found this on the WWW which covers what you need to know.
**True RMS vs average**
One important element of a digital multimeter specification is whether the DMM is able to measure true RMS values. Today many high end digital multimeters include this facility, although the lower end instruments will typically not incorporate it. True RMS measurements provide a measure of the actual heating effect of the waveform. The reading will therefore take account of the AC waveform, although with any harmonics and distortion, as well as any DC components. It is worth looking carefully at the DMM spec to ascertain whether it does provide a true RMS measurement, and if so whether it is able to incorporate any DC components that may be present.
Those digital multimeters that do not provide a true RMS measurement measure the average of the absolute value of AC voltage and are calibrated so that the readings are corrected to that of the RMS value for a sine wave. This normally works well but errors occur if harmonics are present with the effect becoming progressively worse as the harmonic content increases. For example, if a triangular waveform is measured this will give an error of nearly -4%, while for a square wave signal the error is just over +11%. The presence of any DC will then introduce further errors. Pulse waveforms will also be difficult to measure and the error will depend upon the duty cycle of the signal.
In many instances an average reading calibrated for RMS values is quite satisfactory. However it is necessary to choose the correct DMM to buy new or as used test equipment, or when choosing test equipment rental. It also helps when interpreting the results that are obtained if the limitations of the digital multimeter are fully understood.
Cheers,
Richard
