ASTM E824-94
(Test Method)Standard Test Method for Transfer of Calibration From Reference to Field Radiometers
Standard Test Method for Transfer of Calibration From Reference to Field Radiometers
SCOPE
1.1 The method described in this standard applies to the transfer of calibration from reference to field radiometers to be used for measuring and monitoring outdoor radiant exposure levels. This standard has been harmonized with ISO 9847.
1.2 This test method is applicable to field radiometers regardless of the radiation receptor employed, but is limited to radiometers having approximately 180° (2[pi] Steradian), field angles.
1.3 The calibration covered by this test method employs the use of natural sunshine as the source.
1.4 Calibrations of field radiometers may be performed at tilt as well as horizontal (at 0° from the horizontal to the earth). The essential requirement is that the reference radiometer shall have been calibrated at essentially the same tilt from horizontal as the tilt employed in the transfer of calibration.
1.5 The primary reference instrument shall not be used as a field instrument and its exposure to sunlight shall be limited to calibration or intercomparisons. Note 1-At a laboratory where calibrations are performed regularly it is advisable to maintain a group of two or three reference radiometers that are included in every calibration. These serve as controls to detect any instability or irregularity in the standard reference instrument.
1.6 Reference standard instruments shall be stored in a manner as to not degrade their calibration.
1.7 The method of calibration specified for total solar pyranometers shall be traceable to the World Radiometric Reference (WRR) through the calibration methods of the reference standard instruments (Method E913 and Test Method E941), and the method of calibration specified for narrow- and broad-band ultraviolet radiometers shall be traceable to the National Institute of Standards and Technology (NIST), or other internationally recognized national standards laboratories.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
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Standards Content (Sample)
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Designation: E 824 – 94
Standard Test Method for
Transfer of Calibration From Reference to Field
Radiometers
This standard is issued under the fixed designation E 824; 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 solar and solar ultraviolet irradiance are required in: (1)
the determination of the energy incident on surfaces and specimens during exposure outdoors to
various climatic factors that characterize a test site, (2) the determination of solar irradiance and
radiant exposure to ascertain the energy available to solar collection devices such as flat-plate
collectors, and (3) the assessment of the irradiance and radiant exposure in various wavelength bands
for meteorological, climatic and earth energy-budget purposes. The solar components of principal
interest include total solar radiant exposure (all wavelengths) and various ultraviolet components of
natural sunlight that may be of interest, including both total and narrow-band ultraviolet radiant
exposure.
This test method for transferring calibration from reference to field instruments is only applicable
to pyranometers and radiometers whose field angles closely approach 180° . instruments which
therefore may be said to measure hemispherical radiation, or all radiation incident on a flat surface.
Hemispherical radiation includes both the direct and sky (diffuse) geometrical components of sunlight,
while global solar irradiance refers only to hemispherical irradiance on a horizontal surface such that
the field of view includes all of the hemispherical sky dome.
For the purposes of this test method, the terms pyranometer and radiometer are used interchange-
ably.
1. Scope 1.5 The primary reference instrument shall not be used as a
field instrument and its exposure to sunlight shall be limited to
1.1 The method described in this standard applies to the
calibration or intercomparisons.
transfer of calibration from reference to field radiometers to be
used for measuring and monitoring outdoor radiant exposure
NOTE 1—At a laboratory where calibrations are performed regularly it
levels. This standard has been harmonized with ISO 9847. is advisable to maintain a group of two or three reference radiometers that
are included in every calibration. These serve as controls to detect any
1.2 This test method is applicable to field radiometers
instability or irregularity in the standard reference instrument.
regardless of the radiation receptor employed, but is limited to
radiometers having approximately 180° (2p Steradian), field
1.6 Reference standard instruments shall be stored in a
angles.
manner as to not degrade their calibration.
1.3 The calibration covered by this test method employs the
1.7 The method of calibration specified for total solar
use of natural sunshine as the source.
pyranometers shall be traceable to the World Radiometric
1.4 Calibrations of field radiometers may be performed at
Reference (WRR) through the calibration methods of the
tilt as well as horizontal (at 0° from the horizontal to the earth).
reference standard instruments (Method E 913 and Test
The essential requirement is that the reference radiometer shall
Method E 941), and the method of calibration specified for
have been calibrated at essentially the same tilt from horizontal
narrow- and broad-band ultraviolet radiometers shall be trace-
as the tilt employed in the transfer of calibration.
able to the National Institute of Standards and Technology
(NIST), or other internationally recognized national standards
laboratories.
This test method is under the jurisdiction of ASTM Committee G-3 on
Durability of Nonmetallic Materials and is the direct responsibility of Subcommittee
G3.09 on Solar and Ultraviolet Radiation Measurement Standards.
Current edition approved May 15, 1994. Published July 1994.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, 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.
E824–94
1.8 This standard does not purport to address all of the performed over several days duration and that data be taken in
safety concerns, if any, associated with its use. It is the early morning or late afternoon, as well as near solar noon.
responsibility of the user of this standard to establish appro-
NOTE 3—Transfer of calibration to both total and narrow-band ultra-
priate safety and health practices and determine the applica-
violet radiometers may require a larger number of measurement sequences
bility of regulatory limitations prior to use.
in order to account for spectral changes due to changing air mass both
early and late in the day, and to the loss of north-sky ultraviolet when
calibrating at tilts.
2. Referenced Documents
4.5 The data are mathematically ratioed, employing the
2.1 ASTM Standards:
instrument constant of the reference instrument to determine
E 913 Method for Calibration of Reference Pyranometers
the instrument constant of the radiometer being calibrated. The
With Axis Vertical by the Shading Method
E 941 Test Method for Calibration of Reference Pyranom- mean value and the standard deviation are determined.
eters With Axis Tilted by the Shading Method
5. Significance and Use
E 772 Terminology Relating to Solar Energy Conversion
5.1 The methods described represent the preferable means
G 113 Terminology Relating to Natural and Artificial
for calibration of field radiometers employing standard refer-
Weathering Test of Nonmetallic Materials
ence radiometers. Other methods involve the employment of
2.2 Other Standards:
an optical bench and essentially a point source of artificial
ISO 9847 Solar Energy —Calibration of Field Pyranom-
light. While these methods are useful for cosine and azimuth
eters by Comparison to a Reference Pyranometer
correction analyses, they suffer from foreground view factor
and directionality problems. Transfer of calibration indoors
3. Terminology
using artificial sources is not covered by this test method.
3.1 Definitions:
5.2 Traceability of calibration of global pyranometers is
3.1.1 See Terminology E 772 and G 113 for terminology
accomplished when employing the method using a reference
relating to this test method.
global pyranometer that has been calibrated, and is traceable to
the World Radiometric Reference (WRR). For the purposes of
4. Summary of Test Method
this test method, traceability shall have been established if a
4.1 Mount the reference radiometer, or pyranometer, and the
parent instrument in the calibration chain participated in either
field (or test) radiometers, or pyranometers, on a common
the International Pyrheliometric Comparison VI (IPC VI),
calibration table for horizontal calibration (Type A), on a tilted
Davos (held in October 1985), or IPC VII, Davos (held in
platform for calibration at tilt (Type B), or on an altazimuth or
November, 1990). Traceability of calibration of narrow- and
sun-pointing mount for normal-incidence calibration (Type C).
broad-band radiometers is accomplished when employing the
Adjust the height of the photoreceptor, or radiation receptor, of
method using a reference ultraviolet radiometer that has been
all instruments to a common elevation.
calibrated and is traceable to the National Institute of Standards
4.2 Ensure that the pyranometer’s, or radiometer’s, azimuth
and Technology (NIST), or other national standards organiza-
reference marks point in a common direction.
tions. See Zerlaut for a discussion of the WRR, the IPC’s and
their results.
NOTE 2—Current convention is to use the electrical connector as the
azimuth reference and to point it towards the equator and downward. The 5.2.1 The reference global pyranometer (for example, one
reasons are (1) this convention diminishes the possibility of moisture
measuring hemispherical solar radiation at all wavelengths)
intrusion into the connector, and (2) it ensures that instruments with
shall have been calibrated by the shading-disk method against
disparities in the hemispherical domes, or with domes not properly
one of the following instruments:
centered over the receptor, are not operated in such a manner that they
5.2.1.1 An absolute cavity pyrheliometer that participated in
amplify deviations from the cosine law.
one of the above IPC’s (and therefore possesses a WRR
4.3 For a transfer of calibration to a field instrument that
reduction factor),
will be used in a tilted position the following conditions must
5.2.1.2 A WMO First Class pyrheliometer that was cali-
be met: The reference instrument must have a calibration at the
brated by direct transfer from such an absolute cavity.
desired tilt angle; both instruments must be oriented at the tilt
5.2.2 Alternatively, the reference pyranometer may have
angle and facing the equator.
been calibrated by direct transfer from a World Meteorological
4.4 The analog voltage signal from each radiometer is
Organization (WMO) First Class pyranometer that was cali-
measured, digitized, and stored using a calibrated data-
brated by the shading-disk method against an absolute cavity
acquisition instrument, or system. A minimum of fifteen
pyrheliometer possessing a WRR reduction factor, or by direct
10-min measurement sequences are obtained, each sequence
transfer from a WMO Standard Pyranometer (see WMO’s
comprising a minimum of 21 instantaneous readings. It is
Guide WMO—No. 8 for a discussion of the classification of
preferable that a larger number of measurement sequences be
solar radiometers).
Zerlaut, G. A., “Solar Radiation Instrumentation,” Chapter 5 in Solar Re-
Annual Book of ASTM Standards, Vol 06.01. sources, The MIT Press, Cambridge, MA, 1989, pp. 173–308.
3 6
Annual Book of ASTM Standards, Vol 12.02. WMO—No. 8, “Guide to Meteorological Instruments and Methods of Obser-
Available from International Standards Organization (ISO), 1 Rue De Varem- vation,” Fifth Ed., World Meteorological Organization, Geneva, Switzer-
bre, Geneva, Switzerland CH-1211 20. land, 1983.
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.
E824–94
NOTE 4—Any of the absolute radiometers participating in the above
6.1.1 The spectral response of both the reference and test
intercomparisons and being within 60.5 % of the mean of all similar
radiometers must be identical.
instruments compared in any of those intercomparisons, shall be consid-
6.2 Sky Conditions—The measurements selected in deter-
ered suitable as the primary reference instrument.
mining the instrument constant shall be for periods of essen-
5.2.3 The reference ultraviolet radiometer, regardless of
tially uniform rates of change of radiation (either cloudless or
whether it measures total ultraviolet solar radiation, or narrow-
overcast conditions). Periods selected shall be for 10 to 20-min
band UV-A or UV-B radiation, or a defined narrow band
segments. Measurements selected under varying cloudy con-
segment of ultraviolet radiation, shall have been calibrated by
ditions may result in erroneous calibrations if the reference and
one of the following:
test radiometers possess significantly different response times
5.2.3.1 By comparison to a standard source of spectral
(see also 5.6).
irradiance that is traceable to NIST or to the appropriate
national standards organizations of other countries (using
7. Apparatus
appropriate filter factors),
7.1 Data Acquisition Instrument—A digital voltmeter or
5.2.3.2 By comparison to the spectral irradiance in the
data logger capable of repeatability to 0.1 % of average
appropriate wavelength band of a spectroradiometer that has
reading, and an uncertainty 0f 60.2 % with an input imped-
itself been calibrated against such a standard source of spectral
ance of at least 1 MV may be employed. Data loggers having
irradiance,
printout must be capable of a measurement frequency of at
5.2.3.3 By comparison to a spectroradiometer that has
least two per minute. A data logger having three-channel
participated in a regional or national Intercomparison of
capacity may be useful.
Spectroradiometers, the results of which are of reference
7.2 Fixed-Angle Calibration Table—A precision calibration
quality.
table required for all horizontal and fixed angle tilt tests that is
NOTE 5—The calibration of reference ultraviolet radiometers using a
level at 0° horizontal and that is adjustable in tilt over a suitable
spectroradiometer, or by direct calibration against standard sources of
range of angles from the horizontal.
spectral irradiance (for example, deuterium or 1000 W tungsten-halogen
7.3 Tracking Calibration Table—A precision calibration
lamps) are the subject of ASTM Standards under development.
table required for normal incident calibrations and capable of
5.3 The calibration method employed assumes that the
tracking the sun to within 60.5°.
accuracy of the values obtained are independent of time of year
within the constraints imposed by the test instrument’s tem-
8. Procedure
perature compensation (neglecting cosine errors). The method
8.1 Mount reference and test radiometers on a common
permits the determination of possible tilt effects on the sensi-
calibration table in sunlight. Adjust both instruments to a
tivity of the test instrument’s light receptor.
common elevation facing south for which a calibration value is
5.4 The principal advantage of outdoor calibration of radi-
available. Ensure that the azimuth reference marks point in a
ometers is that all types of radiometers are related to a single
common direction: Also ensure that the electrical connector is
reference under realistic irradiance conditions.
pointed down (to preclude moisture intrusion), and that it is
5.5 The principal disadvantages of the outdoor calibration
pointing to the equator (that is, south-facing in the northern
method are the time required and the fact that the natural
hemisphere) if used as the azimuth reference.
environment is not subject to control (but the calibrations
8.2 Connect both the reference and test instruments to their
therefore include all of the instrumental characteristics of both
respective, or common, data acquisition instrument, using low
the reference and test radiometers that are influenced simulta-
capacitance, shielded cable of at least 20 gage and
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