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Average measuring multimeters more precise than TrueRMS counterparts?

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nightfire:
Just browsed through some Fluke manuals, and stumbled upon the 27II/28II series of multimeters, that, to my understanding are basically 83V/87V in a modified housing and some additional certifications (like mining sites).
The 27 II is average-measuring AC like the Fluke 77 IV, and in the manual https://dam-assets.fluke.com/s3fs-public/2x_2____umger0100.pdf  on page 16 it is stated, that with TrueRMS multimeters it is normal, that when shorting the test leads in ACV mode, some remaining values might be displayed.
Also I noted that the ACV accuracy of the 27II is noted with 0.5% + 3 digits, the 28 II is noted with 0.7% + 4 digits for frequencies around 50 Hz.

Question here: Are those Average-measuring meters more precise due to the fact they do not have to deal with the whole TrueRMS stuff, or the compensation techniques to get rid of induction energy via the test leads?
Might this be a reason why a company like Fluke also provides such meters to the customers (aside from the rumors that the military is a driving force or buyer for the 77 IV)?

alm:

--- Quote from: nightfire on January 21, 2023, 12:05:16 pm ---Just browsed through some Fluke manuals, and stumbled upon the 27II/28II series of multimeters, that, to my understanding are basically 83V/87V in a modified housing and some additional certifications (like mining sites).
The 27 II is average-measuring AC like the Fluke 77 IV, and in the manual https://dam-assets.fluke.com/s3fs-public/2x_2____umger0100.pdf  on page 16 it is stated, that with TrueRMS multimeters it is normal, that when shorting the test leads in ACV mode, some remaining values might be displayed.
Also I noted that the ACV accuracy of the 27II is noted with 0.5% + 3 digits, the 28 II is noted with 0.7% + 4 digits for frequencies around 50 Hz.

--- End quote ---
The manual will probably also say that this offset does not affect accuracy above 5% of full scale or something like that. And the specifications will say the AC accuracy is only guaranteed from X% to 100% of full scale. True RMS meters don't perform well at the very bottom end of the scale.


--- Quote from: nightfire on January 21, 2023, 12:05:16 pm ---Question here: Are those Average-measuring meters more precise due to the fact they do not have to deal with the whole TrueRMS stuff, or the compensation techniques to get rid of induction energy via the test leads?
Might this be a reason why a company like Fluke also provides such meters to the customers (aside from the rumors that the military is a driving force or buyer for the 77 IV)?

--- End quote ---
Average responding and True-RMS meters can both be made accurate. The reason Fluke is still selling average-responding meters is that these meters give different readings for non-sinusoidal signals, and some procedures will have been written with this in mind. If there's an F-16 manual saying "measure the signal between pins 1 and 16 on connector A with an average-responding meter and ensure it's between 1.1 V and 1.2 V", then the cheapest thing to do is buy an average-responding meter.

The accuracy of an average-responding meter is of somewhat limited value unless you can ensure the signal is sinusoidal with low distortion. Otherwise the crest factor will likely swamp any uncertainty of the meter.

MadTux:
Perhaps not use an "electrician DMM" when you actually care about ppm RMS???

0.5% is plenty enough for any electrical work, for precicion electronics I'd rather get something like a 12bit DSO, where I can measure RMS signal a a waveform and do the math afterwards....

bdunham7:

--- Quote from: nightfire on January 21, 2023, 12:05:16 pm ---Question here: Are those Average-measuring meters more precise due to the fact they do not have to deal with the whole TrueRMS stuff

--- End quote ---

One major difference between an analog TRMS converter like the AD737 and a typical averaging converter is the internal dynamic range.  The TRMS converter's first operation is to square the input, then it integrates and then there is a logarithmic amp.  The dynamic range of that middle part will be higher than the range of signals at the input.  The TRMS converter may be quite limited by noise and offset at the low end and by crest factor at the top end.  So if you put a similar amount of effort into an averaging converter as you do a TRMS version, you'll probably get better results, especially at the low end of the scale.  The Fluke models you mentioned actually use the same AD737 for the averaging and TRMS models as the chip has both modes.  I don't know if using it in the averaging mode solves the dynamic range issue and I suspect it doesn't.

If you move away from the analog TRMS systems to something like a sampling system (like Keysight's 'TrueVolt' ) this distinction may all but disappear.

HKJ:
Mains voltage is not a perfect sinus anymore, i.e. a average meter will not show correct voltage, but a true rms may.

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