1、BRITISH STANDARD BS7440:1991 ISO9059:1990 Method for Calibrating field pyrheliometers by comparison to a reference pyrheliometerBS7440:1991 This BritishStandard, having been prepared under the directionof the Refrigeration Heating and Air Conditioning Standards Policy Committee, waspublished under t
2、he authorityof the Standards Boardand comes into effect on 31May1991 BSI 01-2000 The following BSI references relate to the work on this standard: Committee reference RHE/25 Draft for comment88/78679 DC ISBN 0 580 19643 7 Committees responsible for this BritishStandard The preparation of this Britis
3、hStandard was entrusted by the Refrigeration Heating and Air Conditioning Standards Policy Committee (RHE/-) to Technical Committee RHE/25, upon which the following bodies were represented: Association for Consumer Research (ACRE) Association of Consulting Engineers British Gas plc British Precast C
4、oncrete Federation Ltd. Chartered Institution of Building Services Engineers Copper Development Association Cranfield Institute of Technology Department of the Environment Department of the Environment (Building Research Establishment) Design Council Institution of Gas Engineers International Solar
5、Energy Society National Centre for Alternative Technology Royal Institute of British Architects Solar Trade Association Swimming Pool and Allied Trades Association Ltd. University College Cardiff Water Byelaws Advisory Service Amendments issued since publication Amd. No. Date CommentsBS7440:1991 BSI
6、 01-2000 i Contents Page Committees responsible Inside front cover National foreword ii Introduction 1 1 Scope 1 2 Normative reference 1 3 Definitions 1 4 Calibration hierarchy of pyrheliometers 2 5 Calibration requirements 2 6 Calibration procedure 3 7 Uncertainty 4 8 Documentation 5 Annex A (infor
7、mative) Calculated percentages of circumsolar radiation contained in direct solar radiation 6 Annex B (informative) Bibliography 7 Table A.1 Circumsolar radiation values for various atmospheric conditions 6 Publication(s) referred to Inside back coverBS7440:1991 ii BSI 01-2000 National foreword This
8、 BritishStandard has been prepared under the direction of the Refrigeration Heating and Air Conditioning Standards Policy Committee and is identical with ISO9059:1990 “Solar energyCalibration of field pyrheliometers by comparison to a reference pyrheliometerr”, published by the International Organiz
9、ation for Standardization (ISO) and prepared by Technical Committee ISO/TC180, Solar heating, in which the UK played an active part. Cross-reference The Technical Committee has reviewed the provisions of ISO9060:1990, to which reference is made in the text, and has decided that they are acceptable f
10、or use in conjunction with this standard. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations.
11、 Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to8, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside f
12、ront cover.BS7440:1991 BSI 01-2000 1 Introduction This International Standard is one of a series of International Standards specifying methods and instruments for the measurement of solar radiation. Pyrheliometers are used to measure direct solar irradiance. The data collected are used for the deter
13、mination of the efficiency of concentrating collectors, the determination of the direct beam resource for concentrating solar energy devices as well as for determining their siting, sizing, etc., and the accurate determination of hemispherical solar radiation as a sum of the measured direct solar an
14、d diffuse solar radiation. The calibration hierarchy of pyrheliometers specified in this International Standard follows the scheme developed by the World Meteorological Organization (WMO) 1, and the classification and specification used are prescribed in ISO9060. During the elaboration of this Inter
15、national Standard, extensive reference was made to ASTM816-812. 1 Scope This International Standard describes the calibration of field pyrheliometers using reference pyrheliometers and sets out the calibration procedures and the calibration hierarchy for the transfer of the calibration. This Interna
16、tional Standard is mainly intended for use by calibration services and test laboratories to enable a uniform quality of accurate calibration factors to be achieved. 2 Normative reference The following standard contains provisions which, through reference in this text, constitute provisions of this I
17、nternational Standard. At the time of publication, the edition indicated was valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent edition of the standard indicated below.
18、 Members of IEC and ISO maintain registers of currently valid International Standards. ISO9060:1990, Solar energySpecification and classification of instruments for measuring hemispherical solar and direct solar radiation. 3 Definitions For the purposes of this International Standard, the following
19、definitions apply. 3.1 direct solar radiation radiation received from a small solid angle centred on the suns disc, on a given plane ISO9060:1990,3.3. 3.2 pyrheliometers radiometers designed for measuring the irradiance which results from the solar radiant flux from a well-defined solid angle the ax
20、is of which is perpendicular to the plane receiver surface NOTE 1It follows from this definition that pyrheliometers are used to measure direct solar radiation at normal incidence. Typical field-of-view angles of pyrheliometers range from5 to10 . Unlike the windowless instruments, the spectral respo
21、nsivity of field pyrheliometers is limited to the range approximately0,34m to34m, depending on the spectral transmittance of the window which protects the receiver surface. However, windowless instruments operate with a loss of energy of less than1% (seeISO9060:1990, note2 to3.3). 3.3 field pyrhelio
22、meters pyrheliometers which are designed and used for long-term field measurements of direct solar radiation. These pyrheliometers are weather-proofed. The receiver is protected from wind, dirt, rain, snow and insects by a window (made of quartz) or alternatively by a system ensuring strong ventilat
23、ion with air 3.4 reference pyrheliometers pyrheliometers of any category serving as a reference in calibration procedures. They are selected and well-tested instruments (seeISO9060:1990, Table2), which have a low rate of yearly change in responsivity. They are controlled on a routine basis by compar
24、isons with other reference pyrheliometers NOTE 2Generally, reference pyrheliometers are operated without protective windows. To achieve the above-mentioned stability of the responsitivity, the use of a reference pyrheliometer should be restricted to comparisons and calibration activities. The instru
25、ment should be stored carefully in a laboratory under moderate ambient conditions. 3.5 primary standard pyrheliometers pyrheliometers, selected from the group of absolute pyrheliometers (also called absolute radiometers), which meet the requirements of ISO9060:1990,5.3.1BS7440:1991 2 BSI 01-2000 3.6
26、 secondary standard pyrheliometers pyrheliometers of high precision and stability whose calibration factors are derived from primary standard pyrheliometers. The group comprises absolute pyrheliometers, which do not fulfil the requirements of primary standard pyrheliometers, as well as compensation
27、pyrheliometers and some other precise but out-dated instruments such as the silver disk pyrheliometer (Seealso ISO9060:1990,5.3.2.) 3.7 first class and second class pyrheliometers pyrheliometers of a lower stability and accuracy than secondary standard pyrheliometers. Generally the group of field py
28、rheliometers belongs to this category (Seealso ISO9060:1990,5.3.2.) NOTE 3These instruments are sometimes used as so-called “working standards” because they are simple to operate and are less dependent on weather conditions than the higher category instruments. Such working standards should be compa
29、red with pyrheliometers of higher category as frequently as possible. 4 Calibration hierarchy of pyrheliometers The following calibration hierarchy of pyrheliometers shall apply. All pyrheliometers shall be referred to the World Radiometric Reference (WRR) (seealso ISO9060:1990,3.6). Primary standar
30、d pyrheliometers shall be referred to the WRR by comparison with selected groups of well-maintained primary standards (seeISO9060:1990,5.3.1). Primary standard pyrheliometers shall be used as reference for the calibration of secondary standard pyrheliometers and may be used as reference for first cl
31、ass and second class pyrheliometers. The reference for calibration of any pyrheliometer in first class or second class categories shall be a pyrheliometer in the same or higher category. If the reference pyrheliometer and the pyrheliometer being calibrated are in the same category, they shall be fro
32、m the same manufacturer and shall be of the same model, and the reference pyrheliometer shall have been calibrated using a higher category pyrheliometer as its reference. NOTE 4It is recommended that more than one reference instrument be used in the case where the reference is of the same category a
33、s the pyrheliometer being calibrated. 5 Calibration requirements The calibration of a field pyrheliometer by means of a reference pyrheliometer is accomplished by exposing the two instruments to the same radiation field and comparing their corresponding measuring signals. The calibration shall meet
34、the requirements with respect to the choice of the radiation source, the limits of meteorological variables acceptable during the calibration procedure, and the choice of measuring equipment. 5.1 Radiation source Pyrheliometers shall be exposed to the radiation field comprising direct solar radiatio
35、n and parts of the circumsolar radiation. The irradiance should be not less than300Wm 2 , but irradiance values exceeding700Wm 2are preferred. The calibration conditions, in terms of the circumsolar contribution, shall be as close as possible to the routine measuring conditions in which the field py
36、rheliometer is used (see5.2.3). 5.2 Meteorological variables 5.2.1 Wind speed The wind speed during the calibration should be low, particularly when the wind is blowing from the direction of the suns azimuth ( 30 ). NOTE 5Because of wind cooling, pyrheliometers with open tubes yield lower measuring
37、values with a higher standard deviation. The magnitude of this effect depends on the type of pyrheliometer, and especially on the design of the diaphragms in the tube. The wind-cooling effect may be reduced by installing wind screens. For instance, it may be beneficial to carry out the measurement f
38、rom a balcony or an open window. A tolerable maximum wind speed for unprotected measurement conditions cannot be specified. 5.2.2 Ambient air temperature In order to determine the temperature dependence of the calibration factor, if it is not already known from laboratory tests, the calibration shou
39、ld be carried out over a range of ambient air temperatures covering a large part of the temperature range which is typical for the field application. 5.2.3 Sky conditions During calibration, clouds should have an angular distance from the sun of greater than15 . Generally, good calibration condition
40、s exist when the cloud cover is less than12,5%. NOTE 6Contrails of aeroplanes that are within15 of the sun may be tolerated if the number of disturbed instantaneous measurements is small compared with the number of undisturbed measurements in a series (see6.3.2).BS7440:1991 BSI 01-2000 3 NOTE 7Atmos
41、pheric water vapour in the pre-condensation phase occasionally causes variable atmospheric transmission. Generally, the scattering of measuring data which is produced by these clusters is acceptable. The atmospheric turbidity during calibration should be close to values typical for the field measuri
42、ng conditions. Generally, the turbidity should be confined to conditions with Linke turbidity factors lower than6 (seeISO9060:1990,3.7). NOTE 8Atmospheric turbidity is produced by scattering and absorption of direct solar radiation by aerosol particles and gases including water vapour. The circumsol
43、ar radiation (aureole) originates from forward scattering of direct solar radiation. It decreases from the limb of the sun to an angular distance of about15 by some orders of magnitude, depending on the type and concentration of the aerosol (see, for example, 3, 4 and 5). The typical amount of circu
44、msolar radiation within an angular distance of5 from the sun represents only a few per cent of the direct solar radiation. If standard and field pyrheliometers have different field-of-view angles, the aerosol may strongly influence the accuracy of calibration. Calculated percentages of circumsolar c
45、ontained in direct solar radiation, for different aerosol types and solar elevation angles 4, are given for information in Table A.1. 5.3 Measuring equipment 5.3.1 Reference pyrheliometer The reference pyrheliometer should be selected in accordance with the calibration hierarchy given in clause4. Fu
46、rthermore, it should have a field-of-view angle and a slope angle (seeISO9060:1990,5.1) similar to those of the field pyrheliometer. 5.3.2 Sun tracker Sun trackers delivering separate movements in elevation and azimuth (altazimuth mount) as well as trackers turning the pyrheliometer in parallel with
47、 the solar equatorial plane (equatorial or parallactic mount) may be used. Pyrheliometers with a rotationally non-symmetric sensor should follow the sun without rotation of the receiver around the tube axis. Therefore, in this case, only altazimuth trackers shall be used. The admissible misalignment
48、 of the sun tracker shall be less than the slope angle of the pyrheliometer minus0,25 . 5.3.3 Data acquisition Digital voltmeters capable of at least0,05% resolution or the maximum pyrheliometer reading shall be used for the read-out of the pyrheliometer signals. They shall have an accuracy, stable
49、over at least1year and including temperature-generated drift, of better than 0,1%. NOTE 9When operating a digital voltmeter outdoors its accuracy may be influenced by the ambient temperature. Protection of the instrument from direct sunlight is recommended. When operating a digital voltmeter indoors, low-noise cabling should be employed owing to the length of the cable. The data logger system should have at least a four-channel capacity including the provision for the recording of temperatures. The read-out of the pyrheliometer signals
