An average responding meter assumes a sinusoidal wave and calculates the average of the highest and lowest parts of that wave, then multiplies it by 1.11, or by the conversion factor. If a waveform is distorted or uneven, this average measurement will be higher or lower than it should be — up to 40% low or 10% high depending on the waveform (see table).
Below are a few scenarios with nonsinusoidal waves — where current occurs in short pulses, rather than smooth waves. In these situations, you’ll want to measure with a true-RMS meter:
- Variable-frequency drives (VFDs).
- Electronic ballasts.
- HVAC control systems.
- Solid-state environments.
How a True-RMS Meter Works
A true-RMS meter can accurately measure those imperfect, nonsinusoidal waves, as well as perfect, sinusoidal waves. “RMS” stands for root-mean-square, which is a calculation used to determine the equivalent DC value of an AC waveform. For example, a 120VAC signal applied to a resistor and a 120VDC battery applied to the same resistor should both heat the resistor to the same temperature.
True-RMS meters use mathematical equations to convert the measurements and give an accurate measurement of both sinusoidal and nonsinusoidal waveforms.
V AC+DC = √ V AC 2 + V DC 2
V High Frequency = √ V Total 2 - V Low Pass 2