Author Topic: Which DMMs can reasonably measure RMS?  (Read 5298 times)

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Offline joeqsmith

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Re: Which DMMs can reasonably measure RMS?
« Reply #25 on: April 26, 2023, 01:48:15 pm »
New users of the vintage Fluke may have to read the manual.   :-DD

Offline joeqsmith

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Re: Which DMMs can reasonably measure RMS?
« Reply #26 on: April 26, 2023, 02:12:31 pm »
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.   

Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #27 on: April 26, 2023, 04:13:00 pm »
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.

Offline Kleinstein

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Re: Which DMMs can reasonably measure RMS?
« Reply #28 on: April 26, 2023, 04:52:16 pm »
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.
 
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Offline BeBuLamar

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Re: Which DMMs can reasonably measure RMS?
« Reply #29 on: April 27, 2023, 12:51:27 am »
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.
« Last Edit: April 27, 2023, 12:56:46 am by BeBuLamar »
 

Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #30 on: April 27, 2023, 04:11:05 am »
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.

That seems right to me from what I've learned so far.

Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #31 on: April 27, 2023, 04:33:05 am »
Alright, well, I did some more careful testing.

I decided to leave the scope in AC coupled mode just to put it into the same point of view of my cheap multimeters. I allowed everything to warm up over an hour and then did some careful measurements.

In order to sort of establish how well things were accurate, relative to each other, I ran a 60 Hz sine wave first. That ought to be something any reasonable meter can measure, right? And indeed, all instruments were under 2% of error, which is pretty good for my bunch. I'm using the HP scope as my reference since it actually arrived calibrated (it's since lapsed, but not that long ago).

I then did a basic sweep to see where each meter fell off 3dB (power). Most of my meters provided plenty of bandwidth for audio-range signals, but the Owon HDS272S surprised me at only 4 KHz. After about 5 KHz it's basically deaf. The Tenma 72-410A went out all the way to 236 KHz before losing 3dB, however, it was not very flat along the way, showing little bumps of a 100mV here and there. The Owon B35T went out to 600 KHz before dropping 3dB, which surprised me. This little meter seems to hold its own pretty well! Even the RadioShack (22-174B) held out to a respectable 60 KHz.

Next came the square wave test. I did this at 60 Hz and at 1 KHz. Given the bandwidth of some of these meters, there was no point going out much further. The HP400EL didn't do so well on the square wave test, despite its massive bandwidth advantage, simply because it is not an RMS meter. It is calibrated to show the RMS level of a sine wave, but it lacks a circuit to actually compute or measure RMS. So despite it's massive bandwidth (well over 10 MHz), it is simply not accurate for a square wave.

As for the other meters, they did pretty well except for the Owon HDS272S, which probably did poor simply due to it's rather pathetic 4 KHz bandwidth. This is somewhat ironic given the scope portion of this meter can read out to 70 MHz... Add to this that the scope lacks an RMS measurement readout. Go figure. It's still a nice meter though, lovely bright display and very handy in tight spots to have a scope.

The standard deviation on the sine wave test was 14.6mV, the 60 Hz square wave was 106mV, and the 1 KHz square wave was 129mV. So you can see that indeed there is far more variation as you move up in frequency and some of these meters start to fall apart. I wouldn't read too far into this though since my meters are not exactly high-quality nor are they even similar to each other.

A little story on the radioshack meter. I got it when I worked at radioshack and it went on clearance for $29.97. With my employee discount it was probably half that price. I thought it was pretty great, it had HFE, capacitance, a dB scale, a bargraph, all the sorts of things a kid in the 90s would consider pretty premium stuff. Years later I was working as a tech for a guy and I noticed that the meter had poor performance over about 10 KHz. The guy I was working for had a basement full of old vacuum tube HP equipment, so I dragged up an enormous HP meter and compared the two, and indeed, the RS meter was off a few dB at the high end. Now comes the fun part. I had a warranty on the meter because, why not, it was dirt cheap as an employee, so I drove to the local radio shack store. I walked in and proceeded to plug in an HP oscillator and an HP meter, and planned to demonstrate how poor the performance was. The poor guy behind the counter had no idea what was going on and just offered to let me pick a similar model to replace it (since they didn't have it anymore). I picked the most expensive one they had. But! When I tested it, it was worse!! So I left with my original meter. That evening, I took the meter apart and discovered several pots inside it. After a few hours of playing around in the basement of HP equipment, I had it flat to 20 KHz (which was all I needed at the time). What surprises me about all this, is that "calibration" happened in around 2003. The meter's plastic melted in my car and several sets of batteries corroded up inside it. And yet, here it is, 20 years later, showing under 2% of error. It still has holes drilled in the back over the pots with little bits of tape over the holes.

Anyway, that's my story! Results are attached.

Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #32 on: April 27, 2023, 05:03:16 am »
btw .. speaking of Tenma 72-410A  and Owon B35T , such devices doesn't have dedicated RMS\DC converter like popular AD637.
so true RMS  in some narrow brackets of frequency and AC waveform.
I'm guessin HDS272S , and RadioShack same story.

most basic dmm AC up 5kHz ,  if something add it would be 20K , maybe 50K,  who claim 1% accuracy up to 100K usually dedicated RMC-DC converter.
like owon bt41+ has such dedicated chip

Hi GigaJoe,

I found the schematic for the Tenma 72-410A. It indeed does have an RMS converter, it uses the Maxim MX536A, a 2 MHz-wide RMS converter. This converter's value then goes into a 14-bit ADC (TC835) running at 1 MHz. I think, given that it was only off by 2-3%, it just needs a calibration.

Also the Owon B35T has an AD8439JCPZ RMS chip, see here: https://lygte-info.dk/review/DMMOwon%20B35T%20UK.html

--E
« Last Edit: April 27, 2023, 03:42:03 pm by W6EL »
 

Offline David Hess

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Re: Which DMMs can reasonably measure RMS?
« Reply #33 on: April 27, 2023, 06:42:23 am »
I have used various RMS voltmeters in combination with a square wave generator for oscilloscope calibration.  From 100 to 1000 Hz, my Tektronix DMM916, Beckman RMS225, and HP3478A all read correctly, whether in AC averaging mode, AC RMS mode, or AC+DC RMS mode.  The average AC reading should be 11.1% higher than the RMS measurement, and is a little more convenient because it settles faster so adjustment is easier.
 

Offline schmitt trigger

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Re: Which DMMs can reasonably measure RMS?
« Reply #34 on: April 27, 2023, 07:13:20 am »
Pretty interesting story about the Radio Shack meter, W6EL.

Radio Shack’s test equipment was average or below average, but there was the ocasional golden nugget.
Before affordable DMMs, the silver-standard were FET-VOMs, and I worked for several months at a McDonalds to be able to purchase one.
I loved the meter, its average responding AC function frequency response would extend all the way to 1 Mhz.

But then after a few years, the plastic case started to disintegrate. I ignore what the cause was, but the plastic became very brittle and little pieces would fall off. I mended it with tape, until the center post where the rotary switch actually rotated cracked.
 
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Online bdunham7

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Re: Which DMMs can reasonably measure RMS?
« Reply #35 on: April 27, 2023, 02:18:15 pm »
In order to sort of establish how well things were accurate, relative to each other, I ran a 60 Hz sine wave first. That ought to be something any reasonable meter can measure, right? And indeed, all instruments were under 2% of error, which is pretty good for my bunch. I'm using the HP scope as my reference since it actually arrived calibrated (it's since lapsed, but not that long ago).

A couple of things that jumped out at me and I'm curious about them.

First, does the HP 54645D actually give you an explicit VRMS reading or are  you calculating it somehow?  If it gives you a reading, what is its specified accuracy?  Scopes are typically not competitive with good DMMs in basic accuracy and I think the basic DC-gain tolerance is 1.5% or so for this model.

Second, you give readings for the HP 400EL to four significant digits which seems impossible on an analog meter--that last digit corresponds roughly with the thickness of the paint on the needle!  Are you just squinting extra hard or do you have some other method? 

I suspect your hand-tuned Radio Shack meter may be even more accurate than you think. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline PartialDischarge

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Re: Which DMMs can reasonably measure RMS?
« Reply #36 on: April 27, 2023, 02:55:52 pm »
UniT 181A gives 2.8995 for 100Hz, 2.89 for 1KHz and 2.895 for 200KHz.

4.1Vpp square wave with 2.05V offset...

 
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Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #37 on: April 27, 2023, 02:57:48 pm »
In order to sort of establish how well things were accurate, relative to each other, I ran a 60 Hz sine wave first. That ought to be something any reasonable meter can measure, right? And indeed, all instruments were under 2% of error, which is pretty good for my bunch. I'm using the HP scope as my reference since it actually arrived calibrated (it's since lapsed, but not that long ago).

A couple of things that jumped out at me and I'm curious about them.

First, does the HP 54645D actually give you an explicit VRMS reading or are  you calculating it somehow?  If it gives you a reading, what is its specified accuracy?  Scopes are typically not competitive with good DMMs in basic accuracy and I think the basic DC-gain tolerance is 1.5% or so for this model.

Second, you give readings for the HP 400EL to four significant digits which seems impossible on an analog meter--that last digit corresponds roughly with the thickness of the paint on the needle!  Are you just squinting extra hard or do you have some other method? 

I suspect your hand-tuned Radio Shack meter may be even more accurate than you think.

The scope has an RMS "measurement" you can bring up (among other things). I turned on 8x averaging as well. Yes, scopes are not exactly known for their vertical accuracy, but it's the best I can do. That it agrees (sine wave) with the HP-400EL does say something.

The HP 400EL has more sig figs than you might expect, provided you are in the correct range. I used the 3 volt scale. Each of the most minor tic marks near the 2 volt mark is 0.020 volts, and I can discern values in between to a degree. Years of working in laboratories has refined this skill, although with most equipment being digital I don't get to use it as much as I wish. I'll grant you that maybe the 0.005 is generous though.

I looked inside the radioshack meter this evening. Nothing fancy really. A single quad-pack chip seems to handle most of it. I did not see a dedicated RMS chip, although I did not look under the LCD for fear of breaking it. I do note that the six batteries are tapped such that a negative rail is available inside. The PWB reads "Tandy 1992"  :)

Also, the Owon B35T does indeed have an RMS chip (AD8439JCPZ, see here for the insides: https://lygte-info.dk/review/DMMOwon%20B35T%20UK.html).

Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #38 on: April 27, 2023, 03:01:30 pm »
UniT 181A gives 2.8995 for 100Hz, 2.89 for 1KHz and 2.895 for 200KHz.

4.1Vpp square wave with 2.05V offset...

I like the readout showing AC, DC, and the combined result. Fantastic. Tells a lot more of the story!

Offline shakalnokturn

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Re: Which DMMs can reasonably measure RMS?
« Reply #39 on: April 27, 2023, 11:20:23 pm »
The scope has an RMS "measurement" you can bring up (among other things). I turned on 8x averaging as well. Yes, scopes are not exactly known for their vertical accuracy, but it's the best I can do.

Not having the vertical scale set as near as possible to ADC's full scale would add inaccuracy, I'm not sure what the optimal horizontal scaling would be for RMS measurement on a DSO although I'd suspect one full cycle and that it may vary from one scope to another.
 

Offline gamalot

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Re: Which DMMs can reasonably measure RMS?
« Reply #40 on: April 28, 2023, 12:08:38 am »
Those multimeters that have AC+DC mode will do it.
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Offline vk6zgo

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Re: Which DMMs can reasonably measure RMS?
« Reply #41 on: April 28, 2023, 01:28:56 am »
And RMS is RMS and it naturally include also DC. 4.1V DC RMS is 4.1V. If some meter do not display 4.1V RMS for 4.1V DC then designer need doctor. Or  some extra lesson for math.
Or maybe they had one lesson more than you and learnt that calculating the RMS of an AC coupled signal is still RMS. Would you say that if you enable a bandwidth limit on your scope, it's no longer measuring RMS? RMS just means the root of the mean of squared values. It can be for AC or AC+DC. Some meters are marked like that and can measure both. Other meters can only measure the AC part and you have to do the math yourself to add the DC part. And yet other meters can only measure AC+DC, and you need to do math to subtract DC to get the AC value. As long as meters are clearly marked as AC or AC+DC I don't see a problem.

AC coupling balances the signal either side of zero volts, so to all intents & purposes, an ac coupled 0-4.1v square wave becomes
a continuous voltage of 2.05 volts, as power doesn't care about polarity.
A "true RMS" meter should display it as 2.05 volts RMS.

A "non-true RMS" meter will display 0.7071 of peak, yielding 2.899 volts if DC coupled, or 1.496 volts if ac coupled------- if, of course, the correction factor does work for non-sinusoidal signals.
 

Offline Kleinstein

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Re: Which DMMs can reasonably measure RMS?
« Reply #42 on: April 28, 2023, 06:13:35 am »
The scope has an RMS "measurement" you can bring up (among other things). I turned on 8x averaging as well. Yes, scopes are not exactly known for their vertical accuracy, but it's the best I can do.

Not having the vertical scale set as near as possible to ADC's full scale would add inaccuracy, I'm not sure what the optimal horizontal scaling would be for RMS measurement on a DSO although I'd suspect one full cycle and that it may vary from one scope to another.
Which data are used for the RMS calculation depends and cause some trouble. Some use all the data in the memory and some only the data on the screen. Multiple full periods may be less sensitive to a rest of a partial period.
Using averaging mode will remove some of the noise, but the noise in the signal is also part of the RMS value. So averaging (over multiple triggered sections) mode is more like a bad idea.
 

Offline alm

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Re: Which DMMs can reasonably measure RMS?
« Reply #43 on: April 28, 2023, 08:22:31 am »
Many scopes (Tektronix and Lecroy scopes I have used, but also Siglent I believe) have a cycle RMS/cRMS option that just measures the RMS of a single cycle. This is less sensitive to partial cycles and horizontal scaling. For this you should adjust the horizontal scale for a bit more than one period of the signal.
 
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Online 2N3055

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Re: Which DMMs can reasonably measure RMS?
« Reply #44 on: April 28, 2023, 08:25:15 am »
The scope has an RMS "measurement" you can bring up (among other things). I turned on 8x averaging as well. Yes, scopes are not exactly known for their vertical accuracy, but it's the best I can do.

Not having the vertical scale set as near as possible to ADC's full scale would add inaccuracy, I'm not sure what the optimal horizontal scaling would be for RMS measurement on a DSO although I'd suspect one full cycle and that it may vary from one scope to another.
Which data are used for the RMS calculation depends and cause some trouble. Some use all the data in the memory and some only the data on the screen. Multiple full periods may be less sensitive to a rest of a partial period.
Using averaging mode will remove some of the noise, but the noise in the signal is also part of the RMS value. So averaging (over multiple triggered sections) mode is more like a bad idea.

Many scopes calculate RMS on whole buffer or screen AND have Cycle RMS and Cycle Stdev (AC RMS), that will calculate for either one single full period OR each period on screen.

Problem with accuracy on RMS calculation on full screen is what gets into calculation and it is more complicated with nonperiodic signals.. If you set timebase so it chops off piece of period RMS is going to be wrong..   If you have signal with large crest factor (large pause and then short spike) you have to capture exactly from same point at beginning and end of one period so you have exactly one period.

This is where Cycle RMS is useful, or you use gating and select which part of the curve you are measuring. Also, if scope can do it, use measurement cursors connected to measurement so scope will show you what exactly it is calculating on...

Or you simply go with deep memory, and set timebase slower to put hundreds of periods on screen. That will push down errors caused by chopping the signal off at beginning and end of the screen. It's good when you pretty much can't see much of the signal and it looks like a continuous block... For nonperiodic signal (like switcher going into discontinuous mode) that is only way to get any real estimate...

As for statistics, some scopes calculate exactly one measurement per screen (usually leftmost on the screen) and calculate stats between triggers (repetitive captures) and some are capable to capture long capture and if you do Cycle RMS, it will calculate it for every cycle it detects, 10s of thousands at the time from single capture....
Both of those will give you good estimate in slightly different way...

Averaging the measurements is good idea, because it does average of measurements of full signal not the measurement of average of signal...

16bit and 12bit scopes will give you excellent RMS results, because they have wider BW than any multimeter and they show you what are you measuring... So you get data and understanding of signal.... Even decent 8bit scopes will give you good accuracy compared to average meters in RMS measurements...

That being said, all my meters do good job in their BW. AC+DC TRMS is reason why I like dual or triple display meters...
"Just hard work is not enough - it must be applied sensibly."
Dr. Richard W. Hamming
 
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Offline gamalot

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Re: Which DMMs can reasonably measure RMS?
« Reply #45 on: April 28, 2023, 02:46:58 pm »
Oscilloscopes have both DC RMS and AC RMS measurements for a reason.
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