Power Quality Analysers make measurements in accordance to 61000-4-30.  This International Standard means that different Power Quality meters use the same definitions and measurement techniques when recording power quality parameters such as sags/dips, swells, frequency, harmonics and flicker,  Class A and Class S compliant meters use a standard method and level of uncertainties for making measurements.

Older, non-compliant instruments often use their own definitions and we categorise these as Class B.  As Class B is manufacturer specific, instruments may produce very different readings for the same measurement.