What a pyranometer class tells you, and what it does not
ISO 9060 classes describe the instrument. The quality of your data depends just as much on what happens after it is bolted down.
When a specification asks for a "Class A" pyranometer, it is referring to ISO 9060:2018, the standard that sorts pyranometers into classes A, B and C according to limits on response time, zero offsets, stability, directional response, temperature response and spectral error.
The two extra labels
The 2018 revision added two optional sub-categories that matter more than they first appear.
- Spectrally flat means the sensor responds evenly across the solar spectrum. Thermopile instruments under a glass dome usually qualify. This matters when the instrument is used as a reference or under changing sky conditions.
- Fast response means the reading settles in under half a second, which matters when tracking passing cloud.
A tender that asks for "Class A" without saying "spectrally flat" may be met by an instrument that is not suitable as a reference.
What the class does not cover
The class describes the instrument on the day it left the factory. It says nothing about the things that most often spoil a record in the field:
- A dome coated in dust or bird droppings
- A sensor that has drifted out of level
- Exhausted desiccant letting moisture inside
- A calibration certificate several years out of date
- A mast shading the sensor at low sun angles
What to write into a project
Alongside the instrument class, specify the cleaning schedule, who checks the level, the recalibration interval, and how gaps and suspect values will be flagged in the data. A well-kept Class B station will give a better record than a neglected Class A one.