Power Quality Analysers should 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 which is defined in the latest edition of IEC61000-4-30:Edition 3.
Some older instruments used their own definitions and accuracys and were categorise as Class B in the now obsolete Edition 2 of the IEC61000-4-30 standard. As Class B allowed manufacturers to specify their own measurement accuracys, different manufacturers instruments might produce very different readings for the same measurement when directly compared with each other.