ASTM E816-15(2023)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E816 − 15 (Reapproved 2023)
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Accurate and precise measurement of the direct (beam) radiation component of sunlight are
required 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) the assessment of the direct beam for energy budget analyses, geographic mapping
of solar energy, and as an aid in the determination of the concentration of aerosol and particulate
pollution, and water vapor effects.
This test method requires calibration to the World Radiometric Reference (WRR), maintained by
the World Meteorological Organization (WMO), Geneva. The Intercomparison of Absolute Cavity
Pyrheliometers, also called Absolute Cavity Radiometers, on which the WRR depends, is covered by
procedures adopted by WMO and by various U.S. Organizations who occasionally convene such
intercomparisons for the purpose of transferring the WRR to the United States, and to maintaining the
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 μm to 10 μm
primary standard pyrheliometer, and the other is the transfer of
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
pyrheliometers. This test method prescribes the calibration
0.3 μm to 4 μm wavelength.
procedures and the calibration hierarchy, or traceability, for
transfer of the calibrations. 1.6 The primary and secondary reference pyrheliometers
NOTE 1—It is not uncommon, and is indeed desirable, for both the
shall not be field instruments and their exposure to sunlight
reference and field pyrheliometers to be of the same manufacturer and
shall be limited to calibration or intercomparisons. These
model designation.
reference instruments shall be stored in an isolated cabinet or
1.3 This test method is relevant primarily for the calibration
room equipped with standard laboratory temperature and
of reference pyrheliometers with field angles of 5° to 6°, using
humidity control.
as the primary reference instrument a self-calibrating absolute
NOTE 2—At a laboratory where calibrations are performed regularly, it
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
This test method is under the jurisdiction of ASTM Committee G03 on
using natural sunshine only.
Weathering and Durabilityand is the direct responsibility of Subcommittee G03.09
on Radiometry.
Current edition approved March 15, 2023. Published March 2023. Originally
approved in 1981. Last previous edition approved in 2015 as E816 – 15. DOI: Angstrom, A., and Rodhe, B., “Pyrheliometric Measurements with Special
10.1520/E0816-15R23. 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 − 15 (2023)
1.8 This standard does not purport to address all of the 3.1.4 field pyrheliometer, n—pyrheliometers that are de-
safety concerns, if any, associated with its use. It is the signed and used for long-term field measurements of direct
responsibility of the user of this standard to establish appro- solar radiation. These pyrheliometers are weatherproof and
priate safety, health, and environmental practices and deter- therefore possess windows, usually quartz, at the field aperture
mine the applicability of regulatory limitations prior to use. that pass all solar radiation in the range from 0.3 μm to 4 μm
1.9 This international standard was developed in accor- wavelength.
dance with internationally recognized principles on standard-
3.1.5 opening angle, n—with radius of field aperture de-
ization established in the Decision on Principles for the
noted by R and the distance between the field and receiver
Development of International Standards, Guides and Recom-
apertures denoted by l, the opening angle is defined for right
mendations issued by the World Trade Organization Technical
circular cones by the equation:
Barriers to Trade (TBT) Committee.
Z 5 tan R/l (1)
o
2. Referenced Documents
The field angle is double the opening angle.
2.1 ASTM Standards:
3.1.6 primary standard pyrheliometers, n—pyrheliometers,
E772 Terminology of Solar Energy Conversion
selected from the group of absolute pyrheliometers (see self-
E824 Test Method for Transfer of Calibration From Refer-
calibrating absolute cavity pyrheliometer).
ence to Field Radiometers
3.1.7 reference pyrheliometer, n—pyrheliometers of any
G90 Practice for Performing Accelerated Outdoor Weather-
category serving as a reference in calibration transfer proce-
ing of Materials Using Concentrated Natural Sunlight
dures. They are selected and well-tested instruments (see
G167 Test Method for Calibration of a Pyranometer Using a
Table 2 of ISO 9060), that have a low rate of yearly change in
Pyrheliometer
responsivity. The reference pyrheliometer may be of the same
2.2 ISO Standards:
type, class, and manufacturer as the field radiometers in which
ISO 9059 Calibration of Field Pyrheliometers by Compari-
case it is specially chosen for calibration transfer purposes and
son to a Reference Pyrheliometer
is termed a secondary standard pyrheliometer (see ISO 9060),
ISO 9060 Specification and Classification of Instruments for
or it may be of the self-calibrating cavity type (see self-
Measuring Hemispherical Solar and Direct Solar Radia-
calibrating absolute cavity pyrheliometer).
tion
3.1.8 secondary standard pyrheliometer, n—pyrheliometers
ISO TR 9673 The Instrumental Measurement of Sunlight for
of high precision and stability whose calibration factors are
Determining Exposure Levels
derived from primary standard pyrheliometers. This group
ISO 9846 Calibration of a Pyranometer Using a Pyrheliom-
comprises absolute cavity pyrheliometers that do not fulfill the
eter
requirements of a primary standard pyrheliometer as described
2.3 WMO Standard:
in 3.1.6.
Guide to Meteorological Instruments and Methods of
Observation, Seventh ed., WMO-No. 8 3.1.9 self-calibrating absolute cavity pyrheliometer, n—a
radiometer consisting of either a single- or dual-conical heated
3. Terminology
cavity that, during the self-calibration mode, displays the
power required to produce a thermopile reference signal that is
3.1 Definitions:
identical to the sampling signal obtained when viewing the sun
3.1.1 The relevant definitions of Terminology E772 apply to
with an open aperture. The reference signal is produced by the
the calibration method described in this test method.
thermopile in response to the cavity irradiance resulting from
3.1.2 absolute cavity pyrheliometer, n—see self-calibrating
heat supplied by a cavity heater with the aperture closed.
absolute cavity pyrheliometer.
3.1.10 slope angle, n—with radius of the sensor denoted by
3.1.3 direct radiation, direct solar radiation, and direct
r, the radius of the limiting aperture is denoted by R, and the
(beam) radiation, n—radiation received from a small solid
distance between aperture and sensor denoted by l, the slope
angle centered on the sun’s disk, on a given plane whose
angle equation is defined as:
normal (perpendicular to the plane) points to the center of the
sun’s disk (see ISO 9060). That component of sunlight is the
S 5 arcTan ~R 2 r!⁄l (2)
beam between an observer, or instrument, and the sun within a
3.2 Acronyms:
solid conical angle centered on the sun’s disk and having a total
3.2.1 ACR—Absolute Cavity Radiometer
included planar field angle of, for the purposes of this test
3.2.2 ANSI—American National Standards Institute
method, 5° to 6°.
3.2.3 ARM—Atmospheric RadiationMeasurement Program
3.2.4 DOE—Department of Energy
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
3.2.5 GUM—(ISO) Guide to Uncertainty in Measurements
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
3.2.6 IPC—International Pyrheliometer comparison
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. 3.2.7 ISO—International Standards Organization
Available from International Organization for Standardization (ISO), 1, ch. de
3.2.8 NCSL—National Council of Standards Laboratories
la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
5 3.2.9 NIST—National Institute of Standards and Technology
Available from World Meterological Organization, 7bis, avenue de la Paix, CP.
2300, CH-1211 Geneva 2, Switzerland, www.wmo.int. 3.2.10 NREL—National Renewable Energy Laboratory
E816 − 15 (2023)
3.2.11 PMOD—Physical Meteorological Observatory Da- measurements. With respect to time of year, the requirement
vos for normal incidence dictates a tile angle from the horizontal
3.2.12 SAC—Singapore Accreditation Council that is dependent on the sun’s zenith angle and, thus, the air
3.2.13 SINGLAS—Singapore Laboratory Accreditation Ser- mass limits for that time of year and time of day.
vice
3.2.14 UKAS—United Kingdom Accrediation Service 5. Interferences
3.2.15 WRC—World Radiation Center
5.1 Radiation Source—Transfer of calibration from refer-
3.2.16 WRR—World Radiometric Reference
ence to secondary standard or field pyrheliometers is accom-
3.2.17 WMO—World Meteorological Organization
plished by exposing the two instruments to the same radiation
field and comparing their corresponding measurands. The
4. Significance and Use
−2
direct irradiance should not be less than 300 W·m , but
4.1 Though the sun trackers employed, the number of −2
irradiance values exceeding 700 W·m is preferred.
instantaneous readings, and the data acquisition equipment
5.2 Sky Conditions—The measurements made in determin-
used will vary from instrument to instrument and from labo-
ing the instrument constant shall be performed only under
ratory to laboratory, this test method provides for the minimum
conditions when the sun is unobstructed by clouds for an
acceptable conditions, procedures, and techniques required.
incremental data-taking period. The most acceptable sky con-
4.2 While the greatest accuracy will be obtained when
ditions should be such that the direct irradiance is not less than
calibrating pyrheliometers with a self-calibrating absolute
80 % of the hemispherical irradiance measured with a pyra-
cavity pyrheliometer that has been demonstrated by intercom-
nometer aligned with its axis vertical and calibrated in accor-
parison to be within 60.5 % of the mean irradiance of a family
dance with Test Method G167. Also, no cloud formation may
of similar absolute instruments, acceptable accuracy can be
be within 15° of the sun during the period data are taken for
achieved by careful attention to the requirements of this test
record when either transferring calibration to a secondary
method when transferring calibration from a secondary refer-
standard pyrheliometer (to be used as a reference pyrheliom-
ence to a field pyrheliometer.
eter) from an absolute cavity pyrheliometer, or when transfer-
4.3 By meeting the requirements of this test method, trace- ring calibration from a secondary reference pyrheliometer to
field pyrheliometers. Generally, good calibration conditions
ability of calibration to the World Radiometric Reference
(WRR) can be achieved through one or more of the following exist when the cloud cover is less than 12.5 %.
recognized intercomparisons:
NOTE 3—Contrails of airplanes that are within 15° of the sun can be
4.3.1 International Pyrheliometric Comparison (IPC) VII,
tolerated providing the ratio of so affected measurements to unaffected
Davos, Switzerland, held in 1990, and every five years measurements is small in any series.
NOTE 4—Atmospheric water vapor in the pre-condensation phase
thereafter, and the PMO-2 absolute cavity pyrheliometer that is
occasionally causes variable atmospheric transmission. Generally, the
the primary reference instrument of WMO.
scattering of measuring data that is produced by these clusters is
4.3.2 Any WMO-sanctioned intercomparison of self-
acceptable.
calibrating absolute cavity pyrheliometers held in WMO Re-
5.2.1 The atmospheric turbidity during transfer of calibra-
gion IV (North and Central America).
tion should be close to values typical for the field measuring
4.3.3 Any sanctioned or non-sanctioned intercomparison
conditions. Generally, the turbidity should be confined to
held in the United States the purpose of which is to transfer the
conditions with Linke turbidity factors lower than six (see ISO
WRR from the primary reference absolute cavity pyrheliom-
9059 and ISO 9060).
eter maintained as the primary reference standard of the United
5.2.2 The circumsolar radiation (aureole) originates from
States by the National Oceanic and Atmospheric Administra-
7 forward scattering of direct solar radiation. It decreases from
tion’s Solar Radiation Facility in Boulder, CO.
the limb of the sun to an angular distance of about 15° by
4.3.4 Any future intercomparisons of comparable reference
several orders of magnitude, depending on the type and
quality in which at least one self-calibrating absolute cavity
2,8,9
concentration of the aerosol. The typical amount of circum-
pyrheliometer is present that participated in IPC VII or a
solar radiation within an angular distance of 5° of the sun
subsequent IPC, and in which that pyrheliometer is treated as
represents only a few percent of the direct solar radiation. If
the intercomparison’s reference instrument.
standard and field pyrheliometers have different field-of-view
4.3.5 Any of the absolute radiometers participating in the
angles, the aerosol may strongly influence the accuracy of the
above intercomparisons and being within 60.5 % of the mean
transfer of calibration. Calculated percentages of circumsolar
of all similar instruments compared in any of those intercom-
contained in direct solar radiation, for different aerosol types
parisons.
and solar elevation angles, are given for information in
4.4 The calibration transfer method employed assumes that
Appendix X1.
the accuracy of the values obtained are independent of time of
year and, within the constraints imposed, time of day of
Eiden, R., “ Calculations and Measurements of the Spectral Radiance of the
WRCD, “Results, Seventh International Pyrheliometer Comparisons,” Working Solar Aureole,” Tellus , Vol 20, No. 3, 1968, pp. 380–399.
Report No. XX, Swiss Meteorological Institute, Zurich, Switzerland, Month, 1991. Thomalla, E., Köpke, P., Müller, H., and Quenzel, H., “Circumsolar Radiation
Currently (2005) the TMI/Kendall Absolute Cavity Radiometer, SN 67502 and Calculated for Various Atmospheric Conditions,” Solar Energy, Vol 30, No. 6, 1983,
Eppley Laboratory Model AHF SN 28553. pp. 575–587.
E816 − 15 (2023)
5.3 Differences in Geometry—If the pyrheliometers being presented in this test method using a self-calibrating absolute
compared do not have similar opening angles, atmospheric cavity pyrheliometer as the primary standard.
turbidity will introduce errors into the calibration.
6.4 Digital Electronic Readout (for Data Acquisition), a
5.4 Wind Conditions—Wind conditions are known to affect digital voltmeter, or data logger, capable of resolution repeat-
instruments differently, particularly some self-calibrating abso- able to 0.05 % of the maximum pyrheliometer reading, with an
lute cavity pyrheliometers, particularly when the wind is input impedance with an input impedance of at least 1 MΩ and
blowing from the direction of the sun’s azimuth (630°). an uncertainty of 60.2 % of at least 1 MΩ. Data loggers must
Measurements affected by wind conditions should be rejected. have the capability of recording, or printing, a measurement
A tolerable maximum wind speed for unprotected measure- with a frequency of every 30 s, or better, and shall be stable
ment conditions cannot be specified. over a period of at least one year, including temperature-
generated drift, of better than 60.1 %.
NOTE 5—Pyrheliometers with open apertures will yield lower measured
6.4.1 The data logger should have at least a four-channel
values and a higher standard deviation under adverse wind conditions. The
capacity and shall have the capacity to synchronously capture
magnitude of these effects depends on the type and design of the
diaphragms in the pyrheliometer tube. Wind effects may be reduced by data from all channels within 1 s. If the data logger is capable
installing wind screens or insulating blankets around the tube, or both.
of delivering integrated pyrheliometer signals, the minimum
integration time shall not be longer than about 10 min.
6. Apparatus
NOTE 6—For a discussion of the various types of equipment, apparatus
7. Procedure
and instruments required in practicing this test method, reference is made
to ISO 9060, ISO TR 9673, and Zerlaut.
7.1 Transfer of Calibration from Primary to Secondary
6.1 Sun Tracker(s), whether a clock-driven equatorial Standard Reference Pyrheliometers:
mount or a servo-operated altazimuth mount, to maintain both
7.1.1 Mount the self-calibrating absolute cavity
the reference and the field (test) pyrheliometer normal to the pyrheliometer, hereinafter designated the primary standard,
sun for the entire test period. Equatorial and altazimuth and the secondary reference pyrheliometer to be calibrated,
astronomical and specially constructed sun-following mounts hereinafter designated the secondary standard, either together
may also be used. However, the admissible misalignment of or separately on one or two sun-tracking mounts. Ensure that
the sun tracker shall be maintained less than the slope angle (S) the alignment is within less than 0.5° from true in accordance
of the pyrheliometer minus 0.25°. with either 7.1.1.1 or 7.1.1.2. If separate mounts are used,
NOTE 7—For a discussion of the various types of equipment, apparatus
ensure that the distance between pyrheliometers is not greater
and instruments required in practicing this test method, reference is made
than 20 m.
to ISO 9060, ISO TR 9673, and Zerlaut.
NOTE 8—Large distances between instruments can influence results
6.2 Self-Calibrating Absolute Cavity Pyrheliometer, em-
because of the inhomogeneity of the direct irradiance due to structured
ployed as the primary standard pyrheliometer, when used as the
turbidity elements in the atmosphere.
reference pyranometer to calibrate secondary standard
7.1.1.1 If the tracker is an equitorial mount, align the
pyrheliometers, shall be selected in accordance with the criteria
trackers in azimuth to coincide with solar noon (south refer-
presented in 3.1.3 and the hierarchy presented in Annex A1.
ence) and in elevation to coincide with the local latitude. Align
6.3 Secondary Standard (Reference) Pyrheliometer, em-
the pyrheliometers coaxially with the solar line-of-sight using
ployed as the reference pyrheliometer for the purposes of
the diopters or other mechanisms provided.
transferring calibration to field pyrheliometers shall be of
7.1.1.2 If the tracker is an altazimuth mount, set the tracking
suitable quality in terms of linearity and stability of its
mechanism and ensure that the pyrheliometer is aligned with
instrument constant, sensitivity, and temperature compensation
its axis perpendicular to the plane of the tracking platform
that it meets or exceeds the specifications of a WMO High
using the diopters or other mechanisms provided.
Quality/ISO First Class Pyrheliometer in accordance with ISO
7.1.2 Connect each instrument to its respective, or common,
9060 and WMO No. 8.
digital voltmeter, or data logger. Check for electrical
6.3.1 The principal additional requirement is that it shall
continuity, sign of the signal, and normal signal strength and
have been calibrated within six months by the procedures
stability.
7.1.2.1 Allow at least 30 min for the pyrheliometers to reach
Zerlaut, G. A., “Solar Radiation Instrumentation,” Chapter 5, Solar Resources, temperature equilibrium with ambient. Allow a minimum 30
R. L. Hulstron, ed., MIT Press, Cambridge, MA, 1989, pp. 173–308.
min warm-up of the data logger and the control unit of the
Suitable trackers are manufactured by the Eppley Laboratories, Inc., 12
absolute cavity pyrheliometer prior to taking measurements.
Sheffield Ave., Newport, RI 02840, and Kipp and Zonen USA, 390 Central Avenue,
Ensure that both the data logger, or digital voltmeter, and the
Bohemia, NY 11716.
Suitable self-calibrating absolute cavity pyrheliometers are the Eppley Model
primary standard pyrheliometer’s control unit are shaded from
HF manufactured by The Epply Laboratories, Inc., Newport, RI 02840, the TMI
direct sunlight.
Mark VI manufactured by Technical Measurements, Inc. Box 838, LaCanada, CA
7.1.3 If required, install wind screens around and in front of
91011, and the PMO-6 and later series of absolute radiometers available from
the pyrheliometers, or an insulating blanket, to preclude
Compagnie Industrielle Radioelectrique, Switzerland.
Suitable secondary reference pyrheliometers are the Eppley Model NIP
high-velocity winds from impacting the aperture areas and
manufactured by The Eppley Laboratory (see Footnote 13), and the EKO Instru-
tubes of the pyrheliometers. This may be doubly important
ments Model MS-53 available from SC-International Inc., 346 W. Pine Valley
when performing calibration transfers under winter-time con-
Drive, Phoenix, AZ and EKO Instruments Trading Co., Ltd., 21-8 Hatagaya
1-chome, Shibuya-ku, Tokyo 151 JAPAN. ditions.
E816 − 15 (2023)
7.1.4 Using the diopters or sighting mechanisms provided, where:
perform alignment checks and adjustments of both pyrheliom-
E (i, j) = i
...