rms reading

[mpj111]

Some multimeters been advertised as it gives true rms reading.
What is true RMS reading and normal RMS reading. What is the difference.
My understanding is there is only one measurements RMS reading.
RMS reading = Peake/sq rt of 2

Any ideas, highly appreciated.

 

[dknguyen]

You are correct in everything you say. DMMs that say true RMS measure RMS values while meters that do not measure peak values when measuring AC. So there is no difference between RMS and true RMS, except that true RMS is an advertisement thing to differentiate a peak-measuring multi-meter from an RMS measuring one.

 

[zevon8]

I think you will find that in the case of properly designed meters, there is a big difference between TRMS and RMS. The only time the two measurements are the same is when measuring a pure 360 degree AC sine wave. Once you start measuring square, triangle, phase controlled AC ( very common in industry) you can end up with wildly different readings. Also, just for fun, check a meters accuracy for AC over a given frequency range. You will be surprised just how narrow a range of frequencies some cheap meters can measure on their AC ranges.

 

[Dean Huster]

Also, most meters measure the AVERAGE value and display the result as RMS. Peak measuring meters were pretty much limited to vacuum tube voltmeters and most of these also had peak-to-peak scales on them as well as RMS scales so that readings could be correlated to scope measurements in the TV repair industry.

In all cases, non-true-RMS meters assume a sine-wave input for their accuracy.

Regarding the frequency response issue: I've checked some Fluke DMMs to find that their upper end rarely made it above 1KHz, others going to 10KHz or more. I've checked the venerable Simpson 260 VOM to 100KHz and found it pretty decent! AC voltmeters (they measure AC only, usually down into the millivolt level) usually typically have frequency response somewhere between 1MHz and 10MHz, depending upon whether they come from Asia or Hewlett-Packard/Agilent. RF millivoltmeters have responses extending beyond 1GHz, although limited in maximum voltage input.

Dean