Introduction to True RMS Multimeters
The quantities used to describe the magnitude of alternating current include the peak value (and peak-to-peak value), average value and effective value. The peak value refers to the maximum value; the average value means the average of the rectified alternating current over one complete cycle. The effective value is also known as the root-mean-square (RMS) value.
Definition of effective value: Pass direct current and alternating current separately through two identical resistive components. If they generate equal amounts of heat within the same time period, the voltage and current values of this direct current are defined as the effective values of the alternating current. For this reason, when people refer to the magnitude of voltage or current without special specification, they generally mean the effective value.
There are fixed relationships between these three values: The ratio of the effective value to the average value is called the Form Factor (Kf); The ratio of the peak value to the effective value is called the Crest Factor (Kp).
For sine waves: Crest Factor Kp = √2 (constant); Form Factor Kf = π/(2√2) (constant). These coefficients will change for non-sinusoidal waveforms.
Evidently, the effective value is the parameter people care most about, yet it is difficult to measure directly. In contrast, average value and peak value can be measured easily. Therefore, ordinary multimeters (including voltmeters) indirectly calculate the effective value by measuring the average or peak value, based on the conversion coefficients for sine waves. For example, if the measured peak voltage is 10 V, the meter will display 7.07 V (the corresponding RMS value).
Although these types of meters show RMS readings, they measure a "pseudo-RMS" value. They deliver relatively accurate results when measuring sine waves, but produce incorrect readings for non-sinusoidal waveforms.
High-precision meters can measure the true RMS value directly via multiple methods. Such meters measure and display the actual RMS value, so accurate readings can be obtained for any waveform. To distinguish them from conventional meters, they are named True-RMS Multimeters.
