http://enginova.com/true_rms_volts.htm

 

Modern Digital Multimeters Often Give an Incorrect Voltage Reading

By Doug Criner 

You might not be able to read the lettering above the LCD display in the above photo, but it’s labeled “True RMS Multimeter.”  However, like most other such meters, it measures the true RMS of a periodic waveform only under special conditions–namely, if the waveform is symmetric about the zero axis.