Question: Why should I buy a True RMS (TRMS) reading instrument?
Answer: A quick answer is TRMS instruments are accurate on any AC waveform. There are two basic AC measuring systems. One is "Average Sensing, RMS Indicating." This has been the most common system, since it is low in cost and easy to manufacture. An instrument with this type of measuring system measures the average of the sine wave, then multiplies it by 1.112, to allow the instrument to indicate RMS. This measuring system is called "Average Sensing, calibrated to the RMS of a sine wave," and is accurate ONLY ON SINE WAVES!
The second measuring method is called "RMS Sensing, RMS Indicating." An instrument with this type of measuring system computes the RMS value of the measured waveform (sine or otherwise). This is referred to as "True RMS" reading. This measuring system is accurate for ANY waveform, sine or otherwise. When an Average sensing instrument is used on a non-sinusoidal waveform, measurement accuracy could be as much as 50% off! Since you may not know what type of AC waveform you are measuring, a TRMS instrument should always be used.