Re: true RMS meter vs. oscilloscope


"mike" <spamme0@xxxxxxxxxxxx> wrote in message
news:432CFBAA.6050503@xxxxxxxxxxxxxxx
> Robert Baer wrote:
>> mike wrote:
>>
>>> Robert Baer wrote:
>>>
>>>> phillip.liu@xxxxxxxxxxxxx wrote:
>>>>
>>>>> Hi, I am trying to measure the rms voltage for a slightly distorted AC
>>>>> sine waveform. I used a "True RMS" multimeter as well as an
>>>>> oscilloscope. Expecting both rms readings to be the same, but
>>>>> different. Any idea why? Thanks.
>>>>>
>>>> Using the scope, one can only measure the peak or peak-to-peak
>>>> voltage, then convert to RMS.
>>>> Not any different than the way AC scales on most DVMs are
>>>> implimented.
>>>
>>>
>>>
>>> I must be confused...cause he said he used a "True RMS" meter. A True
>>> RMS meter, by definition, measures the RMS value of the waveform. If
>>> the waveform is only "slightly distorted", a TRUE RMS meter should
>>> measure within the specs of the meter.
>>>
>>> I don't know about your scope, but mine has a button that calculates the
>>> RMS value of the waveform. I didn't bother to look up the accuracy
>>> spec.
>>>
>>> It would be interesting to know the exact equipment used and the
>>> magnitude of the discrepancy.
>>>
>>> One common source of such problems is DC offset of the AC waveform.
>>>
>>> mike
>>>
>> You are fortunate to have the "read in RMS" option on your scope.
>> But, i bet that it works by measuring peak or Peak-to-peak and
>> converting that to RMS.

Which waveform has the greatest discrepancy between average and rms values?

Norm Strong


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