheliometer, absolute (self-calibrating)—a solar
Geneva
radiometer in a pyrheliometer configuration having a field of
2.3 ISO Standards:
view of approximately 5° and a slope angle of from 0.75 to
ISO 9060:1990 Solar Energy - Specification and Classifica-
0.8°, and possessing a blackened conical cavity receiver for
tion of Instruments for Measuring Hemispherical Solar
absorption of the incident radiation; the measured electrical
and Direct Solar Radiation
power to a heater wound around the cavity receiver constitutes
ISO 9846:1993 Solar Energy - Calibration of a Pyranometer the method of self-calibration from first principles and trace-
Using a Pyrheliometer ability to absolute SI units; the self-calibration principle relates
to the sensing of the temperature rise of the receiving cavity by
an associated thermopile when first the sun is incident upon the
3. Terminology
receiver and subsequently when the same thermopile signal is
3.1 Definitions:
induced by applying precisely measured power to the heater
3.1.1 See Terminology E 772.
with the pyrheliometer shuttered from the sun.
3.2 Definitions of Terms Specific to This Standard:
3.2.14 shading-disk device—a device which allows move-
3.2.1 altazimuth mount—a tracking mount capable of rota-
ment of a disk in such a way that the receiver of the
tion about orthogonal altitude and azimuth axes; tracking may
pyranometer to which it is affixed, or associated, is shaded
be manual or by a follow-the-sun servomechanism.
from the sun with the cone formed between the origin of the
3.2.2 calibration of a radiometer—determination of the
receiver and the disk representing a subtendence of the sun that
responsivity (or the calibration factor, as its reciprocal) of a
closely matches the field of view of the pyrheliometer against
radiometer under well-defined measurement conditions.
3.2.3 direct solar radiation—that component of solar radia- which it is compared.Alternatively, and increasingly preferred,
tion within the solid angle subtended at the observer by the is the use of a sphere rather than a disk; use of a sphere
sun’s solar disk plus arbitrarily defined portion of the circum- eliminates the need to continuously ensure the proper align-
solar radiation. ment of the disk normal to the sun.
3.2.4 diffuse solar radiation—that component of solar ra-
diation reaching the earth as a result of being scattered by the 4. Significance and Use
air molecules, aerosol particles, cloud and other particles.
4.1 The pyranometer is a radiometer designed to measure
3.2.5 equitorial mount—see Terminology E 772.
the sum of directly solar radiation and sky radiation in such
3.2.6 field of view angle of a pyrheliometer—full angle of
proportions as solar altitude, atmospheric conditions and cloud
the cone which is defined by the center of the receiver surface
cover may produce. When tilted to the equator, pyranometers
(see ISO 9060, 5.1) and the border of the aperture, if the latter
measureonlyhemisphericalradiationfallingintheplaneofthe
are circular and concentric to the receiver surface; if not,
radiation receptor.
effective angles may be calculated.
4.2 This test method represents the only practical means for
3.2.7 global solar radiation—combined direct and diffuse
calibration of a reference pyranometer. While the sun-trackers,
solar radiation falling on a horizontal surface; solar radiation
theshadingdisk,thenumberofinstantaneousreadings,andthe
incident on a horizontal surface from the hemispherical sky
electronic display equipment used will vary from laboratory to
dome, or from 2p Sr.
laboratory, the method provides for the minimum acceptable
3.2.8 hemispherical radiation—combined direct and diffuse
conditions, procedures and techniques required.
solar radiation incident from a virtual hemisphere, or from 2p
Sr, on any inclined surface. 4.3 While, in theory, the choice of tilt angle is unlimited, in
practice, satisfactory precision is achieved over a range of tilt
3.2.8.1 Discussion—The limiting case of a horizontal sur-
face is denoted global solar radiation (3.2.7). angles close to the zenith angles used in the field.
3.2.9 pyranometer—see Terminology E 772.
4.4 The at-tilt calibration as performed in the tilted position
3.2.10 pyranometer, field—a pyranometer essentially meet-
relates to a specific tilted position and in this position requires
ing WMO Second Class or better (that is, First Class) appro-
no tilt correction. However, a tilt correction may be required to
priate to field use and typically exposed continuously.
relate the calibration to other orientations, including axis
3.2.11 pyranometer, reference—a pyranometer (see also
vertical.
ISO 9060), used as a reference to calibrate other pyranometers,
NOTE 1—WMOFistClasspyranometers,orbetter,generallyexhibittilt
errors of less than 1 % to tilts of 50° from the horizontal.
4.5 Traceability of calibrations to the World Radiometric
Available from American National Standards Institute, 11 W. 42nd St., 13th
Reference (WRR) is achieved through comparison to a refer-
Floor, New York, NY 10036.
ence absolute pyrheliometer that is itself traceable to the WRR
Angström, A. and Rodhe, B., “Pyrheliometric Measurements with Special
Regards to the Circumsolar Sky Radiation,” Tellus, XVII (1), 1966. through one of the following:
G167–00
4.5.1 One of the International Pyrheliometric Comparisons calibrating absolute cavity pyrheliometer and when the refer-
held in Davos, Switzerland in either 1990 (IPC VII) or 1995 ence pyranometer has itself been calibrated over a range of air
(IPC VIII). mass by the component summation (continuous shade)
4.5.2 Any like intercomparison held in the United States, method. Such a reference pyranometer must have been cali-
CanadaorMexicoandsanctionedbytheWorldMeteorological brated under conditions in which the continuously shaded
Organization as a Regional Intercomparison of Absolute Cav- pyranometer had been itself calibrated by the alternating shade
ity Pyrheliometers. method.
4.5.3 Intercomparison with any absolute cavity pyrheliom-
5.3 Comparison of the Alternating and Continuous Shade
eter that has participated in either and IPC or a WMO-
Methods:
sanctioned intercomparison within the past five years and
5.3.1 The disadvantage of the continuous, or component-
which was found to be within 6 0.4 % of the mean of all
summation shade method, is that two radiometers must be
absolute pyrheliometers participating therein.
employed as reference.a pyrheliometer and a continuously
4.6 The calibration method employed in this test method
shaded pyranometer.
assumes that the accuracy of the values obtained are indepen-
5.3.2 The disadvantage of the component-summation
dent of time of year with the constraints imposed and by the
method is the complexity of the apparatus to effect a continu-
test instrument’s temperature compensation circuity (neglect-
ously moving, that is, tracking, shaded disk with respect to the
ing cosine errors).
reference pyranometer’s receiver.
5.3.3 The advantage of the component-summation method
5. Selection of Shade Method
is that any number of co-planer pyranometers may be cali-
5.1 Alternating Shade Method:
brated at the same time.
5.1.1 The alternating shade method is required for a primor-
5.3.4 Calibrations performed using the component-
dial calibration of the reference pyranometer used in the
summation method have the advantage of much lower uncer-
Continuous, Component-Summation Shade Method described
tainties under conditions of moderately high to high ratios of
in 5.2.
direct to diffuse solar radiation.
5.1.2 The pyranometer under test is compared with a
pyrheliometer measuring direct solar irradiance. The voltage NOTE 2—If an absolute pyrheliometer with a typical uncertainty of
0.5 % is used to measure the direct solar radiation when the direct
values from the pyranometer that correspond to direct solar
component is 80 % of the global radiation (as an example), and a
irradiance are derived from the difference between the mea-
pyranometer with an uncertainty of 4 % is used to measure 20 % of the
sured values of hemispherical solar irradiance and the diffuse
solar radiation, resultant uncertainties can be as low as 1.2 % (as opposed
solar irradiance. These values are measured periodically by
to nearly 4 % for the alternating shade method).
means of a movable sun shade disk. For the calculation of the
responsivity, the difference in the components of irradiance is
6. Interferences and Precautions
divided by the measured direct solar irradiance normal to the
6.1 Sky Conditions—The measurements made in determin-
receiver plane of the pyranometer.
ing the instrument constant shall be performed only under
5.1.3 For meteorological purposes, the solid angle from
conditions when the sun is unobstructed by clouds for an
which the scattered radiative fluxes that represent diffuse
incrementaldatatakingperiod.Theminimumacceptabledirect
radiation are measured shall be the total sky hemisphere,
solar irradiance on the tilted surface, given by the product of
excluding a small solid angle around the sun’s disk.
the pyrheliometric measurement and the cosine of the incident
5.1.4 In addition to the basic method, modifications of this
angle, shall be 80 % of the global solar irradiance. Also, no
method that are considered to improve the accuracy of the
cloudformationshallbewithin30°ofthesunduringtheperiod
calibration factors, but which require more operational expe-
that data are taken for record.
rience, are presented in Appendix X1.
6.2 Instrument Orientation Corrections—The irradiance
5.2 Continuous Sun-and-Shade Method (Component Sum-
calibration of a pyranometer is influenced by the tilt angle and
mation):
the azimuthal orientation of the instrument about its optical
5.2.1 The pyranometer is compared with two reference
axis. Orientation effects are minimized by using an altazimuth
radiometers, one of which is a pyrheliometer and the
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