What does "TRUE RMS" mean?

Sure, it's called electrical substitution, and it's been used in optical power meters since before the invention of rocks. I understand the concept perfectly well, I just disagree with you about the clarity of the nomenclature. "True RMS" gives me a lot more confidence that the meter is doing what I think it is.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Most meters still do it that way. Seems to me that it would be better, these days, to use a fast sampling ADC, digitize whatever's there, and do the math.

John

The problem with electrical substitution in general is that its dynamic range is the pits. On the other hand, you don't have to worry about some buggy firmware giving you wrong readings for particular waveforms.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

All you do with fast ADC samples is square, average, root, display. Autozero on the front end maybe. The only serious mistake you can make is letting the ADC clip on peaks, and that's easily detected.

The analog RMS converters are expensive and usually have mediocre bandwidth. The wideband ones (like 100 KHz or so) wind up having frequency compensation tweaks. All that's a lot of work to replace some essentially free firmware.

There are lots of "True RMS" DVMs around, with analog converters in the front end, that are good to 0.03% or whatever on DC but are rotten on AC, both accuracy and frequency range... numbers like 1% and 1 KHz.

John

That is if you trust the meter or what it claims it can do.

A friend got their first meter- one of those dollar store looking ones. I flat out told them, don't even try to use this to test an outlet.

I think it had a 600 volt AC range too. I think the leads were the fuse. I would not be surprised if it claimed to be a true RMS meter.

You mean, Won Hung Lo meters aren't as good as Flukes? Who knew?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Except that whenever I see the phrase "True xxx", I automatically assume that someone is lying:-(

I'm sure AD could make a chip with a DSP on it that would do just that, just as they effectively incorporate DSPs into some of their high-end ADCs.

Not sure there's much demand for really high accuracy RMS measurements though... for the ones I've done a few tenths of a percent is gross overkill.

I have a really old fluke and wavetek meter that I trust. I have to do the sanity check when I use anybody else's meters though. A no-name meter with old batteries is always sketchy.

It would be nice if meters had an indicator for "you blew your current ranges".

"John Larkin" "Phil Allison"

** Try doing the costing and making it work on a 9v battery at under 1mA drain.

DMMs with " true rms " sell for under $50.

... Phil

I'm doing that with a PIC, but for power line frequencies. I take the = square=20 of each sample and add that to a running sum of squares, using integer = math.=20 Then when I want to display the true RMS, I just shift the result to, 16 =

bits and then take a quick square root which is about 8 bits, which is = still=20

0.4% accurate. My samples are typically 100 to 200 mSec. But I also=20 calculate the true RMS value of much longer pulses, such as are = encountered=20 in circuit breaker testing for short and long time delays, which can be=20 anywhere from about 0.5 seconds to 200 seconds. So I still just maintain = the=20 total of squares, which may be an unsigned long 32 bit number, which is=20 enough to hold 29 minutes of 10 bit data at 2400 samples per second.

This is needed because the current can vary during the test, due to = heating=20 and other effects, so the breaker tends to react to the true RMS value = of=20 the entire waveform, especially if it has a thermal delay element. = Reclosers=20 are even more challenging, because their impedance changes by a factor = as=20 high as 3, causing a huge difference in the RMS value of the current = from=20 the beginning to the end of its stroke. This is an effect of low voltage =

testing, and the change would be negligible on an actual fault with 14.4 = kV.=20 Series resistance can be added to reduce the drop-off effect, but only = so=20 much, so the RMS value must be computed for the entire waveform. This = has=20 been shown to match the expected trip times very accurately.

I chose my Fluke 45 multimeter as the best choice for true RMS reading = at=20 the time I bought it about 20 years ago. It has an accuracy of 0.2% + =

100=20 counts from 50 Hz to 10 kHz, but that means that on the 100 mV range,=20 specified for 15,000 to 99,999 counts, it may be an additional 100/15000 = or=20 0.67%. It seems to be actually much better than that, although it has an =

offset of about 0.240 mV with the input shorted. I have been using this = to=20 calibrate my Ortmasters, which are rated at 1% from full scale ranges of =

50A=20 to 10,000A, using a 1000A 100 mV shunt. So when I calibrate the 50A = range, I=20 am reading about 5.000 mV, which could be as much as 100 counts off, or = 2%.=20 The Ortmaster typically shows an offset of 0.10 A on that range, which = is=20 only 10 uV. So I may need to get a better standard!

The Fluke 8520A has a 1 year accuracy of 0.15% reading plus 0.05% FS, = which=20 in this case is 1.99999 V, so the true error is as much as 1 mV. So its=20 accuracy at 5 mV is only really 20%! No winner here.

The Fluke 289 has a true-RMS accuracy of 0.3% + 25 counts, 45-65 Hz, on = the=20

50.000 mV range, which gives me an effective accuracy on a 5 mV reading = of=20 0.8%. An improvement, but I'd really like 0.5% or better.

The Fluke 8808A is similar to the Fluke 45, but it does not seem to be = true=20 RMS.

The Fluke 87 has a true-RMS accuracy of 0.7% +2 digits and 0.1 mV=20 resolution.

The HP 3478 has a true-RMS accuracy of 0.46% + 163 counts for the =

300.000 mV=20 range.

I really have not found a meter that is significantly better than the = Fluke=20

1. Any suggestions?

Paul=20

"P E Schoen"

The Fluke 289 has a true-RMS accuracy of 0.3% + 25 counts, 45-65 Hz,

** This spec refers to sine wave inputs in the narrow range of 45 to 65 z - so it really only gives you the AC voltage accuracy at mains frequencies.

True RMS accuracy is another ball game where measurement bandwidth is everything.

For the 289, Fluke claim a 100kHz bandwidth which is the 3dB down bandwidth at some level. The frequency response of the RMS to DC converters used is level dependant, falling at lower levels, the full spec is in the handbook.

Scopes like the Rigol supply true RMS readings of the wave on the screen which are pretty good, long as the bandwidth of the signal does not exceed that of the timebase range in use.

... Phil

"John Fields"

** Thankfully, it only takes milliseconds to delete your asinine posts.

... Phil

better,

Much depends on the frequency response you need. For most power line=20 frequency applications, a low cost, low power PIC can be used to sample = at=20 up to about 50 kHz, so even a 10 kHz signal can be read with reasonable=20 accuracy. So the basic components for the computed true-RMS function=20 basically come free with the processor which also takes care of driving = the=20 display and reading the keys and switch positions. Everything else, and=20 probably the most expensive, is high accuracy front end analog = components,=20 controls, packaging, and safety. And the basic system I have designed, = and=20 now build, is much more accurate and sensitive than any generally = available=20 true-RMS meter, and the parts required are probably no more than $20 in=20 quantity.

It seems that it is just "easier" to toss in a true-RMS IC and keep=20 everything else the same.

I have even made a true-RMS meter with an incandescent lamp and a = photocell.=20 But it had problems with aging and temperature effects. That was for a = very=20 simple, self-powered analog meter, however, which used thermocouples = which=20 were very expensive. Now, however, I would use a matched lamp/photocell=20 pair, and drive one with the input signal (through a variable gain=20 amplifier), and the other from a DC current source, until the photocells =

registered equal resistance. The bandwidth would be limited only by the=20 input amp. And for a very narrow range, if high impedance were not = needed,=20 the lamp could be driven directly through a resistor. For most power = line=20 measurements, drawing a few mA is not a problem.

Paul=20

"John Fields"

** Does the term psychopathic stalker mean anything to you ?

There's no reason for the sample rate to be greater than the signal frequency. You just need an ADC with a narrow s/h time.

You don't want the sample rate to alias with the signal, so it's good to sample sort of randomly. HP did this 45 years ago.

They did average reading, but could have done RMS if they'd had a uP or something available.

John

Looking for a high accuracy True-RMS DMM, I found the HP 3458A, which = has=20 several modes for AC measurement, including the random sampling = technique:

Nice! About $7500 new, as low as $4000 on eBay. Too rich for my blood!

Paul

Since you were looking in a mirror when you wrote that crap, yes.

--- Sure.

In the current context you want to pretend that you made no error and that poor little innocent you is being stalked by someone who wants to do you in.

Clue: You're not that important and you should make an effort to acknowledge your errors when they're pointed out to you.

-- JF