The Stetzer meter simply plugs into any standard UK socket and measures GS (Graham-Stetzer) units of dirty electricity in your mains electricity supply, displayed as a number in the range of 0-1999. Overreading is indicated by a single 1.
The meter functions by measuring the rate of change of voltage, in the frequency range 2kHz to 150kHz.
Less than 25 GS units = good
25 to 50 GS units = average
More than 50 = undesirable
*“Dirty Electricity” is a term used by electrical and electronic engineers to describe distortions in the mains supply which should a pure sine wave but often has interference from electronic power supplies.