what is true rms mean
A true-rms device (rms = root mean square) is one of three tools that can measure alternating current (ac) or ac voltage: 1  True-rms digital multimeters (or clamp meter).2  Average-responding digital multimeter (or clamp meter). more sophisticated true-rms meter can accurately measure both pure waves and the more complex nonsinusoidal waves. Waveforms can be distorted by nonlinear loads such as variable speed drives or computers. An averaging meter attempting to measure distorted waves can be up to 40% low or 10% high in its calculations.