ASTM E816-15

4.1 Though the sun trackers employed, the number of instantaneous readings, and the data acquisition equipment used will vary from instrument to instrument and from laboratory to laboratory, this test method provides for the minimum acceptable conditions, procedures, and techniques required.

4.2 While the greatest accuracy will be obtained when calibrating pyrheliometers with a self-calibrating absolute cavity pyrheliometer that has been demonstrated by intercomparison to be within ±0.5 % of the mean irradiance of a family of similar absolute instruments, acceptable accuracy can be achieved by careful attention to the requirements of this test method when transferring calibration from a secondary reference to a field pyrheliometer.

4.3 By meeting the requirements of this test method, traceability of calibration to the World Radiometric Reference (WRR) can be achieved through one or more of the following recognized intercomparisons:

4.4 The calibration transfer method employed assumes that the accuracy of the values obtained are independent of time of year and, within the constraints imposed, time of day of measurements. With respect to time of year, the requirement for normal incidence dictates a tile angle from the horizontal that is dependent on the sun's zenith angle and, thus, the air mass limits for that time of year and time of day.

1.1 This test method has been harmonized with, and is technically equivalent to, ISO 9059.

1.2 Two types of calibrations are covered by this test method. One is the calibration of a secondary reference pyrheliometer using an absolute cavity pyrheliometer as the primary standard pyrheliometer, and the other is the transfer of calibration from a secondary reference to one or more field pyrheliometers. This test method prescribes the calibration procedures and the calibration hierarchy, or traceability, for transfer of the calibrations.

1.3 This test method is relevant primarily for the calibration of reference pyrheliometers with field angles of 5 to 6°, using as the primary reference instrument a self-calibrating absolute cavity pyrheliometer having field angles of about 5°. Pyrheliometers with field angles greater than 6.5° shall not be designated as reference pyrheliometers.

1.4 When this test method is used to transfer calibration to field pyrheliometers having field angles both less than 5° or greater than 6.5°, it will be necessary to employ the procedure defined by Angstrom and Rodhe.2

1.5 This test method requires that the spectral response of the absolute cavity chosen as the primary standard pyrheliometer be nonselective over the range from 0.3 to 10 μm wavelength. Both reference and field pyrheliometers covered by this test method shall be nonselective over a range from 0.3 to 4 μm wavelength.

1.6 The primary and secondary reference pyrheliometers shall not be field instruments and their exposure to sunlight shall be limited to calibration or intercomparisons. These reference instruments shall be stored in an isolated cabinet or room equipped with standard laboratory temperature and humidity control.

1.7 This test method is applicable to calibration procedures using natural sunshine only.