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Which DMMs can reasonably measure RMS?
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joeqsmith:
New users of the vintage Fluke may have to read the manual.   :-DD
joeqsmith:
It you like glitter,  the UNI-T may be a good choice.  Be VERY careful with the front end as it has been my experience that products from this company are not robust.  One small ESD may end up trashing the meter.   

Anymore, if I pull it out, the battery is dead (it only gets used for these demonstrations).  Here it is, 10 hours on the charger and it's ready to use.  Of course, you can't use the meter while it is charging.  Oh, and a bit of advice about charging.  What ever you do, DO NOT CONNECT THE CHARGER UNTIL YOU HAVE THE METER'S SELECTOR SET TO THE CHARGE MODE!!!   The charger uses the low current safety fuse and if you set the meter to current, you will blow that fuse with the charger attached!!   Ask me how I learned that right after I bought the meter..  :-DD 

Still, it is very stable over temperature and accurate.  It also has a few improvements over the Fluke 289 (its a copy if you didn't notice).  Drives just like the 289.  You can buy a BLE interface for remote data logging but sadly, there are no working applications for it.  I started to hack the protocol and ran into a few others who had already been down that path who helped me out.  There is now a document that covers it.   

Cost is about $400 now.  If the meter had a few improvements, it may be worth it but I would rather use the Brymen BM869s.   It's just a nice basic meter and one of the more robust meters I have looked at.   
W6EL:
Well, this is all very interesting.

I think we can decouple the bandwidth argument pretty easily. Of course a square wave has an infinite number of terms, so there is no way to measure it (or even create it) perfectly. But we get to 20 dB down within about 4 or 5 terms, so for low frequency square waves, your garden-variety DMM should be acceptable bandwidth-wise, at least down to 1% (power-bandwidth wise, not accuracy). Many meters have surprisingly wide bandwidth. I saw very little difference with my listed equipment across the spectrum of my testing. This is not to say it is not a factor, rather, it is not a major limiting factor in this particular test. 

There was in fact a little DC offset on the low-side of the output of my generator. Just about 150mV above zero. Thank you to those that pointed this likely issue out.

Since this test has become more interesting than I anticipated, I'll run it again but with more attention to detail. I can now appreciate the AC vs DC coupling issue in how the DUT sees the incoming voltage. With that in mind, a purely AC measurement produces a different result than a DC measurement, but this is acceptable so long as the operator is aware of it.

It's also worth mentioning that any RMS measurement has a large dependence on the integration time of the reading. For high frequency signals, short integration times are probably just fine, but for lower frequency waveforms, the integration time needs to be sufficient so as to capture the full period of whatever nonsense is going on in the signal. For those using thermal conversion measurements, this is essentially the heat capacity and the thermal conductance of the module to the environment which sets the time constant.
Kleinstein:
The cheaper meters may not have a hard integration time for the AC mode. With the analog RMS converters there is more like a 1st or 2nd order response and thus possibly relatively slow settling. This is especially true for diconnecting from an AC source.  So RMS converters (e.g. AD737 /736) can be awfully slow in this case.
There is a compromise between response time and the ability to measure low frequencies / accuracy at low frequencies (like < 30 Hz)
The digital RMS type (e.g. part of the DMM chip set) has usually much faster response, more similar to the DC mode. They may still be more accurate with low frequencies.
BeBuLamar:
I don't know if it's right or wrong but with my Fluke 189 and a square wave 0-4.1V it reads a bit more than 2V (2.09V) on AC. It reads 2.9V on AC+DC mode.
The meter manual said"
" Your meter features
true rms readings, which are accurate for sinewaves and
other wave forms (with no dc offset) such as square
waves, triangle waves, and staircase waves. For ac with
dc offset, use ac+dc."

So a square wave with the minimum at 0 and max at 4.1 it's considered to have 2.05 VDC offset.
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