Figure 1

True RMS responding multimeters measure the "heating" potential of an applied voltage. Unlike an "average responding" measurement, a true RMS measurement is used to determine the power dissipated in a resistor. The power is proportional to the square of the measured true RMS voltage, independent of waveshape. An average responding ac multimeter is calibrated to read the same as a true RMS meter for sinewave inputs only. For other waveform shapes, an average responding meter will exhibit substantial errors as shown in the figure to the right.

A multimeter typically uses a dc blocking capacitor to only measure ac component of a signal. Only the "heating value" of the ac components of the input waveform are measured (dc is rejected). For sinewaves, triangle waves, and square waves, the ac and ac+dc values are equal since these waveforms do not contain a dc offset. Non-symmetrical waveforms, such as pulse trains, contain dc voltages, which are rejected by ac-coupled true RMS measurements.

Want even more details on True RMS measurements - Including the different types of technology used to measure RMS voltages and a measurement-troubleshooting guide? See Make Better RMS Measurements with Your DMM (AN 1392)