# What do RMS and True RMS stand for? Here we explain you the differences

February 27, 2019.
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RMS stands for Root Mean Square and TRMS (True RMS) for True Root Mean Square. The TRMS instruments are much more accurate than the RMS when measuring AC current. This is why all the multimeters in PROMAX catalog have True RMS measurement capabilities.

The difference between the measurement using a standard multimeter and a True RMS one

### Why the RMS or True RMS measurement are necessary?

Because the voltage of a sine wave varies over time, meaning it is not equal to its peak voltage. The effective voltage of an AC signal is its equivalence in the form of DC voltage and can only be calculated by RMS or True RMS measurement instruments.

170 V sine wave and 120 V effective voltage delivered

### What is RMS?

RMS stands for Root Mean Square which is the mathematical formula internally computed by the RMS-capable instruments to take measurements. This formula is simplified by the instrument to take into account only the positive peak value of the sine wave:

Mathematical formula applied by a RMS multimeter

The RMS measurement is reliable when the sine wave is perfect, because this measure only takes into account the peak value of the waveform.

So, what happens when the sine wave is not perfect? This means, what happens in the REAL world?

### What is True RMS? The most reliable measurement

RMS measurements cannot be seen as reliable because nowadays any installation has multiple sources of noise causing the AC waveform to never be perfect.

The True RMS measurement (True Root Mean Square) uses more complex mathematical formulas that allow getting a value closer to reality than the RMS. In addition to peak values, they take several samples of values along each cycle.

Mathematical formula applied by a True RMS multimeter

A perfect sine wave (left) and a wave closer to reality (right) affected by noise

All this noise is induced by drives, variable switches, appliances, computers and cheap power supplies of routers, battery chargers, etc.

For this reason, because sine wave is never perfect, the reliable measurements in AC current installations are only those taken by True RMS multimeters.

### A brief overview to PROMAX multimeters

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PROMAX is a leading manufacturer of test and measurement systems, broadcast and TV signal distribution equipment. Our product lines include measuring instruments for cable TV, satellite TV, broadcast, wireless and fibre optics networks, FTTH and GPON analyzers. DVB-T modulators, IP streamers or IP converters (ASI, DVB-T) are among the company’s latest developments.