I recently purchased 2 little voltmeters they look like the sort that would go in a control or instrument panel they are connected with just 2 wires which provide the operating supply ( they light up green and red) however the green one states it will work between 20and 500 volts and the red one between 60 and 480 volts. When they are both on the green one indicates normally around 241 volts the red one shows 235 volts why the discrepancy I know it's not much but makes you wonder if one of them is lying. Secondly I've noticed that the green one tracks voltage changes faster than the red one and that a few times the green one jumps down to 238 then up to 241 multiple times while the red one stays the same and I think can see a slight flicker in my filament lamps when this is happening incidentally both meters are connected to the same plug a 2 pin 5 amp one
If you want a more accurate reading you need to buy a "true RMS" meter of high accuracy. This measures the exact voltage at many points of the waveform and then calculates the proper RMS value. There is still some inaccuracy in the voltage measurement inherent in the digitisation process, but it can be very small, perhaps 5 parts per million for a 6 1/2 digit meter. This all comes at a considerable price! All other kinds of meters are usually called average reading, RMS calibrated, which are sensitive to any waveform distortion from a pure sine wave, in other words harmonics. The true RMS meter can cope with a "crest value", typically 10 times when it calculates the resulting voltage, so large waveform distortions are accurately measured. Some Fluke multimeters are true RMS, which are labelled on the front.
If you want a more accurate reading you need to buy a "true RMS" meter of high accuracy. This measures the exact voltage at many points of the waveform and then calculates the proper RMS value. There is still some inaccuracy in the voltage measurement inherent in the digitisation process, but it can be very small, perhaps 5 parts per million for a 6 1/2 digit meter. This all comes at a considerable price! All other kinds of meters are usually called average reading, RMS calibrated, which are sensitive to any waveform distortion from a pure sine wave, in other words harmonics. The true RMS meter can cope with a "crest value", typically 10 times when it calculates the resulting voltage, so large waveform distortions are accurately measured. Some Fluke multimeters are true RMS, which are labelled on the front.
