ASTM E816-95(2002)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:E816–95 (Reapproved 2002)
Standard Test Method for
Calibration of Pyrheliometers by Comparison to Reference
Pyrheliometers
This standard is issued under the fixed designation E816; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Accurateandprecisemeasurementofthedirect(beam)radiationcomponentofsunlightarerequired
in (1) the calibration of reference pyranometers by the shading disk or optical occluding methods, (2)
determination of the energy collected by concentrating solar collectors, including exposure levels
achieved in use of Practice G90 dealing with Fresnel-reflecting concentrator test machines, and (3)
theassessmentofthedirectbeamforenergybudgetanalyses,geographicmappingofsolarenergy,and
asanaidinthedeterminationoftheconcentrationofaerosolandparticulatepollution,andwatervapor
effects.
ThistestmethodrequirescalibrationtotheWorldRadiometricReference(WRR),maintainedbythe
World Meteorological Organization (WMO), Geneva. The Intercomparison of Absolute Cavity
Pyrheliometers, also calledAbsolute Cavity Radiometers, on which the WRR depends, is covered by
procedures adopted by WMO and by various U.S. Organizations who occasionally convene such
intercomparisonsforthepurposeoftransferringtheWRRtotheUnitedStates,andtomaintainingthe
WRR in the United States. These procedures are not covered by this test method.
1. Scope 1.4 When this test method is used to transfer calibration to
field pyrheliometers having field angles both less than 5° or
1.1 This test method has been harmonized with, and is
greater than 6.5°, it will be necessary to employ the procedure
technically equivalent to, ISO 9059.
defined by Angstrom and Rodhe.
1.2 Two types of calibrations are covered by this test
1.5 This test method requires that the spectral response of
method. One is the calibration of a secondary reference
the absolute cavity chosen as the primary standard pyrheliom-
pyrheliometer using an absolute cavity pyrheliometer as the
eter be nonselective over the range from 0.3 to 10-µm
primarystandardpyrheliometer,andtheotheristhetransferof
wavelength. Both reference and field pyrheliometers covered
calibration from a secondary reference to one or more field
by this test method shall be nonselective over a range from 0.3
pyrheliometers. This test method proscribes the calibration
to 4-µm wavelength.
procedures and the calibration hierarchy, or traceability, for
1.6 The primary and secondary reference pyrheliometers
transfer of the calibrations.
shall not be field instruments and their exposure to sunlight
NOTE 1—It is not uncommon, and is indeed desirable, for both the
shall be limited to calibration or intercomparisons. These
reference and field pyrheliometers to be of the same manufacturer and
reference instruments shall be stored in an isolated cabinet or
model designation.
room equipped with standard laboratory temperature and
1.3 Thistestmethodisrelevantprimarilyforthecalibration
humidity control.
of reference pyrheliometers with field angles of 5 to 6°, using
NOTE 2—At a laboratory where calibrations are performed regularly, it
as the primary reference instrument a self-calibrating absolute
is advisable to maintain a group of two or three secondary reference
cavity pyrheliometer having field angles of about 5°. Pyrheli-
pyrheliometers that are included in every calibration. These serve as
ometers with field angles greater than 6.5° shall not be
controls to detect any instability or irregularity in the standard reference
designated as reference pyrheliometers.
pyrheliometer.
1.7 This test method is applicable to calibration procedures
1 using natural sunshine only.
This test method is under the jurisdiction of ASTM Committee G3 on
DurabilityofNonmetallicMaterialsandisthedirectresponsibilityofSubcommittee
G03.09on Solar and Ultraviolet Radiation Measurement Standards.
Current edition approved April 15, 1995. Published July 1995. Originally Angstrom, A., and Rodhe, B., “Pyrheliometric Measurements with Special
published as E816–81. Last previous edition E816–81. Regard to the Circumsolar Sky Radiation,” Tellus, Vol 18, 1966, pp. 25–33.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E816–95 (2002)
2. Referenced Documents 3.1.6 primary standard pyrheliometers—pyrheliometers,
selected from the group of absolute pyrheliometers (see self-
2.1 ASTM Standards:
3 calibrating absolute cavity pyrheliometer).
E772 Terminology Relating to Solar Energy Conversion
3.1.7 reference pyrheliometer—pyrheliometers of any cat-
E824 Test Method for Transfer of Calibration from Refer-
3 egory serving as a reference in calibration transfer procedures.
ence to Field Radiometers
They are selected and well-tested instruments (see Table 2 of
E913 Method for Calibration of Reference Pyranometers
ISO 9060), that have a low rate of yearly change in responsi-
with Axis Vertical by the Shading Method
bility. The reference pyrheliometer may be of the same type,
E941 Test Method for Calibration of Reference Pyranom-
class, and manufacturer as the field radiometers in which case
eters with Axis Tilted by the Shading Method
it is specially chosen for calibration transfer purposes and is
G90 Practice for Performing Accelerated Outdoor Weath-
termed a secondary standard pyrheliometer (see ISO 9060), or
ering of Nonmetallic Materials Using Concentrated Natu-
it may be of the self-calibrating cavity type (see self-
ral Sunlight
calibrating absolute cavity pyrheliometer).
2.2 ISO Standards:
3.1.8 secondary standard pyrheliometer—pyrheliometersof
ISO 9059 Calibration of Field Pyrheliometers by Compari-
high precision and stability whose calibration factors are
son to a Reference Pyrheliometer
derived from primary standard pyrheliometers. This group
ISO 9060 Specification and Classification of Instruments
comprises absolute cavity pyrheliometers that do not fulfill the
for Measuring Hemispherical Solar and Direct Solar
requirements of a primary standard pyrheliometer as described
Radiation
in 3.1.6.
ISO TR 9673 The Instrumental Measurement of Sunlight
3.1.9 self-calibrating absolute cavity pyrheliometer—a ra-
for Determining Exposure Levels
diometer consisting of either a single- or dual-conical heated
ISO 9846 Calibration of a Pyranometer Using a Pyrheliom-
cavity that, curing the self-calibration mode, displays the
eter
powerrequiredtoproduceathermopilereferencesignalthatis
2.3 WMO Standard:
identicaltothesamplingsignalobtainedwhenviewingthesun
Guide to Meteorological Instruments and Methods of Ob-
with an open aperture.The reference signal is produced by the
servation, Fifth ed., WMO-No. 8 thermopile in response to the cavity irradiance resulting from
heat supplied by a cavity heater with the aperture closed.
3. Terminology
4. Significance and Use
3.1 Definitions:
4.1 Though the sun trackers employed, the number of
3.1.1 The relevant definitions of Terminology E772 apply
instantaneous readings, and the data acquisition equipment
to the calibration method described in this test method.
used will vary from instrument to instrument and from labo-
3.1.2 absolute cavity pyrheliometer—see self-calibrating
ratorytolaboratory,thistestmethodprovidesfortheminimum
absolute cavity pyrheliometer.
acceptable conditions, procedures, and techniques required.
3.1.3 direct radiation, direct solar radiation, and direct
4.2 While the greatest accuracy will be obtained when
(beam) radiation—radiation received from a small solid angle
calibrating pyrheliometers with a self-calibrating absolute
centered on the sun’s disk, on a given plane (see ISO 9060).
cavity pyrheliometer that has been demonstrated by intercom-
That component of sunlight is the beam between an observer,
parisontobewithin 60.5%ofthemeanirradianceofafamily
orinstrument,andthesunwithinasolidconicalanglecentered
of similar absolute instruments, acceptable accuracy can be
on the sun’s disk and having a total included planar field angle
achieved by careful attention to the requirements of this test
of, for the purposes of this test method, 5 to 6°.
method when transferring calibration from a secondary refer-
3.1.4 field pyrheliometer—pyrheliometers that are designed
ence to a field pyrheliometer.
and used for long-term field measurements of direct solar
4.3 By meeting the requirements of this test method, trace-
radiation.Thesepyrheliometersareweatherproofandtherefore
ability of calibration to the World Radiometric Reference
possess windows, usually quartz, at the field aperture that pass
(WRR) can be achieved through one or more of the following
all solar radiation in the range from 0.3 to 4-µm wavelength.
recognized intercomparisons:
3.1.5 opening angle—with radius of field aperture denoted
4.3.1 International Pyrheliometric Comparison (IPC) VII,
by R and the distance between the field and receiver apertures
Davos, Switzerland, held in 1990, and every five years there-
denoted by l, the opening angle is defined for right circular
after, and the PMO-2 absolute cavity pyrheliometer that is the
cones by the equation:
primary reference instrument of WMO.
Z 5tan R/l (1)
o
4.3.2 Any WMO-sanctioned intercomparison of self-
The field angle is double the opening angle.
calibrating absolute cavity pyrheliometers held in WMO Re-
gion IV (North and Central America).
4.3.3 Any sanctioned or non-sanctioned intercomparison
heldintheUnitedStatesthepurposeofwhichistotransferthe
Annual Book of ASTM Standards, Vol 12.02.
Annual Book of ASTM Standards, Vol 06.01.
Available from American National Standards Institute, 11 W. 42nd St., 13th
Floor, New York, NY 10036. WRCD,“Results,SeventhInternationalPyrheliometerComparisons,” Working
Available from World Meterological Organization, Geneva, Switzerland. Report No. XX, Swiss Meteorological Institute, Zurich, Switzerland, Month, 1991.
E816–95 (2002)
WRR from the primary reference absolute cavity pyrheliom- conditions. Generally, the turbidity should be confined to
etermaintainedastheprimaryreferencestandardoftheUnited conditions with Linke turbidity factors lower than six (see ISO
States by the National Oceanic and Atmospheric Administra- 9059 and ISO 9060).
tion’s Solar Radiation Facility in Boulder, CO. 5.2.2 The circumsolar radiation (aureole) originates from
4.3.4 Any future intercomparisons of comparable reference forward scattering of direct solar radiation. It decreases from
quality in which at least one self-calibrating absolute cavity the limb of the sun to an angular distance of about 15° by
pyrheliometer is present that participated in IPC VII or a several orders of magnitude, depending on the type and
, ,
subsequent IPC, and in which that pyrheliometer is treated as concentration of the aerosol. The typical amount of
the intercomparison’s reference instrument. circumsolar radiation within an angular distance of 5° of the
4.3.5 Any of the absolute radiometers participating in the sun represents only a few percent of the direct solar radiation.
above intercomparisons and being within 60.5% of the mean If standard and field pyrheliometers have different field-of-
of all similar instruments compared in any of those intercom- viewangles,theaerosolmaystronglyinfluencetheaccuracyof
parisons. the transfer of calibration. Calculated percentages of circum-
4.4 The calibration transfer method employed assumes that solar contained in direct solar radiation, for different aerosol
the accuracy of the values obtained are independent of time of types and solar elevation angles, are given for information in
year and, within the constraints imposed, time of day of Appendix X1.
measurements. With respect to time of year, the requirement 5.3 Differences in Geometry—If the pyrheliometers being
for normal incidence dictates a tile angle from the horizontal compared do not have similar opening angles, atmospheric
that is dependent on the sun’s zenith angle and, thus, the air turbidity will introduce errors into the calibration.
mass limits for that time of year and time of day. 5.4 Wind Conditions—Wind conditions are known to affect
instrumentsdifferently,particularlysomeself-calibratingabso-
5. Interferences
lute cavity pyrheliometers, particularly when the wind is
5.1 Radiation Source—Transfer of calibration from refer-
blowing from the direction of the sun’s azimuth (630°).
ence to secondary standard or field pyrheliometers is accom- Measurements affected by wind conditions should be rejected.
plished by exposing the two instruments to the same radiation
A tolerable maximum wind speed for unprotected measure-
field and comparing their corresponding measurands. The ment conditions cannot be specified.
−2
direct irradiance should not be less than 300 W·m , but
NOTE 5—Pyrheliometerswithopenapertureswillyieldlowermeasured
−2
irradiance values exceeding 700 W·m is preferred.
valuesandahigherstandarddeviationunderadversewindconditions.The
5.2 Sky Conditions—The measurements made in determin-
magnitude of these effects depends on the type and design of the
ing the instrument constant shall be performed only under
diaphragms in the pyrheliometer tube. Wind effects may be reduced by
conditions when the sun is unobstructed by clouds for an installing wind screens or insulating blankets around the tube, or both.
incremental data-taking period. The most acceptable sky con-
6. Apparatus
ditions should be such that the direct irradiance is not less than
6.1 For a discussion of the various types of equipment,
80% of the hemispherical irradiance measured with a pyra-
apparatus and instruments required in practicing this test
nometer aligned with its axis vertical and calibrated in accor-
method, reference is made to ISO 9060, ISO TR 9673, and
dance with Test Method E913.Also, no cloud formation may
Zerlaut.
be within 15° of the sun during the period data are taken for
6.2 Sun Tracker(s),whetheraclock-drivenequitorialmount
record when either transferring calibration to a secondary
or a servo-operated altazimuth mount, to maintain both the
standard pyrheliometer (to be used as a reference pyrheliom-
reference and the field (test) pyrheliometer normal to the sun
eter) from an absolute cavity pyrheliometer, or when transfer-
for the entire test period. Equitorial and altazimuth astro-
ring calibration from a secondary reference pyrheliometer to
nomical and specially constructed sun-following mounts may
field pyrheliometers. Generally, good calibration conditions
alsobeused.However,theadmissiblemisalignm
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