ASTM G167 – 00

Designation: G 167 – 00 Standard Test Method for Calibration of a Pyranometer Using a Pyrheliometer1 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 1.1 This test method covers an integration of Test Method E 913 dealing with the calibration of pyranometers with axis vertical and Test Method E 941 on calibration of pyranometers with axis tilted. This amalgamation of the two methods essentially harmonizes the methodology with ISO 9846. 1.2 This test method is applicable to all pyranometers regardless of the radiation receptor employed, and is applicable to pyranometers in horizontal as well as tilted positions. 1.3 This test method is mandatory for the calibration of all secondary standard pyranometers as defined by the World Meteorological Organization (WMO) and ISO 9060, and for any pyranometer used as a reference pyranometer in the transfer of calibration using Test Method E 842. 1.4 Two types of calibrations are covered: Type I calibra- tions employ a self-calibrating, absolute pyrheliometer, and Type II calibrations employ a secondary reference pyrheliom- eter as the reference standard (secondary reference pyrheliom- eters are defined by WMO and ISO 9060). 1.5 Calibrations of reference pyranometers may be per- formed by a method that makes use of either an altazimuth or equitorial tracking mount in which the axis of the radiometer’s radiation receptor is aligned with the sun during the shading disk test. 1.6 The determination of the dependence of the calibration factor (calibration function) on variable parameters is called characterization. The characterization of pyranometers is not specifically covered by this method. 1.7 This test method is applicable only to calibration pro- cedures using the sun as the light source. 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 appro- priate safety and health practices and determine the applica- bility of regulatory limitations prior to use. 2. Referenced Documents 2.1 ASTM Standards: E 772 Terminology Relating to Solar Energy Conversion2 E 824 Method for Transfer of Calibration from Reference to Field Radiometers3 E 913 Test Method for Calibration of Reference Pyranom- eters with Axis Vertical by the Shading Method3 1 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. Current edition approved Feb. 10, 2000. Published June 2000. 2 Annual Book of ASTM Standards, Vol 12.02. 3 Annual Book of ASTM Standards, Vol 14.02. 1 Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. E 941 Test Method for Calibration of Reference Pyranom- eters with Axis Tilted by the Shading Method3 2.2 WMO Document: World Meteorological Organization (WMO), “Measure- ment of Radiation” Guide to Meteorological Instruments and Methods of Observation, fifth ed., WMO-No. 8, Geneva4 2.3 ISO Standards: ISO 9060:1990 Solar Energy - Specification and Classifica- tion of Instruments for Measuring Hemispherical Solar and Direct Solar Radiation4 ISO 9846:1993 Solar Energy - Calibration of a Pyranometer Using a Pyrheliometer4 3. Terminology 3.1 Definitions: 3.1.1 See Terminology E 772. 3.2 Definitions of Terms Specific to This Standard: 3.2.1 altazimuth mount—a tracking mount capable of rota- tion about orthogonal altitude and azimuth axes; tracking may be manual or by a follow-the-sun servomechanism. 3.2.2 calibration of a radiometer—determination of the responsivity (or the calibration factor, as its reciprocal) of a radiometer under well-defined measurement conditions. 3.2.3 direct solar radiation—that component of solar radia- tion within the solid angle subtended at the observer by the sun’s solar disk plus arbitrarily defined portion of the circum- solar radiation. 3.2.4 diffuse solar radiation—that component of solar ra- diation reaching the earth as a result of being scattered by the air molecules, aerosol particles, cloud and other particles. 3.2.5 equitorial mount—see Terminology E 772. 3.2.6 field of view angle of a pyrheliometer—full angle of the cone which is defined by the center of the receiver surface (see ISO 9060, 5.1) and the border of the aperture, if the latter are circular and concentric to the receiver surface; if not, effective angles may be calculated.5 3.2.7 global solar radiation—combined direct and diffuse solar radiation falling on a horizontal surface; solar radiation incident on a horizontal surface from the hemispherical sky dome, or from 2p Sr. 3.2.8 hemispherical radiation—combined direct and diffuse solar radiation incident from a virtual hemisphere, or from 2p Sr, on any inclined surface. 3.2.8.1 Discussion—The limiting case of a horizontal sur- face is denoted global solar radiation (3.2.7). 3.2.9 pyranometer—see Terminology E 772. 3.2.10 pyranometer, field—a pyranometer essentially meet- ing WMO Second Class or better (that is, First Class) appro- priate to field use and typically exposed continuously. 3.2.11 pyranometer, reference—a pyranometer (see also ISO 9060), used as a reference to calibrate other pyranometers, which is well-maintained and carefully selected to possess relatively high stability and has been calibrated using a pyrheliometer. 3.2.12 pyrheliometer—see Terminology E 772 and ISO 9060. 3.2.13 pyrheliometer, absolute (self-calibrating)—a solar radiometer in a pyrheliometer configuration having a field of view of approximately 5° and a slope angle of from 0.75 to 0.8°, and possessing a blackened conical cavity receiver for absorption of the incident radiation; the measured electrical power to a heater wound around the cavity receiver constitutes the method of self-calibration from first principles and trace- 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 receiver and subsequently when the same thermopile signal is induced by applying precisely measured power to the heater with the pyrheliometer shuttered from the sun. 3.2.14 shading-disk device—a device which allows move- ment of a disk in such a way that the receiver of the pyranometer to which it is affixed, or associated, is shaded from the sun with the cone formed between the origin of the receiver and the disk representing a subtendence of the sun that closely matches the field of view of the pyrheliometer against which it is compared. Alternatively, and increasingly preferred, is the use of a sphere rather than a disk; use of a sphere eliminates the need to continuously ensure the proper align- ment of the disk normal to the sun. 4. Significance and Use 4.1 The pyranometer is a radiometer designed to measure the sum of directly solar radiation and sky radiation in such proportions as solar altitude, atmospheric conditions and cloud cover may produce. When tilted to the equator, pyranometers measure only hemispherical radiation falling in the plane of the radiation receptor. 4.2 This test method represents the only practical means for calibration of a reference pyranometer. While the sun-trackers, the shading disk, the number of instantaneous readings, and the electronic display equipment used will vary from laboratory to laboratory, the method provides for the minimum acceptable conditions, procedures and techniques required. 4.3 While, in theory, the choice of tilt angle is unlimited, in practice, satisfactory precision is achieved over a range of tilt angles close to the zenith angles used in the field. 4.4 The at-tilt calibration as performed in the tilted position relates to a specific tilted position and in this position requires no tilt correction. However, a tilt correction may be required to relate the calibration to other orientations, including axis vertical. NOTE 1—WMO Fist Class pyranometers, or better, generally exhibit tilt errors of less than 1 % to tilts of 50° from the horizontal. 4.5 Traceability of calibrations to the World Radiometric Reference (WRR) is achieved through comparison to a refer- ence absolute pyrheliometer that is itself traceable to the WRR through one of the following: 4 Available from American National Standards Institute, 11 W. 42nd St., 13th Floor, New York, NY 10036. 5 Angström, A. and Rodhe, B., “Pyrheliometric Measurements with Special Regards to the Circumsolar Sky Radiation,” Tellus, XVII (1), 1966. G 167 – 00 2 4.5.1 One of the International Pyrheliometric Comparisons held in Davos, Switzerland in either 1990 (IPC VII) or 1995 (IPC VIII). 4.5.2 Any like intercomparison held in the United States, Canada or Mexico and sanctioned by the World Meteorological Organization as a Regional Intercomparison of Absolute Cav- ity Pyrheliometers. 4.5.3 Intercomparison with any absolute cavity pyrheliom- eter that has participated in either and IPC or a WMO- sanctioned intercomparison within the past five years and which was found to be within 6 0.4 % of the mean of all absolute pyrheliometers participating therein. 4.6 The calibration method employed in this test method assumes that the accuracy of the values obtained are indepen- dent of time of year with the constraints imposed and by the test instrument’s temperature compensation circuity (neglect- ing cosine errors). 5. Selection of Shade Method 5.1 Alternating Shade Method: 5.1.1 The alternating shade method is required for a primor- dial calibration of the reference pyranometer used in the Continuous, Component-Summation Shade Method described in 5.2. 5.1.2 The pyranometer under test is compared with a pyrheliometer measuring direct solar irradiance. The voltage values from the pyranometer that correspond to direct solar irradiance are derived from the difference between the mea- sured values of hemispherical solar irradiance and the diffuse solar irradiance. These values are measured periodically by means of a movable sun shade disk. For the calculation of the responsivity, the difference in the components of irradiance is divided by the measured direct solar irradiance normal to the receiver plane of the pyranometer. 5.1.3 For meteorological purposes, the solid angle from which the scattered radiative fluxes that represent diffuse radiation are measured shall be the total sky hemisphere, excluding a small solid angle around the sun’s disk. 5.1.4 In addition to the basic method, modifications of this method that are considered to improve the accuracy of the calibration factors, but which require more operational expe- rience, are presented in Appendix X1. 5.2 Continuous Sun-and-Shade Method (Component Sum- mation): 5.2.1 The pyranometer is compared with two reference radiometers, one of which is a pyrheliometer and the other a well-calibrated reference pyranometer equipped with a track- ing shade disk to measure diffuse solar radiation. The reference pyranometer shall be either calibrated using the alternating sun-and shade method described in 5.1, or shall be compared against such a pyranometer in accordance with Test Method E 842. 5.2.2 Global solar irradiance (or hemispherical solar irradi- ance for inclined pyranometers) is determined by the sum of the direct solar irradiance measured with a pyrheliometer, and the diffuse solar irradiance measured with a shaded reference pyranometer. 5.2.3 The smallest uncertainty realized in the calibration of pyranometers will occur when the pyrheliometer is a self- calibrating absolute cavity pyrheliometer and when the refer- ence pyranometer has itself been calibrated over a range of air mass by the component summation (continuous shade) method. Such a reference pyranometer must have been cali- brated under conditions in which the continuously shaded pyranometer had been itself calibrated by the alternating shade method. 5.3 Comparison of the Alternating and Continuous Shade Methods: 5.3.1 The disadvantage of the continuous, or component- summation shade method, is that two radiometers must be employed as reference . . . a pyrheliometer and a continuously shaded pyranometer. 5.3.2 The disadvantage of the component-summation method is the complexity of the apparatus to effect a continu- ously moving, that is, tracking, shaded disk with respect to the reference pyranometer’s receiver. 5.3.3 The advantage of the component-summation method is that any number of co-planer pyranometers may be cali- brated at the same time. 5.3.4 Calibrations performed using the component- summation method have the advantage of much lower uncer- tainties under conditions of moderately high to high ratios of direct to diffuse solar radiation. NOTE 2—If an absolute pyrheliometer with a typical uncertainty of 0.5 % is used to measure the direct solar radiation when the direct component is 80 % of the global radiation (as an example), and a pyranometer with an uncertainty of 4 % is used to measure 20 % of the solar radiation, resultant uncertainties can be as low as 1.2 % (as opposed to nearly 4 % for the alternating shade method). 6. Interferences and Precautions 6.1 Sky Conditions—The measurements made in determin- ing the instrument constant shall be performed only under conditions when the sun is unobstructed by clouds for an incremental data taking period. The minimum acceptable direct solar irradiance on the tilted surface, given by the product of the pyrheliometric measurement and the cosine of the incident angle, shall be 80 % of the global solar irradiance. Also, no cloud formation shall be within 30° of the sun during the period that data are taken for record. 6.2 Instrument Orientation Corrections—The irradiance calibration of a pyranometer is influenced by the tilt angle and the azimuthal orientation of the instrument about its optical axis. Orientation effects are minimized by using an altazimuth platform and mounting the pyranometer with the cable con- nection mounted downward. When calibrating a pyranometer with its axis vertical, the sun angle changes through a range of azimuths. Hence, the azimuthal angle between the sun and the direction of the cable connector or other reference mark may be significant. 6.3 Cosine Corrections—This test method permits the pyra- nometer to be tested either with axis vertical (with the pyranometer mounted in an exactly horizontal plane), or with the axis directed toward the sun by employing an altazimuth platform. With the pyranometer’s axis vertical, the zenith and incident angles are the same and never smaller than z 5 L – d (1) G 167 – 00 3 where: z = the zenith (or incident angle), L = the latitude of the site, and d = the solar declination for the day. 6.3.1 The range of minimum incident angles available for test due to the range of latitudes available in the continental U.S. is 2.4 and 24.6° at the summer solstice, and 49.2 and 71.4° at the winter solstice, for Miami and Seattle, respectively. The flux calibration is derived from flux measurements made at incident angles of convenience but referred to the value the calibration would have if the measured flux were incident along the pyranometer axis. Therefore, since each calibration involves the cosine and azimuth correction of the pyranometer at each incident angle, the accuracy of the calibration is limited by the cosine and azimuth correction uncertainty. 6.3.2 When the pyranometer is calibrated with its axis pointing toward the sun, there are no cosine errors either during calibration or during use as a transfer instrument in the tilted mode. The incident angles and hence the cosine corrections are small in most applications and essentially can be ignored. 6.3.3 When the pyranometer is calibrated at a fixed tilt from the horizontal (and at a fixed azimuth direction), the calibration factor includes the instrument constant and the cosine and azimuth correction of the pyranometer at each incident angle. The accuracy of the calibration is therefore limited by the cosine and azimuth correction uncertainty. 6.4 Environmental Conditions—Under general conditions of both calibration and use, the pyranometer signal is a function of many parameters which may affect calibration factors or data derived from use to a significant degree. Many of these parameters are beyond the scope of this test calibration method and the control of the practitioner. 6.5 Reference Radiometers—Both the reference pyrheliom- eter or pyranometer(s) shall not be used as a field instrument and its exposure to sunlight shall be limited to calibration or to intercomparisons. NOTE 3—At a laboratory where an absolute cavity pyrheliometer is not available, it is advisable to maintain a group of two or there pyrheliom- eters which are included in every calibration. These serve as controls to detect any instability or irregularity in any of the reference instruments. It is also advisable to maintain a set of two or three reference pyranometers for the same reasons. 6.5.1 Reference radiometers shall be stored in such a manner as to not degrade their calibration. Exposure to excessive temperature or humidity can cause instrumental drift. 6.5.2 The distance between the reference radiometer(s) and the field pyranometer(s) being calibrated shall be no more than 30 m, otherwise both the reference and field radiometers may not be similarly affected by the same atmospheric events such as, for example, structured turbidity elements. 6.6 Physical Environment—Precautions shall be taken to ensure that the horizon is substantially free of natural or manmade objects that obscure more than 5 % of the sky at the horizon. Special emphasis shall be given to ensure that any objects that do exist above the horizon do not reflect sunlight onto the calibration facility. When calibrating at tilt angles from the horizontal, the foreground shall be selected so as not reflect sunlight onto the test facility from materials, objects or the ground (for example, snow, sand, etc.). 6.6.1 During calibration, wind conditions are also impor- tant, since absolute cavity pyrheliometers operating with open tubes are disturbed by strong wind speeds, especially gusts coming from the sun’s azimuthal direction. Under such condi- tions, it may be necessary to operate with wind screen