The Differences Between Average Response Multimeters and True RMS Multimeters
Fluke's digital multimeters and clamp meters come in two types: average response and true RMS. For example, the 110 and 170 series are labeled as true RMS multimeters, while the 15B and 17B are simply described as digital multimeters. What are the differences between them, and how should users choose?
What Is RMS (Root Mean Square)?
The RMS value of an alternating current i is defined as the direct current I that produces the same amount of heat in a pure resistive circuit R over one cycle T. In other words, RMS represents the equivalent DC value of an AC signal in terms of power dissipation.
Principles of Average Response Measurement
For sinusoidal waves:
Peak value = 1.414× RMS value.
RMS value = 1.11× average value (this is the form factor of a sine wave).
Measurement method:
The meter measures the average value of the sinusoidal wave and multiplies it by 1.11 to calculate the RMS value. This is called "average reading, calibrated to RMS."
Limitation:
This technique only works for pure sine waves. It fails to accurately measure non-sinusoidal waveforms.
Principles of True RMS Measurement
For non-sinusoidal waveforms (e.g., square waves, triangular waves, or distorted signals):
The form factor (RMS/average) can deviate significantly from 1.11. For example, a typical non-sinusoidal wave might have a form factor of 1.82.
