What is secondary standard in metrology?

In metrology, a secondary standard is a measurement standard whose value is derived from a primary standard. It's not directly traceable to a fundamental physical constant or a natural phenomenon like a primary standard is. Instead, its value is determined by comparison to a primary standard or another already calibrated secondary standard.

Think of it like this:

* Primary Standard: The ultimate source of accuracy, often based on a fundamental physical constant (e.g., the definition of the kilogram based on the Planck constant). This is rarely used directly for everyday measurements due to its fragility and complexity.

* Secondary Standard: A highly accurate instrument or artifact calibrated against the primary standard. It's used to calibrate working standards and other instruments.

* Working Standard: Used for everyday measurements. These are regularly checked against the secondary standard to ensure accuracy.

Essentially, secondary standards act as a bridge between the ultimate accuracy of primary standards and the practical needs of everyday measurement. They are more readily available and easier to handle than primary standards, allowing for wider dissemination of accurate measurement capabilities. However, they require periodic recalibration against a higher-level standard to maintain their accuracy.