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Nice video about RMS measurments ..

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Kiriakos-GR:


Thats a great video ... helping in understanding of RMS power ...

I do repairs , and I repair also UPS back-up  units ...
In some cheap UPS back-up units , the output in volts was looking truly out of specs ...

I own an  APC 1000 XL , the output voltage was always correct , when was measured with my ordinary multimeter ..

After looking the above video , I just realized , that the quality of AC volt ( clean enough ),
could  effect the readings, of one ordinary multimeter , by allot !!

I do not know If I must fly from joy , because I just got an Fluke 87-5 (V) ,
and probably I will never see again , misleading readings from bad quality AC ( all sorts of inverters).

But hey, I just found a good reason to feel good about ..  ;D 
     
 

Simon:
hm thats nice to know, pity my newly aquired amprobe AM220 is calibrated and not true RMS, I suppose for non sinewave stuff it is best to use an oscilloscope which in the case of the digital ones will tell you the RMS voltage. Luckily I work in DC so I'm not too bothered about AC unless its the mains anyhow

RayJones:
One caution with true RMS meters - even the some Flukes - is the bandwidth of the true RMS measurement.

We have equipment at work that uses a 20kHz filament heater voltage, that needs to be set correctly, nominally 5.5V.
The waveform is not sinusoidal, so you need a true RMS meter.
The budget Fluke true RMS models will read this voltage as <2V.

A Fluke 189 will however read as the correct 5.5V.

The true RMS bandwidth of the Fluke 189 is speced as 100kHz.
The other Fluke meter is only speced as true RMS, but no BW mentioned

Caveat Emptor.

RayJones:
Just found the spec for the Fluke 87V

AC Bandwidth   20kHz*
*with low pass filter; 3db @ 1kHz

Huh?
3dB down at 1kHz, how much dB is it down at 20kHz?

I'm thinking this would read the <2V mentioned above!

And checking for the 189 - it is now discontinued - so Fluke 289 instead:

Fluke 289: True-rms AC bandwidth  100 kHz

desolatordan:

--- Quote from: RayJones on April 09, 2010, 11:32:05 pm ---Just found the spec for the Fluke 87V

AC Bandwidth   20kHz*
*with low pass filter; 3db @ 1kHz

Huh?
3dB down at 1kHz, how much dB is it down at 20kHz?

--- End quote ---

Well, the low pass filter can be enabled/disabled. With it off, it'll be -3dB at 20kHz.

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