Average Responding vs True RMS Multimeters
Fluke digital multimeters and clamp meters are available in average-responding and true RMS versions. For example, the documentation describes the 110 Series and 170 Series as true RMS multimeters, while the 15B and 17B are simply referred to as digital multimeters. What are the differences between them, and how should users choose?
What is RMS Value? If the heat generated by an alternating current i flowing through a pure resistance circuit R over one period T is equal to the heat produced by a direct current I through the same resistor for the same duration, the magnitude I is defined as the RMS value of current i.
Principle of Average-Responding Measurement For a sine wave, the peak value is 1.414 times the RMS value, and the RMS value is 1.11 times the average value. This factor is also the form factor of a sine wave. Therefore, the average rectification principle can be used to measure the RMS value for sine waves. The measured average value is multiplied by 1.11 to obtain the RMS value. This technique is known as "average reading, RMS-calibrated". The limitation is that this measurement method only works for pure sine waves.
Principle of True RMS Measurement For the waveform shown in the figure below, the form factor = RMS value / average value = 1.82. If the average-responding method is used, the average value will still be multiplied by 1.11, resulting in a large deviation from the true RMS value. Hence, the true RMS method must be adopted. Expressed by formula, this measurement principle enables direct RMS measurement for waveforms of all characteristics.
Conclusion For pure sine waves, both true RMS and average-responding instruments can perform accurate measurements. However, for distorted waveforms or typical non-sinusoidal waveforms such as square waves, triangular waves and sawtooth waves, only true RMS instruments can deliver accurate measurements.
