Yes, RMS has only one physical definition, but in the case of DMMs the actual implementation is obfuscated.
"true" RMS in a DMM context is an RMS calculation that does not assume a sine wave - most cheaper DMMs do indeed assume a sine wave input. Then there are "true RMS" (and indeed "ordinary" RMS) DMMs that may or may not include any DC component, or at least in the Tek case, give you the choice. Old meters indeed did use to measure the heat produced in a resistor - the definition of the "RMS value" used was that of the DC voltage that would give the equivalent heating effect to the signal under inspection. Nick On Tuesday, March 6, 2012 2:16:45 PM UTC, GastonP wrote: > > Actually there is only a definition of RMS, not subject to > "trueness" :) > > http://en.wikipedia.org/wiki/Root_mean_square > > AFAIK, the old instruments that gave a true-"true RMS" output measured > the heat generated by the signal when applied to a resistor. That way > the waveform shape did not affect the measurement, and they were able > to measure with the DC component included, something fake-"True RMS" > instruments can't do. > Many of the existing instruments assume sinusoidal signals and thus > are subject to gross errors. > > Gaston > > On Mar 5, 6:15 am, Nick <[email protected]> wrote: > > On Monday, March 5, 2012 8:46:42 AM UTC, Cobra007 wrote: > > > > > Yes, you're right Nick, the Fluke is indeed AC coupled. I didn't > > > expect that to be honest as it undermines the definition of "true RMS" > > > but a simple battery test shows 0V RMS :-). > > > > Its not a commonly known problem, even among professional EEs. One of my > > DMMs, a Tektronix DMM916, has the option to include/exclude any DC > > component as required. I've had "forthright" discussions with some over > > what theoretically constitutes true-RMS vs. what they expect/want in > > actuality. > > > > Nick -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To view this discussion on the web, visit https://groups.google.com/d/msg/neonixie-l/-/cOKZXWW5GXwJ. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
