Volt Meters  Diagnosing Symptoms
The most important factor in selecting and using a multimeter is the calculating method it uses. All of the commonly used meters are calibrated to give an RMS indication for the measured signal. However, a number of different methods are used to calculate this RMS value. The three most common methods are:
 Peak Method. The meter reads the peak of the signal and divides the result by 1.414 to obtain the RMS.
 Averaging Method. The meter determines the average value of a rectified signal. For a clean sinusoidal signal, this value is related to the RMS value by a constant.
 True RMS. The RMS value of a signal is a measure of the heating which will result if the voltage is impressed across a resistive load. One method of detecting the true RMS value is to use a thermal detector to measure a heating value. More modern digital meters use a digital calculation of the RMS value. They obtain this by squaring the signal on a sample by sample basis, averaging over a period of time, and then taking the square root of the result.
These methods all give the same result for a clean, sinusoidal signal, but can give significantly different answers for distorted signals. This is very important because significant distortion levels are quite common, especially for the phase and neutral currents within a facility.
