Overview
ISO 9059:1990 - Solar energy: Calibration of field pyrheliometers by comparison to a reference pyrheliometer specifies procedures and a calibration hierarchy for transferring accurate calibration from reference pyrheliometers to field instruments. The standard is intended mainly for calibration services and test laboratories to achieve uniform, traceable calibrations of instruments that measure direct solar radiation at normal incidence.
Key topics and requirements
- Calibration hierarchy: Defines primary, secondary and reference pyrheliometer roles for traceable transfer of calibration (references ISO 9060:1990 classification).
- Instrument categories: Distinguishes primary, secondary, first-class, second-class and field pyrheliometers and recommends choosing a reference of equal or higher category.
- Measurement conditions: Guidance on acceptable meteorological variables:
- Irradiance generally not less than 300 W·m⁻2 (values above 700 W·m⁻2 preferred).
- Cloud cover preferably below 12.5%; clouds should be >15° from the sun.
- Atmospheric turbidity comparable to field conditions; Linke turbidity factor typically below 6 recommended.
- Wind speed considerations (wind cooling can bias open-tube instruments); use wind screens as needed.
- Calibration procedure: Preparation, alignment and equilibration (instruments warmed and stabilized ~30 min), sun-tracking alignment, cleaning windows of enclosed instruments, and performing series of measurements.
- Data sampling: minimum of 10 measuring series (preferably 28) of 10–20 minutes each, with at least 10 instantaneous readings per series (or integrated values).
- Synchronous readout within 1 s and appropriate integration limits (integration times ≤ ~20 min where applicable).
- Checks: zero point, polarity and reference voltages; reject disturbed data (e.g., deviations > ±2% in series).
- Uncertainty and documentation: The standard requires estimating calibration uncertainty and documenting calibration factors, environmental conditions, and measurement logs.
- Circumsolar radiation: Annex A provides calculated percentages of circumsolar radiation contributions for different aerosol types and solar elevations - important because different field and reference instruments have different fields-of-view.
Applications and users
ISO 9059:1990 is practical for:
- Calibration laboratories and national metrology institutes
- Solar resource assessment teams and meteorological services
- Manufacturers and maintenance teams for pyrheliometers and concentrating solar collectors
- Researchers conducting precision solar irradiance measurements
Typical uses include determining direct-beam solar resource for concentrating systems, assessing concentrating collector efficiency, and producing accurate hemispherical solar radiation by combining direct and diffuse measurements.
Related standards
- ISO 9060:1990 - Specification and classification of instruments for measuring hemispherical and direct solar radiation (referenced for instrument classes).
- World Meteorological Organization (WMO) calibration schemes and historical references (e.g., ASTM methods cited in the development notes).
Keywords: ISO 9059:1990, pyrheliometer calibration, reference pyrheliometer, solar energy measurement, direct solar radiation, calibration hierarchy, calibration procedure, circumsolar radiation.
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