Measuring True RMS

Tech Note

Just like using a hand-held multimeter to read AC voltages and currents, you can install a DIN package "Instrument Class(tm)" module in your control system to measure and present that same RMS data. Dataforth's DSCA33 True RMS input module provides a single channel of AC input which is converted to its True RMS dc value, filtered, isolated, amplified, and converted to a standard process voltage or current output.

RMS, or Root Mean Square, is a principle measurement of an AC signal. In practical terms, it is the equivalent dc signal that produces the same amount of heat in the same load. Therefore, it directly relates to signal power. For purely sinusoidal signals, the RMS value is the familiar peak amplitude divided by the square root of two. However, for signals comprised either of harmonics or of noncoherent content, this expression becomes increasingly invalid as the harmonic content or noncoherency increases. Typically this is given in terms of a waveform's Crest Factor (C.F.), which is the ratio of its peak value to RMS value. Crest factors start at one for square waves and generally increase for more "pointed" signals. True RMS conversion reduces the dependency on sinusoidal purity, thus allowing accurate measurement of signals ranging from pulses to complex nonperiodic waveforms.

Previously, industrial users requiring true RMS conversion were forced to use 'brick' type converters, in-house designs, or those not well suited for typical industrial conditions. The RMS conversion core circuit used in the DSCA33 is identical to those used in high-end digital voltmeters costing thousands of dollars. In addition, the module provides 1500Vrms continuous protection to computer-side equipment and personnel.

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