Standards Development

Scope

The object of this document is to promote the uniform application of reliable methods to calibrate pyranometers by comparison to a reference pyrheliometer.

Three methods are specified in this document:The Alternating Sun and Shade Method uses a shading disc to alternately shade and unshade a pyranometer to compare with the tracking pyrheliometer. The test pyranometer can be horizontal, on a fixed tilt or tracking alongside the pyrheliometer.

The Continuous Sun and Shade Method requires a shaded calibrated reference pyranometer in addition to the reference pyrheliometer. The test pyranometer can be horizontal, on a fixed tilt or tracking alongside the reference pyrheliometer, but the reference pyranometer must be mounted in the same plane as the test pyranometer (most often on the horizontal).

The Collimation Tube Method requires the test pyranometer be mounted on a solar tracker and be equipped with a collimation tube designed to allow the test pyranometer to have the same geometric view as the reference pyrheliometer for a direct comparison of the two instruments.

In general, all pyranometers may be calibrated using the described methods provided that a proper uncertainty evaluation is performed. Unlike spectrally flat pyranometers, non-spectrally flat pyranometers have a sensitivity that strongly depends on the solar spectrum. Therefore, the calibration result may be valid under a more limited range of conditions.

The result of a calibration is the instrument sensitivity accompanied by an uncertainty. This document includes suggestions for uncertainty evaluation.