Dataforth has just published Application Note 128, RMS Revisited. The 14-page document revisits the definition of RMS, shows RMS values for various time functions, and explores some RMS measurement issues including the frequency response and crest (or form) factor. The object of the Application Note is to provide a better understanding of some of the subtle characteristics of RMS; four appendices offer additional information.

When we purchase electrical equipment, appliances, light bulbs, motors, and such, somewhere on the “name plate” are electrical specifications including voltage, current, frequency, power, etc. For alternating (AC) current, the values for voltage and current are always shown as RMS values, though this is rarely stated on the name plate.

The Application Note takes the reader through an initial experiment that demonstrates the derivation of RMS. It begins with the requirement that the function of time is “well” behaved (bounded with a finite number of discontinuities), repetitive at some fundamental frequency, and available for as long as needed. The name RMS comes from the mathematics of this experiment, which shows that the effective value is the square Root of the Mean (average) value of a time varying voltage Squared.

From this beginning, the document moves on to some RMS examples, a discussion of Parceval’s Theorem, and measuring RMS. Information provided for Dataforth RMS modules includes nine commonly understood analog specifications as well as the less well-known frequency response and crest factor specs, both of which are examined in greater detail. Fourier’s theorem is used to demonstrate the effects of frequency on RMS measurements and four examples of non-sinusoidal time measurements are given.

RMS Revisited joins the Dataforth library of Application Notes; the full list of offerings is available at www.dataforth.com/application-notes.aspx Dataforth was established in 1984 and is the world leader in data acquisition and control, signal conditioning, and data communication products for industrial applications. Worldwide, our products provide rugged signal and data integrity and wide spectrum accuracy. All Dataforth products are manufactured in the USA and have been RoHS Compliant since 2006. The Dataforth Quality Management System is ISO9001:2008 registered.

For additional information, call 800-444-7644 toll-free or email sales@dataforth.com.

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