GB/T 33865-2017 in English

1. Standard Overview

GB/T 33865—2017 "Calibration Method for Photosynthetically Active Pyrheometers" specifies the calibration conditions, methods and uncertainty assessment of photosynthetically active pyrheometers, and is applicable to the calibration of the sensitivity of hemispherical photosynthetically active pyrheometers. This standard was proposed by the National Meteorological Administration to ensure measurement accuracy and reliability.

2. Calibration Conditions

2.1 Environmental Conditions

• Open space with no obstructions;
• Sunny weather with a solar altitude angle of no less than $30^{\circ}$;
• Temperature range: $10^{\circ}C \sim 30^{\circ}C$, relative humidity ≤80%, wind speed ≤5 m/s.

2.2 Standard instrument and supporting equipment

The standard photosynthetically active pyranometer must meet the following requirements: uncertainty ≤ 6%, cosine response error ≤ 10%, azimuth response error ≤ 5%.

3. Calibration method

The calibration method is divided into two categories: the working-level standard pyranometer method and the spectroradiometer method. The former is used for the calibration of business-use pyranometers, and the latter is applicable to working-level standards.

3.1 Working-level standard method

1. Environmental conditions: sunny weather, solar altitude angle ≥$30^{\circ}$;
2. Instrument layout: the standard instrument and the instrument to be calibrated are placed on the same horizontal plane, with the terminal facing north;
3. Data acquisition: synchronous data acquisition, lasting $3~4$ hours, with an interval of $1$ minutes.

3.2 Spectroradiometer method

The irradiance integral value is measured by a spectroradiometer to calculate the sensitivity of the instrument to be calibrated. This method requires the use of an optical sensor equipped with a cosine corrector.

4. Uncertainty assessment of calibration results

Refer to JJF 1059.1-2012, by establishing a mathematical model and sensitivity coefficient analysis, the uncertainty components of each input quantity are integrated to calculate the synthetic standard uncertainty. When the relative expanded uncertainty is finally determined, the inclusion factor $k=2$ is usually selected, and the corresponding confidence probability is about 95%.

5. Implementation suggestions

• Regular calibration: The recalibration interval should be 2 years;
• Environmental monitoring: Ensure that the temperature, humidity and wind speed are within the specified range;
• Data processing: Strictly follow the formulas in the standard to calculate sensitivity and uncertainty to ensure measurement accuracy.