Nice video about RMS measurments ..

[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 ..   
     
 

[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]

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?

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

[RayJones]

Ah OK, the difference is now obvious!

Nevertheless, for our application where the signal can head slightly north of 20kHz the Fluke 87 is not to be trusted.

Somebody labelled the one at work "It's a fluke if this thing reads filament volts correctly"

[Kiriakos-GR]

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?

Low Pass Filter (87)
The 87 is equipped with an ac low pass filter. When measuring ac voltage
or ac frequency, press A to activate the low pass filter mode (K).
The Meter continues measuring in the chosen ac mode, but now the signal
diverts through a filter that blocks unwanted voltages above 1 kHz, refer to
Figure 2. The lower frequency voltages pass with reduced accuracy to the
measurement below 1 kHz. The low pass filter can improve measurement
performance on composite sine waves that are typically generated by
inverters and variable frequency motor drives.
Thats from the book ..

Just email them your question.