ASTM E816-95

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 816 – 95
Standard Test Method for
Calibration of Pyrheliometers by Comparison to Reference
Pyrheliometers
This standard is issued under the fixed designation E 816; 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 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 G 90 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 greater than 6.5°, it will be necessary to employ the procedure
defined by Angstrom and Rodhe.
1.1 This test method has been harmonized with, and is
1.5 This test method requires that the spectral response of
technically equivalent to, ISO 9059.
the absolute cavity chosen as the primary standard pyrheliom-
1.2 Two types of calibrations are covered by this test
eter be nonselective over the range from 0.3 to 10-μm
method. One is the calibration of a secondary reference
wavelength. Both reference and field pyrheliometers covered
pyrheliometer using an absolute cavity pyrheliometer as the
by this test method shall be nonselective over a range from 0.3
primary standard pyrheliometer, and the other is the transfer of
to 4-μm wavelength.
calibration from a secondary reference to one or more field
1.6 The primary and secondary reference pyrheliometers
pyrheliometers. This test method proscribes the calibration
shall not be field instruments and their exposure to sunlight
procedures and the calibration hierarchy, or traceability, for
shall be limited to calibration or intercomparisons. These
transfer of the calibrations.
reference instruments shall be stored in an isolated cabinet or
NOTE 1—It is not uncommon, and is indeed desirable, for both the
room equipped with standard laboratory temperature and
reference and field pyrheliometers to be of the same manufacturer and
humidity control.
model designation.
NOTE 2—At a laboratory where calibrations are performed regularly, it
1.3 This test method is relevant primarily for the calibration
is advisable to maintain a group of two or three secondary reference
of reference pyrheliometers with field angles of 5 to 6°, using
pyrheliometers that are included in every calibration. These serve as
as the primary reference instrument a self-calibrating absolute
controls to detect any instability or irregularity in the standard reference
cavity pyrheliometer having field angles of about 5°. Pyrheli-
pyrheliometer.
ometers with field angles greater than 6.5° shall not be
1.7 This test method is applicable to calibration procedures
designated as reference pyrheliometers.
using natural sunshine only.
1.4 When this test method is used to transfer calibration to
field pyrheliometers having field angles both less than 5° or
2. Referenced Documents
2.1 ASTM Standards:
This test method is under the jurisdiction of ASTM Committee G-3 on
Durability of Nonmetallic Materialsand is the direct responsibility of Subcommittee
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 E 816 – 81. Last previous edition E 816 – 81. Regard to the Circumsolar Sky Radiation,” Tellus, Vol 18, 1966, pp. 25–33.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 816
E 772 Terminology Relating to Solar Energy Conversion They are selected and well-tested instruments (see Table 2 of
E 824 Test Method for Transfer of Calibration from Refer- ISO 9060), that have a low rate of yearly change in responsi-
ence to Field Radiometers bility. The reference pyrheliometer may be of the same type,
E 913 Method for Calibration of Reference Pyranometers class, and manufacturer as the field radiometers in which case
with Axis Vertical by the Shading Method it is specially chosen for calibration transfer purposes and is
E 941 Test Method for Calibration of Reference Pyranom- termed a secondary standard pyrheliometer (see ISO 9060), or
eters with Axis Tilted by the Shading Method it may be of the self-calibrating cavity type (see self-
G 90 Practice for Performing Accelerated Outdoor Weath- calibrating absolute cavity pyrheliometer).
ering of Nonmetallic Materials Using Concentrated Natu- 3.1.8 secondary standard pyrheliometer—pyrheliometers of
ral Sunlight high precision and stability whose calibration factors are
2.2 ISO Standards: derived from primary standard pyrheliometers. This group
ISO 9059 Calibration of Field Pyrheliometers by Compari- comprises absolute cavity pyrheliometers that do not fulfill the
son to a Reference Pyrheliometer requirements of a primary standard pyrheliometer as described
ISO 9060 Specification and Classification of Instruments in 3.1.6.
for Measuring Hemispherical Solar and Direct Solar 3.1.9 self-calibrating absolute cavity pyrheliometer—a ra-
Radiation diometer consisting of either a single- or dual-conical heated
ISO TR 9673 The Instrumental Measurement of Sunlight cavity that, curing the self-calibration mode, displays the
for Determining Exposure Levels power required to produce a thermopile reference signal that is
ISO 9846 Calibration of a Pyranometer Using a Pyrheliom- identical to the sampling signal obtained when viewing the sun
eter with an open aperture. The reference signal is produced by the
2.3 WMO Standard: thermopile in response to the cavity irradiance resulting from
Guide to Meteorological Instruments and Methods of Ob- heat supplied by a cavity heater with the aperture closed.
servation, Fifth ed., WMO-No. 8
4. Significance and Use
3. Terminology
4.1 Though the sun trackers employed, the number of
3.1 Definitions:
instantaneous readings, and the data acquisition equipment
3.1.1 The relevant definitions of Terminology E 772 apply
used will vary from instrument to instrument and from labo-
to the calibration method described in this test method.
ratory to laboratory, this test method provides for the minimum
3.1.2 absolute cavity pyrheliometer—see self-calibrating
acceptable conditions, procedures, and techniques required.
absolute cavity pyrheliometer.
4.2 While the greatest accuracy will be obtained when
3.1.3 direct radiation, direct solar radiation, and direct
calibrating pyrheliometers with a self-calibrating absolute
(beam) radiation—radiation received from a small solid angle
cavity pyrheliometer that has been demonstrated by intercom-
centered on the sun’s disk, on a given plane (see ISO 9060).
parison to be within 60.5 % of the mean irradiance of a family
That component of sunlight is the beam between an observer,
of similar absolute instruments, acceptable accuracy can be
or instrument, and the sun within a solid conical angle centered
achieved by careful attention to the requirements of this test
on the sun’s disk and having a total included planar field angle
method when transferring calibration from a secondary refer-
of, for the purposes of this test method, 5 to 6°.
ence to a field pyrheliometer.
3.1.4 field pyrheliometer—pyrheliometers that are designed
4.3 By meeting the requirements of this test method, trace-
and used for long-term field measurements of direct solar
ability of calibration to the World Radiometric Reference
radiation. These pyrheliometers are weatherproof and therefore
(WRR) can be achieved through one or more of the following
possess windows, usually quartz, at the field aperture that pass
recognized intercomparisons:
all solar radiation in the range from 0.3 to 4-μm wavelength.
4.3.1 International Pyrheliometric Comparison (IPC) VII,
3.1.5 opening angle—with radius of field aperture denoted
Davos, Switzerland, held in 1990, and every five years there-
by R and the distance between the field and receiver apertures
after, and the PMO-2 absolute cavity pyrheliometer that is the
denoted by l, the opening angle is defined for right circular
primary reference instrument of WMO.
cones by the equation:
4.3.2 Any WMO-sanctioned intercomparison of self-
Z 5 tan R/l (1) calibrating absolute cavity pyrheliometers held in WMO Re-
o
gion IV (North and Central America).
The field angle is double the opening angle.
4.3.3 Any sanctioned or non-sanctioned intercomparison
3.1.6 primary standard pyrheliometers—pyrheliometers,
held in the United States the purpose of which is to transfer the
selected from the group of absolute pyrheliometers (see self-
WRR from the primary reference absolute cavity pyrheliom-
calibrating absolute cavity pyrheliometer).
eter maintained as the primary reference standard of the United
3.1.7 reference pyrheliometer—pyrheliometers of any cat-
States by the National Oceanic and Atmospheric Administra-
egory serving as a reference in calibration transfer procedures.
tion’s Solar Radiation Facility in Boulder, CO.
Annual Book of ASTM Standards, Vol 12.02.
Annual Book of ASTM Standards, Vol 06.01.
5 7
Available from American National Standards Institute, 11 W. 42nd St., 13th WRCD, “Results, Seventh International Pyrheliometer Comparisons,” Working
Floor, New York, NY 10036. Report No. XX, Swiss Meteorological Institute, Zurich, Switzerland, Month, 1991.
6 8
Available from World Meterological Organization, Geneva, Switzerland. Currently the TMI/Kendall Absolute Cavity Radiometer, SN 67502.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 816
, ,
2 9 10
4.3.4 Any future intercomparisons of comparable reference concentration of the aerosol. The typical amount of
quality in which at least one self-calibrating absolute cavity circumsolar radiation within an angular distance of 5° of the
pyrheliometer is present that participated in IPC VII or a sun represents only a few percent of the direct solar radiation.
subsequent IPC, and in which that pyrheliometer is treated as If standard and field pyrheliometers have different field-of-
the intercomparison’s reference instrument. view angles, the aerosol may strongly influence the accuracy of
4.3.5 Any of the absolute radiometers participating in the the transfer of calibration. Calculated percentages of circum-
above intercomparisons and being within 60.5 % of the mean solar contained in direct solar radiation, for different aerosol
of all similar instruments compared in any of those intercom- types and solar elevation angles, are given for information in
parisons. Appendix X1.
4.4 The calibration transfer method employed assumes that 5.3 Differences in Geometry—If the pyrheliometers being
the accuracy of the values obtained are independent of time of compared do not have similar opening angles, atmospheric
year and, within the constraints imposed, time of day of turbidity will introduce errors into the calibration.
measurements. With respect to time of year, the requirement 5.4 Wind Conditions—Wind conditions are known to affect
for normal incidence dictates a tile angle from the horizontal instruments differently, particularly some self-calibrating abso-
that is dependent on the sun’s zenith angle and, thus, the air lute cavity pyrheliometers, particularly when the wind is
mass limits for that time of year and time of day. blowing from the direction of the sun’s azimuth (630°).
Measurements affected by wind conditions should be rejected.
5. Interferences
A tolerable maximum wind speed for unprotected measure-
ment conditions cannot be specified.
5.1 Radiation Source—Transfer of calibration from refer-
ence to secondary standard or field pyrheliometers is accom-
NOTE 5—Pyrheliometers with open apertures will yield lower measured
plished by exposing the two instruments to the same radiation
values and a higher standard deviation under adverse wind conditions. The
field and comparing their corresponding measurands. The
magnitude of these effects depends on the type and design of the
−2
diaphragms in the pyrheliometer tube. Wind effects may be reduced by
direct irradiance should not be less than 300 W·m , but
−2
installing wind screens or insulating blankets around the tube, or both.
irradiance values exceeding 700 W·m is preferred.
5.2 Sky Conditions—The measurements made in determin-
6. Apparatus
ing the instrument constant shall be performed only under
6.1 For a discussion of the various types of equipment,
conditions when the sun is unobstructed by clouds for an
apparatus and instruments required in practicing this test
incremental data-taking period. The most acceptable sky con-
method, reference is made to ISO 9060, ISO TR 9673, and
ditions should be such that the direct irradiance is not less than
Zerlaut.
80 % of the hemispherical irradiance measured with a pyra-
6.2 Sun Tracker(s), whether a clock-driven equitorial mount
nometer aligned with its axis vertical and calibrated in accor-
or a servo-operated altazimuth mount, to maintain both the
dance with Test Method E 913. Also, no cloud formation may
reference and the field (test) pyrheliometer normal to the sun
be within 15° of the sun during the period data are taken for
for the entire test period. Equitorial and altazimuth astro-
record when either transferring calibration to a secondary
nomical and specially constructed sun-following mounts may
standard pyrheliometer (to be used as a reference pyrheliom-
also be used
...