Author Topic: Which DMMs can reasonably measure RMS?  (Read 5303 times)

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Offline W6ELTopic starter

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Which DMMs can reasonably measure RMS?
« on: April 25, 2023, 05:36:08 pm »
I was surprised the other day when I tried measuring the RMS value of a rather standard TTL square wave on several "True RMS" meters.

The waveform was 0 to ~4.1 volts, 50% duty cycle. I ran the test at several frequencies (100Hz, 2 KHz, 200KHz)  just to make sure. Every meter I had basically failed to even get close to the correct RMS value, which should be ~2.90 volts (Vpp * sqrt(D)). The only gear I have which measured it correctly was my HP 54645D scope (not really surprising).

I figured a TTL waveform has to be one of the more common waveforms people would measure using a handheld meter. After all, TTL appears in many low-speed digital circuits. I wouldn't expect to measure more complex waves accurately or higher-speed waveforms (especially with a handheld unit), but come on!

Of particular disappointment is the more modern Owon meters, which surely could take a few readings and do an actual calculation? One would think? Even the scope mode on the HDS272S (a great handheld scope) lacks an RMS readout, providing only Vpp, Vax, Vmin, and Vamp (which reads the same value as Vpp).

The HP 400EL is excused of course, since it is calibrated to read RMS only for a sine wave (like many analog meters of its time). Somewhat ironically it provided a closer measurement than most of the other equipment though.

I get that RMS requires a calculation. But come on. How much trouble is it to take a few consecutive samples at 5 bits resolution and do the calculation? Surely modern meters can do such a thing?

Does anyone know of a good meter that would pass the TTL test? Maybe one of the EEVBlog models? Agilent? Fluke?

Here are my results at 2 KHz. I have measured using both DC and AC since this is a fully-positive signal:

  • HP 4645D scope: 3.035V RMS, 4.094Vpp
  • HP-400EL: 2.22V (3V scale used, read 74% of full scale)
  • Tenma 72-410A True RMS: 2.224 (AC), 2.289 (DC)
  • Owon B35T TrueRMS meter: 2.076 (AC), 2.278 (DC)
  • RadioShack TrueRMS meter (can't find the model): 2.064 (AC), 2.278 (DC)
  • Owon HDS272S: 1.761 (AC), 2.281 (DC)

I'm quite disappointed. I had thought the so-called "True RMS" would be a bit closer than this. How do the expensive meters stack up against my hobby-lot?

Offline nctnico

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Re: Which DMMs can reasonably measure RMS?
« Reply #1 on: April 25, 2023, 05:43:45 pm »
You have to be very carefull when selecting a DMM that can measure odd waveforms and other frequencies than 50Hz. Most handheld meters are 50Hz only.

But you don't have to buy extremely expensive meters. Vici VC8145 is one that can measure RMS in the audio frequency range because it has a dedicated RMS converter chip inside.
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Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #2 on: April 25, 2023, 05:48:24 pm »
You have to be very carefull when selecting a DMM that can measure odd waveforms and other frequencies than 50Hz. Most handheld meters are 50Hz only.

But you don't have to buy extremely expensive meters. Vici VC8145 is one that can measure RMS in the audio frequency range because it has a dedicated RMS converter chip inside.

Have you actually tested this with something other than a sine wave?

I tried down at 60 Hz as well, same results. I believe at least one of these meters I mentioned has one of those "RMS chips" inside.

Offline Kleinstein

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Re: Which DMMs can reasonably measure RMS?
« Reply #3 on: April 25, 2023, 06:10:48 pm »
The TTL signal is a mixed AC+DC signal.
For the RMS reading there are 2 ways to handle this:
1) have AC coupling and thus only show the AC part. For the 50% square wave this would be the same AC as DC reading (50% of peak voltage if the low voltage is at zero).
2) a combined DC+AC RMS values and thus the AC part + DC part as the geometric sum. In this case 1.41 * the DC reading.

A few meters offer both version and many meter offer only the AC coupled case. Usually the manul will tell.

So the Tenma, Owon B35T and radioshack meters don't look that bad.

Many handheld DMMs don't work to very high frequencies. So 2 kHz square wave can already be a bit on the fast side and thus a lower than expeced reading.
 
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Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #4 on: April 25, 2023, 06:33:16 pm »
The TTL signal is a mixed AC+DC signal.
For the RMS reading there are 2 ways to handle this:
1) have AC coupling and thus only show the AC part. For the 50% square wave this would be the same AC as DC reading (50% of peak voltage if the low voltage is at zero).
2) a combined DC+AC RMS values and thus the AC part + DC part as the geometric sum. In this case 1.41 * the DC reading.

A few meters offer both version and many meter offer only the AC coupled case. Usually the manul will tell.

So the Tenma, Owon B35T and radioshack meters don't look that bad.

Many handheld DMMs don't work to very high frequencies. So 2 kHz square wave can already be a bit on the fast side and thus a lower than expeced reading.

It's not the frequency. I tried even 60 Hz, same results. I'm actually surprised how well all these meters handle higher frequencies. The Tenma bench meter works well above 200 KHz, and the HP 400 really does work fine at 10 MHz! The others are pretty flat in the usual audio range at least, tapering off some as you get higher.

While I can understand RMS being different for AC and DC coupled measurements, the RMS calculation should be conceptually a differential measurement from Vmin to Vmax at duty cycle D.

What would an HP-3400 (a meter that converts the energy to thermal energy) read for a square wave about zero versus one with DC bias?

Offline radiolistener

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Re: Which DMMs can reasonably measure RMS?
« Reply #5 on: April 25, 2023, 07:03:57 pm »
square wave has too many harmonics, so RMS measurement will depends on bandwidth. Since different DMM have different bandwidth, they show different results.
 
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Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #6 on: April 25, 2023, 07:14:37 pm »
square wave has too many harmonics, so RMS measurement will depends on bandwidth. Since different DMM have different bandwidth, they show different results.

Indeed, but a meter that is shown to be flat with a sine wave up to and beyond 20 KHz should be able to handle many harmonics of a 60 Hz square wave with full fidelity, preserving the RMS level to at least three significant figures. Any meter claiming "True RMS" ought to at least be able to handle 60 Hz, right? The tenth harmonic of 60 Hz is only 600 Hz after all. 

Offline Kleinstein

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Re: Which DMMs can reasonably measure RMS?
« Reply #7 on: April 25, 2023, 08:16:48 pm »
With analog RMS it is not just the harmonics, but also an effect of the slew rate. Usually there is some amplification / buffer with a limited slew rate. A 2nd point may be the precision rectifier, that can also have problems with a very high slew rate and this way loose some of the amplitude.  The RMS part is by design nonlinear and thus does not handle the harmonics separate.
 

Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #8 on: April 25, 2023, 08:24:45 pm »
With analog RMS it is not just the harmonics, but also an effect of the slew rate. Usually there is some amplification / buffer with a limited slew rate. A 2nd point may be the precision rectifier, that can also have problems with a very high slew rate and this way loose some of the amplitude.  The RMS part is by design nonlinear and thus does not handle the harmonics separate.

Alright but what would be the use in an RMS measurement that only works on pure sine waves? I mean, any meter that isn't RMS can be calibrated to read the RMS of a sine wave correctly. The entire reason to have RMS circuits is, indeed, to be able to integrate and calculate over some period of measurement. The slew rate for a 60 Hz square wave should not be an issue given how long the period of a 60 Hz signal is. Any op-amp from the 1970s could easily slew up and down for 60 Hz in under 1% of a period.

Offline GLouie

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Re: Which DMMs can reasonably measure RMS?
« Reply #9 on: April 25, 2023, 08:47:28 pm »
The EEVBlog BM235 would no doubt disappoint you. Dave points out that with a limited bandwidth, a few harmonics of 60Hz is not typical with low cost.

Quote from manual:

True RMS
RMS (Root-Mean-Square) is a term used to describe the effective or equivalent DC value of an AC signal. True RMS is the term which identifies a DMM that responds accurately to the effective RMS value regardless of the waveforms such as: square, sawtooth, triangle, pulse trains, spikes, as well as distorted waveforms with the presence of harmonics. Harmonics may cause :
1) Overheated transformers, generators and motors to burn out faster than normal
2) Circuit breakers to trip prematurely
3) Fuses to blow
4) Neutrals to overheat due to the triplen harmonics present on the neutral
5) Bus bars and electrical panels to vibrate

Dave's Note: Whilst this is a True RMS multimeter (that's good), like most lower end multimeters it does not use a separate True RMS converter chip. It relies upon the internal multimeter chipset capability. This gives the meter a low True RMS frequency response of only a few hundred Hz (check the specs), and is typical of other meters in this class. Don't be fooled thinking that “True RMS” automatically means “high frequency range measurement”.
 

Offline nctnico

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Re: Which DMMs can reasonably measure RMS?
« Reply #10 on: April 25, 2023, 09:29:31 pm »
You have to be very carefull when selecting a DMM that can measure odd waveforms and other frequencies than 50Hz. Most handheld meters are 50Hz only.

But you don't have to buy extremely expensive meters. Vici VC8145 is one that can measure RMS in the audio frequency range because it has a dedicated RMS converter chip inside.

Have you actually tested this with something other than a sine wave?
Yes. I can set my generator to RMS and the VC8145 tracks the level nicely. But it measures only the AC part of the signal.
« Last Edit: April 25, 2023, 09:54:25 pm by nctnico »
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Offline Fungus

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Re: Which DMMs can reasonably measure RMS?
« Reply #11 on: April 25, 2023, 09:51:16 pm »
I figured a TTL waveform has to be one of the more common waveforms people would measure using a handheld meter. After all, TTL appears in many low-speed digital circuits.

It's a common signal but nobody measures TRMS of it.  :-DD

Stick to your oscilloscope for looking at TTL signals - shape is more important.
 

Offline bdunham7

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Re: Which DMMs can reasonably measure RMS?
« Reply #12 on: April 25, 2023, 09:59:10 pm »
While I can understand RMS being different for AC and DC coupled measurements, the RMS calculation should be conceptually a differential measurement from Vmin to Vmax at duty cycle D.

What would an HP-3400 (a meter that converts the energy to thermal energy) read for a square wave about zero versus one with DC bias?

I'm not sure what you are trying to get at in the first statement.  The RMS value of a 0 to 4.1V 50% duty cycle square wave would be 2.05V AC and 2.90V AC+DC, or DC coupled, RMS.  In addition, the integrated DC value should also be 2.05VDC, meters not reading close to that on a 2kHz signal as you've described probably have terrrible normal mode rejection.  If you have a meter that does AC and DC, but not AC+DC (or DC-coupled RMS if you prefer) then you can measure DC and AC separately, square the results and add them and the square root of that sum is your AC+DC result.

A thermal transfer measuring instrument would measure the latter if it had an DC-coupled input, but the HP 3400A has an AC-coupled input with a 10Hz lower cutoff (or spec anyway, the cutoff may be lower) so I'd presume it would read the former.  I don't have one here at the moment to confirm that.  I do have a different thermal transfer meter, but there's no room on the bench for it at the moment.  I don't think it is necessary to go to that extreme for such a simple example, but it is probably the only meter I have that is going to read the TRMS of a 200kHz square wave with any accuracy.

Every TRMS meter I have will perform as I've stated and the only other limitation would be the bandwidth, obviously there will not be very many meters that will read a 200kHz square wave accurately.

Quote
How do the expensive meters stack up against my hobby-lot?

Frankly yours are looking like rubbish!  :)

Seriously, they're just wrong.  Or something is wrong in any case.  If your signal is actually 0.00 to 4.10V and there isn't a loading issue, then only the HP 400EL is actually showing what it ought to with any reasonable accuracy.  Every other instrument is off by enough to be considered a fail (IMO) except maybe the scope which would need further discussion to conclude anything.

« Last Edit: April 25, 2023, 10:47:18 pm by bdunham7 »
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Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #13 on: April 25, 2023, 11:05:26 pm »
The TTL signal is a mixed AC+DC signal.
For the RMS reading there are 2 ways to handle this:
1) have AC coupling and thus only show the AC part. For the 50% square wave this would be the same AC as DC reading (50% of peak voltage if the low voltage is at zero).
2) a combined DC+AC RMS values and thus the AC part + DC part as the geometric sum. In this case 1.41 * the DC reading.

A few meters offer both version and many meter offer only the AC coupled case. Usually the manul will tell.

So the Tenma, Owon B35T and radioshack meters don't look that bad.

Many handheld DMMs don't work to very high frequencies. So 2 kHz square wave can already be a bit on the fast side and thus a lower than expeced reading.

I read and re-read what you said. You were exactly correct. The DMM in AC mode is ac-coupled, and thus the result is given for the AC-coupled version of the signal.

Placing the scope's input in AC-coupled mode shows 2.045V RMS, which means that my list was sorted in reverse! The Owon B35T and radioshack meters were almost dead-on, and all others within about 10%. 

This makes a lot of sense. So to sum it up:
1. There was not a problem with bandwidth or high frequency cutoff
2. There was no issue with slewrate

The bottom line is that the DMM places the input in AC coupled mode, and thus, any result must be considered from that point of view. There are still some discrepancies, but they are much less severe once you consider the point of view of the measurement.

Offline shakalnokturn

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Re: Which DMMs can reasonably measure RMS?
« Reply #14 on: April 25, 2023, 11:28:00 pm »
Interesting topic... It got me a little puzzled.
Sorry for my ignorance, I fail to see where the theoretical 2.9V RMS or the 3.035V RMS measured on the DSO come from.

Is this about a "0" to 4.1V amplitude 50% duty cycle square wave?
How far off zero was the real low level?
 

Offline bdunham7

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Re: Which DMMs can reasonably measure RMS?
« Reply #15 on: April 25, 2023, 11:28:44 pm »
What is the source of your signal?  One possible explanation for the discrepancies that I see (discrepancies that seem fairly consistent, so perhaps I've maligned your meters unnecessarily) is that you have about a 135mVDC offset on the signal,  so it is 0.135 to 4.235V. 
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Offline bdunham7

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Re: Which DMMs can reasonably measure RMS?
« Reply #16 on: April 25, 2023, 11:37:21 pm »
Sorry for my ignorance, I fail to see where the theoretical 2.9V RMS or the 3.035V RMS measured on the DSO come from.

50% duty cycle, so half the time it is zero, the other half 4.10V.  So (4.1)2 is 16.81 (the squares), half of that is 8.405 (the mean), the square root of that is 2.90V (the root). 

Quote
How far off zero was the real low level?

I think that's the issue with the readings being off from what I'd expect, but I don't think it was the OPs primary question.  As I wrote, it looks like a ~135mVDC offset would account for all of the errors except the AC readings of the Tenma and the OWON scope-thingy.
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Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #17 on: April 25, 2023, 11:50:24 pm »
Interesting topic... It got me a little puzzled.
Sorry for my ignorance, I fail to see where the theoretical 2.9V RMS or the 3.035V RMS measured on the DSO come from.

Is this about a "0" to 4.1V amplitude 50% duty cycle square wave?
How far off zero was the real low level?

The source was single-ended and grounded at the scope's input, and zero was definitely zero. It's a test generator with a TTL output (among others).

And yes, it's 0 to about 4.1 volts.

The real RMS value is Vmax * sqrt(D) for a square wave, where D is the duty cycle, so basically 4.1 * sqrt(1/2) = 4.1*0.707 = 2.90 volts.

However, the DMM has a capacitor in series when it does AC readings. So the DMM is showing the RMS voltage for an AC coupled version of the input signal. This means it is measuring a square wave with a total amplitude peak to peak of 4.1 volts, centered at zero volts, with vmin = -4.1/2 = -2.05 and vmax = 4.1/2 = +2.05 volts. It's easy to visualize how the RMS is calculated from this data, and it's no wonder that the AC-coupled meters all reported values around 2 volts.

Placing the scope in ac-coupled mode put it at the same vantage point, and read about 2 volts as well.

The lesson here is to do tests just like this and learn more about your test equipment. Who would have thought a DMM from RadioShack purchased in 2002 would be that good, even at 200 KHz?

Offline joeqsmith

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Re: Which DMMs can reasonably measure RMS?
« Reply #18 on: April 26, 2023, 12:20:21 am »
Welcome to the forum.  Assuming that is your ham call sign, your photo of your lab looks nice.  I had one of those 141Ts many years ago with the 1.3ish GHz plug in, and a couple others.  I had the tracking generator for it as well. 

Anyway,  I setup my arb with a 50ohm load.  Peak was measured with my HP34401A at 4.9155 or roughly 3.476 VRMS.   

I measured seven different meters at 100Hz, 2kHz and 200kHz.   Attached showing the measured values and their error relative to the 34401A. 

The CEM is the lowest cost out of the group at $120 on sale.  The Gossen Ultra was by far the most expensive, now rebranded as Prime after my review of it.   The BM789 is an early pre-release and I have done a some rework to bring it up to the latest revision (using factory parts).   The BM689s shown is the first one I purchased several years back.  It was damaged during my testing and I did repair it.  None of these meter have been realigned.   

Once the UT181A with it's odd ball rechargeable battery gets a charge, I will measure it.  Bad design but still one of my favorite products from UNIT.

*** 
The UT181A was allowed to charge to 40% and I retook the first measurement plus the others. 

Because you seem to like old hardware, I tried an old Fluke 97 scope meter but like the Gossen, it could not read the value at 200kHz in DMM mode.   

I saved an old Fluke 8506A Thermal RMS meter from the recycle bin that needed repairs.  I aligned the DC stages using my HP34401A as a reference.   The AC stages are still factory set as I don't have anything near this accurate and thought I would do more harm than good.   For fun, I show the signal at 2MHz compared with the UT181A.   

I'm not sure what accuracy you need but my personal pick of these meters is still the BM869s.   I would take the Fluke 189 if they still offered them new.  The 789 is a nice meter as well.  Has a few things on the BM869s but I like the multi displays.   

Also note the previous percentage was off 100X  and has been corrected.
« Last Edit: April 26, 2023, 01:38:12 am by joeqsmith »
 
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Offline GigaJoe

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Re: Which DMMs can reasonably measure RMS?
« Reply #19 on: April 26, 2023, 02:28:08 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





 

Offline W6ELTopic starter

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Re: Which DMMs can reasonably measure RMS?
« Reply #20 on: April 26, 2023, 04:30:45 am »
Welcome to the forum.  Assuming that is your ham call sign, your photo of your lab looks nice.  I had one of those 141Ts many years ago with the 1.3ish GHz plug in, and a couple others.  I had the tracking generator for it as well. 

Anyway,  I setup my arb with a 50ohm load.  Peak was measured with my HP34401A at 4.9155 or roughly 3.476 VRMS.   

I measured seven different meters at 100Hz, 2kHz and 200kHz.   Attached showing the measured values and their error relative to the 34401A. 

The CEM is the lowest cost out of the group at $120 on sale.  The Gossen Ultra was by far the most expensive, now rebranded as Prime after my review of it.   The BM789 is an early pre-release and I have done a some rework to bring it up to the latest revision (using factory parts).   The BM689s shown is the first one I purchased several years back.  It was damaged during my testing and I did repair it.  None of these meter have been realigned.   

Once the UT181A with it's odd ball rechargeable battery gets a charge, I will measure it.  Bad design but still one of my favorite products from UNIT.

*** 
The UT181A was allowed to charge to 40% and I retook the first measurement plus the others. 

Because you seem to like old hardware, I tried an old Fluke 97 scope meter but like the Gossen, it could not read the value at 200kHz in DMM mode.   

I saved an old Fluke 8506A Thermal RMS meter from the recycle bin that needed repairs.  I aligned the DC stages using my HP34401A as a reference.   The AC stages are still factory set as I don't have anything near this accurate and thought I would do more harm than good.   For fun, I show the signal at 2MHz compared with the UT181A.   

I'm not sure what accuracy you need but my personal pick of these meters is still the BM869s.   I would take the Fluke 189 if they still offered them new.  The 789 is a nice meter as well.  Has a few things on the BM869s but I like the multi displays.   

Also note the previous percentage was off 100X  and has been corrected.

This is great, what fun!

Yes, there is something attractive about the older stuff, I don't even know what it is. At work we have scopes that cost more than my house, but I just don't find it as fun.

Was your square wave with or without DC bias? I am seeing that without DC bias my meters agree much better.

--E

Offline rf-loop

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Re: Which DMMs can reasonably measure RMS?
« Reply #21 on: April 26, 2023, 05:55:26 am »
I was surprised the other day when I tried measuring the RMS value of a rather standard TTL square wave on several "True RMS" meters.

The waveform was 0 to ~4.1 volts, 50% duty cycle. I ran the test at several frequencies (100Hz, 2 KHz, 200KHz)  just to make sure. Every meter I had basically failed to even get close to the correct RMS value, which should be ~2.90 volts (Vpp * sqrt(D)). The only gear I have which measured it correctly was my HP 54645D scope (not really surprising).

I figured a TTL waveform has to be one of the more common waveforms people would measure using a handheld meter. After all, TTL appears in many low-speed digital circuits. I wouldn't expect to measure more complex waves accurately or higher-speed waveforms (especially with a handheld unit), but come on!

Of particular disappointment is the more modern Owon meters, which surely could take a few readings and do an actual calculation? One would think? Even the scope mode on the HDS272S (a great handheld scope) lacks an RMS readout, providing only Vpp, Vax, Vmin, and Vamp (which reads the same value as Vpp).

The HP 400EL is excused of course, since it is calibrated to read RMS only for a sine wave (like many analog meters of its time). Somewhat ironically it provided a closer measurement than most of the other equipment though.

I get that RMS requires a calculation. But come on. How much trouble is it to take a few consecutive samples at 5 bits resolution and do the calculation? Surely modern meters can do such a thing?

Does anyone know of a good meter that would pass the TTL test? Maybe one of the EEVBlog models? Agilent? Fluke?

Here are my results at 2 KHz. I have measured using both DC and AC since this is a fully-positive signal:

  • HP 4645D scope: 3.035V RMS, 4.094Vpp
  • HP-400EL: 2.22V (3V scale used, read 74% of full scale)
  • Tenma 72-410A True RMS: 2.224 (AC), 2.289 (DC)
  • Owon B35T TrueRMS meter: 2.076 (AC), 2.278 (DC)
  • RadioShack TrueRMS meter (can't find the model): 2.064 (AC), 2.278 (DC)
  • Owon HDS272S: 1.761 (AC), 2.281 (DC)

I'm quite disappointed. I had thought the so-called "True RMS" would be a bit closer than this. How do the expensive meters stack up against my hobby-lot?

Brymen BM859S (europa version)
Input
2kHz  4.10 / 0.00V   square wave (no need tell 50% because if it is not 50% then it is not square wave).
Brymen  display RMS 2.8932   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.
« Last Edit: April 26, 2023, 05:59:22 am by rf-loop »
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Offline alm

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Re: Which DMMs can reasonably measure RMS?
« Reply #22 on: April 26, 2023, 07:46:00 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.
« Last Edit: April 26, 2023, 07:47:42 am by alm »
 

Offline joeqsmith

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Re: Which DMMs can reasonably measure RMS?
« Reply #23 on: April 26, 2023, 12:34:01 pm »
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Anyway,  I setup my arb with a 50ohm load.  Peak was measured with my HP34401A at 4.9155 or roughly 3.476 VRMS.   
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Was your square wave with or without DC bias? I am seeing that without DC bias my meters agree much better.
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In the lower left of the spreadsheet is the reference.  It was a squareish sort of waveform with a 50% dutycycle, 0 volt minimum and 4.9155 volt peak.   The calculated RMS is last.   The arb was loaded to 50 ohms and the peak level was measured with my old HP.   

That old thermal RMS meter was set to normal mode when making these measurements.   Nothing was allowed to warmup.  The Arb isn't clean.  Simple test wasn't meant to as a dive down the metrology hole. 

Offline bdunham7

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Re: Which DMMs can reasonably measure RMS?
« Reply #24 on: April 26, 2023, 01:35:39 pm »
Brymen BM859S (europa version)
Input 2kHz  4.10 / 0.00V   square wave (no need tell 50% because if it is not 50% then it is not square wave).
Brymen  display RMS 2.8932   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.

You seem to get a little hung up on semantics and while you have one interpretation of the terms 'square wave' and AC vs DC RMS, others may think differently.  You can argue that they are wrong or you can take care to clarify exactly what is meant.  I prefer the latter approach, the former somehow seems small minded.

So, since your 859s is 'correctly' designed and since you state that any meter that doesn't display what I would call TRMS AC+DC is demented, which of these ranges did you select to get that reading?

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.
 


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