Are RMS multimeters a con? - Page 1
Forum section: electronics / testgear

author: DAIRVINE
author_rank: Contributor
timestamp: March 09, 2015, 09:29:18 am
content: I have an application for measuring phase angle on an electrical supply by measuring voltage, voltage across a resistor for current and the vector sum of both. Use of trigonometry derives phase angle. But you need to use an averaging meter for good results. When the current and volts are in phase the averaging meter will show this, but the rms meter could give a false phase.Does anyone ever need RMS AC voltage measurements? OK, for RF power, but multimeters are no good for this. Most equipment now is electronic and if you want power delivered to load you need to multiply current and volts. RMS volts does not cut it except for a constant resistance load, which barely exists anymore, maybe an electric heater element is as close as it gets, but an averaging meter works fine for this. A non sinusoidal power supply into a resistor load? Never needed that in my career.RMS has complexity and battery life costs and different meters could give different results with high crest factors.Additonal (corrected):Instead of RMS why not true power?Instead of SQRT(average (V^2)) or SQRT(average (I^2))Please could meter manufacturers doAverage ((V*I))

author: electr_peter
author_rank: Unknown
timestamp: March 09, 2015, 10:40:53 am
content: Quote from: DAIRVINE on March 09, 2015, 09:29:18 amYou need simultaneous measurement setup of voltage and current (preferably an oscilloscope) to get a phase angle or power factor. I don't know what you do, but it sounds like TRMS or AVG DMM would make no difference.QuoteYou do understand that for power measurement it is not enough to multiply V and I (whether TRMS or AVG), right? Have you heard about devices called SMPS? They load line with non-sinusoidal load. For simple applications and sine waves TRMS=AVG, so what is the problem?QuoteIt depends, see specs.I don't get your rant. TRMS DMMs are here to provide TRMS results for somewhat non-sinusoidal waveforms because simple AVG meters flat out lie for anything other than sine wave. AVG type can be a benefit for some setups but you need bench type >6 digits DMM with special settings configuration to get any benefit of it. Power factor and phase angle has to be measured by checking voltage and current with high sampling rate, TRMS vs AVG has little difference here.TRMS meters have their place, AVG meters have their place, TRMS is superior in many cases. Choose right meter for the job according to your specs and right measurement techniques.

author: Ericho
author_rank: Regular Contributor
timestamp: March 09, 2015, 10:54:58 am
content: A Con..No not at allIt just tells you what the number on the display means, nothing more.There are plenty of electrician meters for sale that use good old fasion averaging acVIn the field I couldn't care less if the dmm is true rms or not, as long as the number is in the ballpark

author: rsjsouza
author_rank: Super Contributor
timestamp: March 09, 2015, 11:21:23 am
content: Some articles may help understand the benefits of TRMS.http://www.newark.com/pdfs/techarticles/fluke/whyTueRMS.pdfhttp://www.testequipmentdepot.com/application-notes/pdf/hvac/hvac-applications-where-true-rms-rest-tools-matter_an.pdfThese ones are a bit more technically deep:http://www.escoglobal.com/resources/pdf/white-papers/True_G2.pdfhttp://educypedia.karadimov.info/library/5988-6916EN.pdf

author: rs20
author_rank: Super Contributor
timestamp: March 09, 2015, 11:41:33 am
content: Quote from: electr_peter on March 09, 2015, 10:40:53 amEh? The term "phase angle" implies that we're dealing with sinusoidal signals, and with sinusoidal signals, you can indeed (at least theoretically) perform the vector sum of voltage and current and compare it against voltage and current signals alone to derive phase angle. E.g.If the phase angle between voltage and current is 0 degrees, they combine constructively, so the RMS of (voltage + IR) will be (RMS of voltage) + (RMS of IR).Phase angle is 90 degrees, RMS of (voltage + IR) will be hypotenuse ( (RMS of voltage), (RMS of IR))Phase angle is 0 degrees, destructive inteference, RMS of (voltage + IR) will be |(RMS of voltage) - (RMS of IR) |.Everything inbetween is given by the cosine rule. So, with three DMMs, you can measure phase difference. Is it accurate or an advisable way to do it? No commentThe thing I don't get is why using an RMS meter would cause problems in particular. If your signals are truly sinusoidal, the amplitude ratios between the summed and non-summed signals will be the same, whether or not the DMM is RMS. Replace all instances of RMS above with mean-of-absolute-value, and it still works just fine. Any function which obeys k*f(X)=f(k*X) (i.e., double any waveform, and the reported voltage doubles) works fine.EDIT: Just to be crystal clear about this, if you have one signal that the DMM reports as 12V, and another that the DMM reports as 1V, and they're identical modulo scaling ("in phase" in the sinusoidal case), the summed signal will report as 13V, both for an averaging meter and an RMS meter (even though different DMMs might have a different opinion on what a given signal is, they will all [at least theoretically] follow the summing rule I've just stated).

author: DAIRVINE
author_rank: Contributor
timestamp: March 09, 2015, 12:01:42 pm
content: "You need simultaneous measurement setup of voltage and current (preferably an oscilloscope) to get a phase angle or power factor. I don't know what you do, but it sounds like TRMS or AVG DMM would make no difference."Yes I do get that. I am looking at vectors here and using trigonometry. What I am doing is measuring voltage and current spearately and then both together and deriving power factor from trigonometry. For sine waves you can use AVG or RMS, for SMPS with peaky current I think AVG gives a more correct phase angle answer than RMS. So I questioned where RMS is an advantage and don't really see any advantage for RMS."simple AVG meters flat out lie for anything other than sine wave."In the context of power transfer to a resistive load this is correct if you want the equivalent DC voltage. I do understand this, just questioning whether it is still relevant. For anything complex I would use a scope.For DC voltages I am quite happy with average, even for 6.5 digit meters. For measuring battery consumption I would use average so I can work out the battery life. For DC I prefer average even when the load is peaky.Where is RMS important?An article linked by rsjsouza said:"The purpose of RMS value is to simplify power calculations by providing equivalent DC voltage that would develop the same average power in resistive load."When do we ever see resistive loads? RMS is pointless if the load is not resistive."if you have one signal that the DMM reports as 12V, and another that the DMM reports as 1V, and they're in phase, the summed signal will report as 13V, both for an averaging meter and an RMS meter"True if both are sine waves. It think it does not work for RMS is one wave is, for example, smps.

author: rs20
author_rank: Super Contributor
timestamp: March 09, 2015, 12:06:38 pm
content: Quote from: DAIRVINE on March 09, 2015, 12:01:42 pmWhat does phase angle even mean in the context of non-sinusoidal signals? Is that a well-defined concept? I'm curious to see a definition of phase angle and an associated proof that an averaging meter gives a closer value to this definition than the RMS meter.

author: electr_peter
author_rank: Unknown
timestamp: March 09, 2015, 12:43:19 pm
content: Quote from: DAIRVINE on March 09, 2015, 12:01:42 pmI get that method, but I doubt accuracy of such technique for unknown loads.As for TRMS, it does not give you any advantage over the AVG when both methods are appropriate (TRMS=AVG in that case), but TRMS is TRMS for non-sine signals (AVG does not provide correct results for non-sine wave). Also, remember that AVG metersare designedto provide TRMS reading for a sine waves. TRMS type result is useful for AC analysis and measurements.However, for any given signal type or waveform (except very high peaks)TRMS = AVG * constant, so any trigonometric operations are not affected. So I still do not see the problem in your case. If V is sine wave and I is not sine wave, V/I/phase angle method is simply wrong.QuoteYou should differentiate between measuring technique for a single measurement and measurements over time period. High end >6 digits DMM may use AVG(multiple samples for a single measurement) or TRMS circuitry to get TRMS measurement for AC waveform, and then integrate/add/avg/TRMS resultingseries of measurements. Initial measurement type is still TRMS type usually (for a sample time scale).Quote"Pointless" for you, maybe not for others. AVG is considered misleading with some signal types. AVG DMM may miss peaks due to limited BW anyway, so what is the advantage then?Phase angle for SMPS is not important. Power factor is important which consists of phase angle + sin wave distortion which you cannot determine from trigonometry - you need to get real power, voltage and current (many methods, most reliable with oscilloscope).

author: DAIRVINE
author_rank: Contributor
timestamp: March 09, 2015, 12:57:22 pm
content: rs20:My interest is power factor correction. So magnetizing current and other currents are superimposed. So really I am only interested in the phase angle of the fundamental frequency. So, add in power factor capacitors and look for zero phase.I REMOVED an example which was faulty.Even if RMS and AVG gave the same result, what is the advantage of RMS?

author: dom0
author_rank: Super Contributor
timestamp: March 09, 2015, 01:06:38 pm
content: Quote from: electr_peter on March 09, 2015, 10:40:53 amSee attached pic. This is the mains current drawn by a radio set. And it doesn't even have a SMPS, it's just a standard 50 Hz transformer and some DC/DCs for logic supply behind that.As one can easily see the current waveform isn't even close to what you'd expect after reading a text book 'bout transformer-linear-supplies. It just goes like "wardagblablahurrdurrwardasgabl"¹.¹ no, that isn't a German word

author: electr_peter
author_rank: Unknown
timestamp: March 09, 2015, 01:11:39 pm
content: 10 peak with 1% duty cycle:AC AVG = ~0.22AC TRMS = ~0.995AC+DC TRMS = 120 peak with 1% duty cycle is exactly double the value for all methods.When you have to pick one number to describe waveform you are measuring, TRMS is much better than AVG. AVG may be very confusing, TRMS at least indicates something (actual power of signal). TRMS is used in context of AC mostly becauseCode:power = voltage(TRMS) * current(TRMS) * fnc(power factor or phase angle)EquationCode:power = voltage(AVG) * current(AVG) * fnc(power factor or phase angle)is not valid

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2015, 01:29:08 pm
content: Not your application, but in my former day job I needed to measure noise voltage accurately (roughly audio bandwidth) and a true rms meter was essential. I relied on a Marconi 2610, with its variable averaging time (after the "square", before the "root") to obtain stable results.

author: macboy
author_rank: Super Contributor
timestamp: March 09, 2015, 01:32:50 pm
content: Quote from: DAIRVINE on March 09, 2015, 12:57:22 pmHave you ever looked at the waveform of coming out of an electrical socket?If the supply is heavily loaded with devices using rectifer-capacitor supplies (which is most electronics except the rare one with PFC), then the peaks are heavily loaded and flattened.An average responding meter (which has a scaling factor to give an "RMS" result) will give a value which is higher than the true RMS value. A peak repsonding meter (again with scaling for RMS result) will give a value lower than the true RMS value. The True RMS responding meter will give a reading which can be used to calculate the actual power dissipation in a resistive load. The others give a number which is useful for .... very little if anything.

author: rsjsouza
author_rank: Super Contributor
timestamp: March 09, 2015, 02:36:54 pm
content: Quote from: DAIRVINE on March 09, 2015, 12:01:42 pmThe way I learned is: since the amount of current/voltage provided to a resistive load is purely real (no imaginary component), measuring RMS values of non-resistive loads would "automagically" represent the real component of the power triangle in Watts. In Portuguese we call RMS something similar to "effective value" - the amount of energy/power that yields work.We then used a wattmeter that measured what we used to call "true power" in VA and did some simple math to get the apparent or reactive power.If I haven't missed any detail (it's been 15+ years ago), the RMS measurement is still very relevant.

author: rs20
author_rank: Super Contributor
timestamp: March 09, 2015, 09:43:56 pm
content: I have to endorse the OP's question, and point out that it hasn't been answered yet. Linked documents at the start say that averaging meters give an answer that is "wrong by up to 40%", but only under the presupposition that RMS current is the "correct" one. And the only justification given for RMS presupposes a load that is purely resistive/capacitive/inductive; and that is not what an SMPS (or even a classic rectifier-after-transformer approach) does, as illustrated in this thread.How can TRMS ever be useful, when the most common reason for a non-sinusoidal signal is a non-resistive load, and TRMS is founded upon the assumption of a resistive load? The only use I can contrive is I^2 R losses in cables. And measuring square-wave output inverters with a view to powering resistive toasters or incandescent light bulbs. Which seems very niche these days, even a compact fluoro bulb blasts your assumptions to oblivion, and renders TRMS and averaging equally useless.

author: AG6QR
author_rank: Frequent Contributor
timestamp: March 09, 2015, 10:32:20 pm
content: Quote from: rs20 on March 09, 2015, 12:06:38 pmYou can certainly argue that phase angle is an ill-defined concept for non-sinusoidal signals. For many purposes, I'd agree wholeheartedly.But I've seen phase angle defined as the arccosine of the power factor. In other words, the arccosine of the true power divided by apparent power. True power is defined as the integral of instantaneous power over a full AC cycle. Apparent power is defined as Vrms * Arms.This definition of phase angle matches the traditional definition when the waveforms are all sinusoids, but this definition is independent of the waveforms. Voltage, Current, and Power may all be non-sinusoidal. Notice that this definition doesn't distinguish between leading and lagging phase angles, and you can construct non-sinusoidal waveforms for which the concept of "leading" versus "lagging" doesn't make any sense.To measure this definition of phase angle, you've got to have an RMS measurement of voltage and current, and a true measurement of power. The true measurement of power can come from measuring heat dissipated in the load, or can be approximated by measuring instantaneous voltage and current at many points along the power cycle, an integrating them using a numerical method. The RMS voltage and current measurements come from a true RMS meter, thus answering OP's question about why an RMS meter is needed.

author: rs20
author_rank: Super Contributor
timestamp: March 09, 2015, 10:59:02 pm
content: Quote from: AG6QR on March 09, 2015, 10:32:20 pmThe OP responded to say that he was defining phase angle as the phase angle between the fundamentals; which may be nonstandard* but it seems very relevant and well-suited to his (nevertheless questionable) use case of finding the optimum amount of capacitance to add to achieve a maximum power factor. Furthermore, even more interestingly, I've given it some thought and an averaging meter is indeed much more likely to ignore harmonics (which is what the OP wants) than a TRMS meter.* Unless it turns out to be identical to your standard definition through mathematical magic, which I suspect may perhaps be true since everyone's trying to maximize power factor and doesn't care about harmonics** at the end of the day here.** And we don't care about harmonics if either V (as is in this case) or I is harmonic-free.OP: You realise that your method can't distinguish between leading and lagging phase?

author: dacman
author_rank: Frequent Contributor
timestamp: March 10, 2015, 12:46:39 am
content: 1 VRMS Sine WavePeak to Peak: 2.8284Peak: 1.4142RMS: 1.0000Average (Absolute) Value: 0.9003Average Responding; RMS Sine Wave Calibrated; Meter Indication: 1.00001 VRMS Square WavePeak to Peak: 2Peak: 1RMS: 1.0000Average (Absolute) Value: 1.0000Average Responding; RMS Sine Wave Calibrated; Meter Indication: 1.11071 VRMS Triangle WavePeak to Peak: 3.4641Peak: 1.7321RMS: 1.0000Average (Absolute) Value: 0.8660Average Responding; RMS Sine Wave Calibrated; Meter Indication: 0.9619For a waveform, if each point along the waveform is squared, then these squares are averaged, then the square root is taken of this average, the result is the RMS value, for any waveform, including DC. The RMS value is also the heating value. This is important for many reasons, such as the ampere rating for the wiring in a house would usually be limited by heat caused by amperage. A power transformer would be limited by electrical heating. Wiring in an airplane needs to be sufficient to not overheat. A power resistor (or any resistor) is rated so that it is not overheated. A fuse mainly works by heat. Breakers work by heat (usually). The list goes on and on.

author: rs20
author_rank: Super Contributor
timestamp: March 10, 2015, 01:07:13 am
content: Quote from: dacman on March 10, 2015, 12:46:39 amYep, that's a good point. Even if loads are typically much more interesting than they used to be, and therefore RMS is no use in describing how much power draw/heating they experience, the wires, breakers and fuses leadingtothat load do behave according to the RMS current running through them. I think that fully justifies TRMS being standard forcurrentmeasurement.Just for the sake of argument, what's the least contrived reason someone can come up with for a true RMSvoltagemeasurement? I submit an square wave inverter powering a toaster.[ BTW, pedantically, you got the square root and square the wrong way around there. ]

author: mjkuwp
author_rank: Unknown
timestamp: March 10, 2015, 02:36:28 am
content: This is a discussion that comes up a lot. True RMS is not a con but maybe some day it will be a dated term. It just means that the reading will be correct. Older meters used tricks to accommodate lack of affordable technology at the time and would not give a true reading on all types of waveforms - as others have pointed out.Power factor as it is taught in school assumes some idealized and simplified situations that don't often happen in the real world anymore (pure sine waves shifted merely by a phase angle). I think this academic description of power factor causes confusion. With switching power supplies the current drawn by a load is quite a complex waveform. Incandescent light bulbs draw nearly a pure sine wave and 1.0 power factor but these light bulbs are going away, albeit slowly.Power factor is the ratio of Watts / V*APower factor can be negative and this means power is sent back instead of being drawn. (older terminology assigned a sign to power factor to indicate leading or lagging current)Power measuring ics sample current and voltage at the same time - at several khz sample rate and multiply the values together to obtain the active power.

author: dacman
author_rank: Frequent Contributor
timestamp: March 10, 2015, 02:58:56 am
content: Quote from: rs20 on March 10, 2015, 01:07:13 amYou’re right, I got it backwards. I went back and corrected it.One point I was trying to make is that average responding meters indicate about 11% higher than the average so that they will indicate the RMS value of a sine wave.

author: G0HZU
author_rank: Super Contributor
timestamp: March 10, 2015, 03:13:39 pm
content: QuoteI have an old analogue meter here which is designed to 'just' measure Vrms. That's all it does. It works up to a 20MHz bandwidth and can be useful for lots of measurements. eg complex signals, noise, signal to noise ratio and it can measure down to about 30uV with this 20MHz bandwidth.Meters like this have sold in their thousands for many decades. You can do similar measurements with a decent Trms DMM but usually they only work for bandwidths up to a few hundred kHz.

author: dom0
author_rank: Super Contributor
timestamp: March 10, 2015, 04:20:44 pm
content: Yep, even a 3400A from the 60s does a good job at measuring wideband (~25 MHz bw on mine) noise to reasonably low levels.

author: Guni
author_rank: Regular Contributor
timestamp: March 11, 2015, 06:49:12 pm
content: Can you explain difference between RMS DMM and TRMS DMM ?

author: Guni
author_rank: Regular Contributor
timestamp: March 11, 2015, 07:08:05 pm
content: Answer to my own question:TRMS for correct measurement of mixed-mode (AC+DC) voltages and currents,RMS for correct measurement of possibly non-sinusoid AC (without DC offset)http://www.digitek.com.hk/en/newcon.php?id=12It seems that some TrueRMS DMM on the market are RMS only.

author: dom0
author_rank: Super Contributor
timestamp: March 11, 2015, 07:16:41 pm
content: True-RMS has come up when some manufacturers started to label their shit-meters with "RMS" while they didn't measure RMS at all, but measured average or peak value and scaling that with a factor for a pure 50/60 Hz sine wave. Since those times people say RMS when they mean scaled average and True RMS when they actually just mean RMS.Anotherentirely different aspectis bandwidth.Some RMS voltmeters have a bandwidth specification like "10 Hz - 100 kHz", which obviously removes a DC component, since they only measure the RMS from 10 Hz to 100 kHz - that is, they filter the signal before measuring RMS. This is what most multimeters do.Others are "DC - x MHz" and measure the RMS value from DC to x MHz. This is what many dedicated RMS meters do.And yet another aspect is the permissible crest factor. Some meters don't go very far here, like 3 or 4:1. Better meters can handle 10:1, some even more (mainly depends on the measurement method).

author: c4757p
author_rank: Super Contributor
timestamp: March 11, 2015, 07:18:56 pm
content: Acon?No. </thread>One might discuss whether it'suseful.

author: dom0
author_rank: Super Contributor
timestamp: March 11, 2015, 07:22:27 pm
content: Linked above:Quoteugh, for christ's sake,RTFM. I can't stand people claiming wrong results rooted inignoranceof specifications.

author: electr_peter
author_rank: Unknown
timestamp: March 11, 2015, 10:26:07 pm
content: TRMS vs RMS - pure marketing BS, no real difference. It is either TRMS or not. The real question is if it is AC only TRMS or AC+DC TRMS. Specs define such info.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 12, 2015, 06:12:12 am
content: http://www.newark.com/pdfs/techarticles/fluke/whyTueRMS.pdf

author: rs20
author_rank: Super Contributor
timestamp: March 13, 2015, 12:48:24 am
content: Quote from: Wytnucls on March 12, 2015, 06:12:12 amUgh. This was already linked in #3, but more to the point I hate how it's packed full of the word "correct" when they really mean "RMS" (or "True RMS" if you like), as if TRMS is the one true answer to all questions. There's been some good justifications in this thread for why RMS isoftenwhat you want; but also a demonstration by the OP of a use-case which is better served by averaging (albeit even better served by a tool dedicated to the OP's purpose).Writing datasheets that calls RMS "CORRECT" andscaled averaging* other methods "WRONG" with a little tiny footnote saying "* if you're interested in heating effects" is pretty poor form, and promotes ignorance IMHO. But I suppose, what can you expect from marketing material.</broken record sorry>* Edited in response to coppice below

author: coppice
author_rank: Super Contributor
timestamp: March 13, 2015, 01:48:29 am
content: Quote from: rs20 on March 13, 2015, 12:48:24 amRMS isn't the only useful measure of the voltage of an AC signal. For example, peak voltage is often required and a number of instruments can measure that for you. Even the average of the rectified voltage has real uses. However, scaling the rectified average to be like the RMS would be for a pure sine wave is simply a botch without any physical meaning. It is only produced because its easy to produce. Nobody would sit down and try to develop an instrument specifically to output this value.

author: rs20
author_rank: Super Contributor
timestamp: March 13, 2015, 02:46:31 am
content: Quote from: coppice on March 13, 2015, 01:48:29 amFully agree; despite the way my message made it look like I'm trying to defend scaled averaging in particular, I was really just trying to push the point that RMS is just one of many useful measures (a point which your response justifies much better than my message), and calling RMS the one "correct" measurement is just deceptive and simplistic.Pedantically, the equations used by the OP would cancel out the scaling fudge factor -- but what is perhaps more interesting is this question: Scaling aside, how good is a cheap  averaging multimeter at actually getting the averaging right? I'm guessing they don't specify that in the datasheet. How linear is it, what's the bandwidth? How do they implement and integrate abs(x) in cheap circuitry? It's surprising that they scale an average-of-absolute value instead of similarly fudging a peak value, which I would guess to be easier to measure. If only I had a sig gen I'd be doing a few experiments on my multimeters right now...

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 13, 2015, 06:08:30 am
content: Quote from: coppice on March 13, 2015, 01:48:29 amIts meaning is that it is the RMS value of the applied pure sine wave.Quote from: coppice on March 13, 2015, 01:48:29 amThe people who designed this:http://simpson260.com/260-3/simpson_260-3.htmdeveloped just such an instrument.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 13, 2015, 06:16:01 am
content: Quote from: rs20 on March 10, 2015, 01:07:13 amLacking a specialized power factor meter, if one wants to measure the power factor of a load when the grid waveform is distorted (and all grid waveforms these days are distorted; flattened on top by all the non-PF corrected power supplies in computers, etc.) you need a wattmeter to measure the true power and an RMS measurement of the applied voltage and load current to determine volt-amperes.

author: coppice
author_rank: Super Contributor
timestamp: March 13, 2015, 06:21:11 am
content: Quote from: The Electrician on March 13, 2015, 06:08:30 amSo, you don't think they (and pretty much every other passive analogue multimeter maker) ended up with a meter like that because it was too hard to do a true RMS meter in those days?

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 13, 2015, 06:33:20 am
content: Quote from: coppice on March 13, 2015, 06:21:11 amI offered no opinion about whether it was difficult or not. I responded to your claim that there was no physical meaning to the scaled reading of the average rectified value, and your claim that nobody would design an instrument to output this value.

author: N8AUM
author_rank: Regular Contributor
timestamp: March 13, 2015, 09:39:09 am
content: Interesting thread. I barely remember back in the early 80's I had to start up the worlds largest 3 phase UPS system for IBM. After working out all the bugs I got the system going and IBM signed off on it, everybody is happy. Well, then started the random mainframe crashes. IBM is pissed! I would be to after spending millions! The mainframes power supplies had a 50 volt reverence derived from monitoring it's input voltage and frequency "POWER GOOD", if someone farted it would kill the mainframe. Turned out that load variations would change the harmonic content ever so slightly causing the whole problem. IBM finally woke up and increased their tight tolerance and lived happily ever after till the PC market wiped them out.bring back raised floors and vacuum tubes (valves) !

author: rs20
author_rank: Super Contributor
timestamp: March 13, 2015, 09:47:45 am
content: Quote from: The Electrician on March 13, 2015, 06:33:20 amCoppice's point is that theonlyreason it's designed that way is because True RMS is difficult. No-one would willingly design a scaled averaging meterunless budgetary or design constraints force them to, which makes it a strange thing to actually want/go out of your way to get.

author: DAIRVINE
author_rank: Contributor
timestamp: March 13, 2015, 07:11:07 pm
content: Instead of RMS why not true power?Instead of SQRT(average (V^2)) or SQRT(average(I^2))Please could meter manufacturers doAverage ((V*I))Should work for normal DMMs with a common terminal and clamp DMMs.Auto ranging both current and voltage does make it more complicated, but possibly just do it for the highest current range on cheaper meters?True power is much more useful than true RMS. No need to do VA, as that is easy to do by hand.Equation corrected.

author: dom0
author_rank: Super Contributor
timestamp: March 13, 2015, 07:39:14 pm
content: All DMMs have only one ADC.(Well, except for poly-phase meters, but those are actually handheld power analyzers and don't really qualify as a DMM)

author: DAIRVINE
author_rank: Contributor
timestamp: March 13, 2015, 07:47:23 pm
content: Quote from: rs20 on March 09, 2015, 09:43:56 pmFor this use case AVERAGE is more accurate than RMS. With a square wave the volts is in essence constant, so power is proportional to current. The load does not matter.The same argument possibly applies to current into a bridge rectifier/capacitor when the voltage does not change much during current spikes, at least for large capacitors. So power would be approx AVG current *(correction factor*) peak voltage.

author: DAIRVINE
author_rank: Contributor
timestamp: March 13, 2015, 07:50:44 pm
content: Quote from: dom0 on March 13, 2015, 07:39:14 pmYou only need one ADC. Do the average (V*I) before the ADC.Though two ADCs may be better.

author: dom0
author_rank: Super Contributor
timestamp: March 13, 2015, 08:13:13 pm
content: So you'll need an analog multiplier with reasonable linearity and accuracy for, say, 2000 counts or two analog frontends and ADCs. Either way: too little use, too expensive.I am probably repeating myself here, but if you want to do accurate power analysis, go get a power analyzer. The right tool for the right job.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 13, 2015, 08:19:34 pm
content: Quote from: DAIRVINE on March 13, 2015, 07:11:07 pmThis meter does it. Dave even reviewed it:http://www.gossenmetrawatt.com/english/produkte/metrahitenergy.htm

author: rs20
author_rank: Super Contributor
timestamp: March 13, 2015, 09:36:25 pm
content: Quote from: DAIRVINE on March 13, 2015, 07:47:23 pmWrong. Since the absolute voltage of a balanced square wave is essentially constant, the squared voltage is also constant. This means that RMS, average AND peak voltage will all give the same answer and be equally accurate (modulo noise sensitivity). Except, most "averaging" meters are actually scaled average, and will give a meaningless number.Quote from: DAIRVINE on March 13, 2015, 07:11:07 pmJust for the pedantic record, you probably mean sqrt(average(V^2)), etc. (I like to use the mnemonic RMS - Root Mean Square)

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 13, 2015, 09:41:55 pm
content: Quote from: rs20 on March 13, 2015, 09:47:45 amIf that was his point, he wasn't very clear about it.It looks to me like his point is "Nobody would sit down and try to develop an instrument specifically to output this value." BECAUSE "...scaling the rectified average to be like the RMS would be for a pure sine wave is simply a botch without any physical meaning."Furthermore, TRMS is not difficult; you just have to use the right meter movement. Moving iron, thermocouple and electrodynamometer movements inherently respond to TRMS AC+DC.VOMs used a D'Arsonval movement because they wanted greater sensitivity to DC voltage and current, and ohms measurement. Having made that choice of movement, it was necessary to use a rectifier type AC measurement. Back then the waveforms they were measuring were usually sine waves since there were no highly distorted waveforms such as you get from a triac light dimmer, for example, so the rectifier type AC meter did the job.The usual measurements of grid related voltages don't need a meter with a high input impedance, so there is no need to use a high sensitivity D'Arsonval movement. Any movement is suitable for measurements of grid powered loads. Notice that even the VOMs with D'Arsonval movements have fairly low input resistance for AC voltage measurement. For example, the input resistance for AC measurements on the Simpson 260 was 1000 ohms/volt, versus 20,000 ohms/volt for DC voltage measurements. This is not a problem when measuring grid related voltages where there is plenty of power to operate the meter.The reason VOM's AC measurements were made with a rectifier type method is not because TRMS measurements were difficult.When a meter was needed that was not a combination meter like a VOM, but was only a voltmeter, it was easier and cheaper to use a TRMS responding movement such as this, rather than the more complicated rectifier type:Here's an example of a multipurpose analog meter that measures volts and amps, TRMS, with .5% accuracy:

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 13, 2015, 09:54:18 pm
content: Quote from: rs20 on March 13, 2015, 09:36:25 pmIt's not meaningless; it's the average multiplied by 1.1107. If the applied voltage is a pure sine wave, then the value given is the true RMS value of the sine wave; this is not meaningless.For other waveforms, it's also not meaningless. Just divide the reading by 1.1107 and you have the true average of the absolute value.

author: dacman
author_rank: Frequent Contributor
timestamp: March 14, 2015, 12:41:31 am
content: The Fluke 8020A (circa 1979) is average responding and the Fluke 8060A (circa 1982) is RMS responding, but the 8060A has a noise floor that the 8020A does not have (5% of range originally then revised to 10% of range). 10% of range on the 200 mVac range is 20 mVac.One difference between the Fluke 27 II and Fluke 28 II is that the 28 II is RMS responding and is stated to be specified from 3% to 100% of range, where the 27 II is (deduced to be) specified from 0% to 100% of range. 3% of range on the 600 mVac range is 18 mVac.I have a test that I perform which calls out a specific handheld DMM that is average responding. The test limit is below the noise floor of both of these RMS handheld DMMs.

author: DAIRVINE
author_rank: Contributor
timestamp: March 14, 2015, 10:16:13 am
content: Quote from: rs20 on March 13, 2015, 09:36:25 pm