EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Boyd on January 10, 2021, 02:56:49 pm
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Hi!
I looking for recommendations for an "affordable" (but decent) True RMS multimeter.
My application is taking reasonably accurate RMS voltage measurements of audio frequency range (20hz - 20khz) amplifier outputs with complex waveforms, such as 6db crest factor band limited pink noise, to aid correctly setting limiters for pro audio.
I'm not sure what exactly the "count" (ie; 4000 count, 6000 count, 20,000 count) refers to. Presumably there's a sample rate & measurement window involved? The listed device specs seem unclear on this.
I'm not looking for Fluke quality (or expense). The only other things I might use it for are occasional DCR or continuity tests, so really the only thing it needs to do well is True RMS voltage (Peak hold would also be handy).
There seems to be heaps of options on Amazon etc - so many that it's hard to sort the wheat from the chaff.
What specs am I looking for, and which affordable meters would you recommend?
Would something like this be work a look?
https://www.amazon.com/Ranging-Digital-Multimeter-Battery-Alligator/dp/B07R47RYPW/ref=sr_1_228?dchild=1&keywords=true%2Brms%2Bmultimeter&qid=1610284050&s=industrial&sr=1-228&th=1 (https://www.amazon.com/Ranging-Digital-Multimeter-Battery-Alligator/dp/B07R47RYPW/ref=sr_1_228?dchild=1&keywords=true%2Brms%2Bmultimeter&qid=1610284050&s=industrial&sr=1-228&th=1)
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That meter looks a lot like Aneng, and you'll probably find more reviews for that brandname.
If you want a good meter for a reasonable price then have a look into Brymen.
20kHz is also a bit too much for most DMM's, so check the specks if this is really important for you.
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I suspect this meter is a rebrand of the Surpeer AV4 or HY19 - if so, it is an unfinished product.
20kHz is a bit hard to get with less than a Uni-T UT61E (about 60 US dollars).
The lower cost Aneng AN870 (also sold as Richmeters RM219) is an excellent meter for the price, but it does not reach 20kHz.
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This is a pretty good review. Looks interesting:
https://www.amazon.com/hz/reviews-render/mobile-media-feed/B07R47RYPW/ref=cm_cr_dp_mb_crsl_img_3?ie=UTF8&physicalId=71ZL8qAhasL&imageExtension=jpg&reviewId=R8E8XOS0A0TIL (https://www.amazon.com/hz/reviews-render/mobile-media-feed/B07R47RYPW/ref=cm_cr_dp_mb_crsl_img_3?ie=UTF8&physicalId=71ZL8qAhasL&imageExtension=jpg&reviewId=R8E8XOS0A0TIL)
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Based on this review, it is indeed the same meter as I mentioned before. YMMV, but I personally couldn't get past the bad bargraph, the non-working functions and the almost non-existing input protection. I found it is an unfinished product,
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Based on this review, it is indeed the same meter as I mentioned before. YMMV, but I personally couldn't get past the bad bargraph, the non-working functions and the almost non-existing input protection. I found it is an unfinished product,
That's all fair/understandable. It's a little bit in the "in between" category where it's somewhat more expensive than an Aneng 8008 but it isn't to the point of being confidence inspiring for AC/high voltage work - which will likely cost still more. Not sure exactly what the beginning of highly safe is for True RMS DMMs - maybe closer to $100?
I think for low voltage/DC work the 18 Z-III is probably fine - at this level bar graphs are nice to have but not always present and the protections are (should be) generally understood to be questionable to non-existent. The 18 Z-III does seem to have the frequency specs the OP is looking for - just not sure what the OP's voltage requirements are - the 18 Z-III wouldn't be good for vacuum tube work, for example, of course.
I think for a low voltage/DC use case the 18 Z-III is probably fine but that's just a guess.
My general recommendation is that for people looking to build a Test Equipment bench or tool kit it probably makes sense to invest in one serious DMM that can handle AC/voltage at any level likely to be encountered and a second meter that can potentially be less expensive so you can measure voltage and current at the same time. Then it's just a matter of whatever other key requirements the user might have which should be addressed by at least one of the two meters. On the other hand if you are going to be consistently dealing with enough voltage and current and you are going measure both at the same time then you should probably have two serious DMMs. I think for a lot users a Fluke or a Brymen plus an Aneng might be a good start. I like the Aneng 8008 a lot for what it is but I can see why someone might try the 18 Z-III as an alternative to the 8008 or other Anengs.
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My application is taking reasonably accurate RMS voltage measurements of audio frequency range (20hz - 20khz) amplifier outputs with complex waveforms, such as 6db crest factor band limited pink noise, to aid correctly setting limiters for pro audio.
I'm not sure what exactly the "count" (ie; 4000 count, 6000 count, 20,000 count) refers to. Presumably there's a sample rate & measurement window involved? The listed device specs seem unclear on this.
I'm not looking for Fluke quality (or expense). The only other things I might use it for are occasional DCR or continuity tests, so really the only thing it needs to do well is True RMS voltage (Peak hold would also be handy).
Counts simply refer to the number of possible values that it can display. So a common 3.5 digit DMM where the leftmost digit can only be a 1 or 0 can display number from 0000 to 1999, and is thus considered a "2000-count" meter.
Your specs are clear and not easily obtained in an ultra-budget meter. You need a meter with an actual TRMS converter chip (or a sufficiently fast digitizing system, but that is on better bench DMMs) and you need to find a meter that actually specifies its uncertainties at the frequencies you want. High-end or low end, Fluke or not, all that doesn't really matter. You'll need to RTFM (or at least the specs) and ignore any meter that doesn't provide those specs.
Crest factor is typically specified as a simple ratio for DMMs, so you might see 2, 3 or 5 listed, often with extended tolerances. 6db corresponds to a crest factor of 2, which almost any actual TRMS meter can handle just fine.
The new Brymen 786 EEVBlog special can do what you need including the crest (peak) hold, and is about $150.
I believe the Greenlee DMM-820A can also do what you need, crest hold included, and is available in the US with a lifetime warranty for $200 or so from TEquipment.
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You are absolutely right in your recommendation for a cheaper meter for the electronics work, ElectroFan. No contest on your post.
I should have perhaps explained better my beef with the protection. The Surpeer AV4 I tested went completely haywire (locked and reset) with as low as 25~30V @ 6mA applied to its inputs when the wrong range was selected (ohms IIRC). These voltages are not uncommon on the typical electronics bench (an audio amplifier easily goes well beyond that) and, since I used a very low current, the meter survived this test. Given that a mistake on these ranges is easily done, I would perhaps recommend any other similar cost meter that did not show this crazy behaviour under the same test, despite the ones I tested had a bandwidth lower than 20k. One of my top recommendations is the Richmeters RM219 (Aneng 870), which did not show this behaviour on my test and my unit went up to 3.4kHz. The UT61E also did not show this behaviour and goes linearly to more than 60kHz (IIRC), but it is more expensive. Cheaper meters like the RM113D or the RM102Pro (AN113D) also did not show this problem despite being even cheaper than the meter in question.
At any rate, these are just nuggets of experience from a random guy on the internet. :-D
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For sure I won't be buying another Fluke.
I have had it caled and the result was that were no adjustments made and the RMS readings complied.
My 117 cost heaps for what it is, a basic meter.
Despite that it is usless for a low ohms measurement.
When the probes are sorted it typically reads something like -1.7 ohms, what does that mean ?!
I have checked others in shops and they are the same.
I previously had a 78 which was great and faultless and did a massive amount of work over 28 years.
Was the problem due to a change in where they are made, if they are all made there then why pay the crazy price it cost for that performance.
Worse than a cheapy one!
Not happy.
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For sure I won't be buying another Fluke.
I have had it caled and the result was that were no adjustments made and the RMS readings complied.
My 117 cost heaps for what it is, a basic meter.
Despite that it is usless for a low ohms measurement.
When the probes are sorted it typically reads something like -1.7 ohms, what does that mean ?!
I have checked others in shops and they are the same.
I previously had a 78 which was great and faultless and did a massive amount of work over 28 years.
Was the problem due to a change in where they are made, if they are all made there then why pay the crazy price it cost for that performance.
Worse than a cheapy one!
Not happy.
Sorry about your meter. Did it give that reading before the cal? Any reason not to mention it to the cal service provider before or after the cal?
fwiw, just ran the lead to lead resistance test on a Fluke 115. It fluctuated between 0.1 and 0.2 ohms. Also tried a Fluke 179. It mostly reads 0.2 but depending on how well / how still the leads are when touching them together the displayed value can fluctuate quite a bit. I find that when I hold the tips together at their indents I get the most stable reading.
One other fwiw, I tried the same Fluke leads that I used on the 179 on the two wire resistance setting of a Keysight 34465A (which on four wire measurements with Kelvin clips I can get to measure with pretty good accuracy (+/- ~200 microohoms on something I'm pretty sure should read 10 milliohms). Using the Fluke leads for two wire resistance, when I'm very patient with the probes, I can get the meter to settle down to 0.2 ohms (same as on the 179) but it's easy to move the probes just enough to see the reading fluctuate upward by about an ohm (to 1.2 ohms or slightly more). When I change the PLC settings so that the meter samples very rapidly then I can get the meter to show a lot of slightly different values (it's a 6.5 digit meter) but all around 0.2. However, when I slow the sampling down then some of the readings can be well over an ohm. Point being that I think resistance measurements are hard to get at low values (below about 1 ohm) and I think it's the combination of holding the probe leads steady plus the sample rate (speed) of the meter. This doesn't explain why you are getting a negative value but I think it's possible that could happen as part of general fluctuation.
A simple test: use an alligator clip (just one clip, not a full cable) to clip the two leads together so you don't have to hold the leads against each other; just let the test leads being held together by the alligator clip rest on your bench undisturbed. With a good contact being made without any lead movement, what reading do you get?
My guess is that for normal electrical/electrician work the 117 will give accurate enough readings (and be very safe - one of the great strengths of a Fluke) but if you want low to very low ohm electronics level accuracy there are probably better meters.
Edit/Update: Just because this stuff is fun, I got out my 45 cm piece of 18 AWG copper wire which I believe should measure on the order of 9.5 milliohms (give or take a couple hundred microohms) and I checked to see if the Keysight was still on target. It was, so next I tried a DE5000 which I think is roughly as accurate as the Keysight on low ohms (even though it cost about 1/15th the price). On the first measurement it was reading about 22 milliohms, slightly more than twice what I expected. So I double checked everything and found that the alligator clips on the DE5000 (which are 4 wire Kelvin clips) were not making good contact, and when I fixed that the two X reading came back down to X (in this case about 9.2 milliohms). Point being that when you want do do low ohm measurements it's a finicky process that is susceptible to getting a solid and settled connection, and it can also be subject to ambient temperature (although I think temperature is more of an issue at milliohms and below rather than at tenths of an ohm).
All of this is kind of off-topic for the OP but it does show that the selection of a meter should be driven by the particular use case(s).
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Thanks everyone for your comments & suggestions.
To clarify my frequency resolution requirements for TRMS voltage measurements with band limited pink noise, the highest band I might need to measure might be 5khz - 20khz, and the content above 10khz would be fairly insignificant.
I did eventually manage to find the true RMS test in one of the meter shootouts, showing the UT61D (presumably much the same as the UT61E?) performing well at higher frequencies. It's a little more than I was hoping to spend, but definitely on my short list.
Part of the problem I'm having is that few, if any of these meters seem to list any specs pertaining to the resolution / computation of their RMS measurements, so determining if a meter has an "actual TRMS converter chip (or a sufficiently fast digitizing system)" is not obvious.
The Thsinde meter I linked to mentions having a "double integral A/D conversion for core". Is this relevant? A web search only revealed maths that was way over my head.
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Would something like this be work a look?
https://www.amazon.com/Ranging-Digital-Multimeter-Battery-Alligator/dp/B07R47RYPW/ref=sr_1_228?dchild=1&keywords=true%2Brms%2Bmultimeter&qid=1610284050&s=industrial&sr=1-228&th=1 (https://www.amazon.com/Ranging-Digital-Multimeter-Battery-Alligator/dp/B07R47RYPW/ref=sr_1_228?dchild=1&keywords=true%2Brms%2Bmultimeter&qid=1610284050&s=industrial&sr=1-228&th=1)
This meter (Thsinde 18 Z-III) looks like it is the same as this:
https://lygte-info.dk/review/DMMZeast282%20UK.html (https://lygte-info.dk/review/DMMZeast282%20UK.html)
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Hi Electro Fan not sure why you would be sorry about my meter but thanks.
When I bought the 117 I did fully undestand that it was a meter for electrical use but since I can't measure anything under 30 ohms with confidence I guess it fails in that goal.
It did give that reading before the cal, in fact from new.
Its my own meter that I use at home for my ham radio and embedded microprocessor hobbies. It was to replace my trusty 78.
I am a Senior Engineer at the cal lab that did the test. I got an engineer to do it and then did it myself.
We use a Fluke 5522A calibrator.
My work contract prohibits me using the name here.
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Also, how does the voltage level influence the accuracy of TRMS voltage measurements?
In the highest band above 5khz, I might be measuring TRMS voltage at around 25V, in the subwoofer range below 100hz, setting max levels of around 110VRMS, on amplifiers capable of up to ~160VRMS
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It did give that reading before the cal, in fact from new.
That would appear to be an initial defect, no? The spec is 0.0 to 0.2 for the 0 ohm step. Has this issue ever been mentioned to Fluke?
I am a Senior Engineer at the cal lab that did the test. I got an engineer to do it and then did it myself.
We use a Fluke 5522A calibrator.
Then surely you understand what a reading of -1.7 ohms means. Are you saying that this issue persists even after you did the recalibration procedure and set a new constant for the 0 ohms step? I've not observed this as an issue on 110-series meters and I've seen a large pile of them in various states of disrepair and abuse. My own 116 reads 0.0 with a shorting plug.
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Also, how does the voltage level influence the accuracy of TRMS voltage measurements?
Typically not much, but this should be in the specifications. How accurate were you hoping to be? The UT61D is a 1%-ish meter with a 3kHz rated bandwidth. It may be very inaccurate on non-sine signals above 3kHz no matter what some YouTube shootout shows. Meters in that price range are unlikely to have an actual TRMS converter that works at any decent bandwidth--they have a cheaper sampling system that works fairly well up to a few kHz.
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Hi Electro Fan not sure why you would be sorry about my meter but thanks.
When I bought the 117 I did fully undestand that it was a meter for electrical use but since I can't measure anything under 30 ohms with confidence I guess it fails in that goal.
It did give that reading before the cal, in fact from new.
Its my own meter that I use at home for my ham radio and embedded microprocessor hobbies. It was to replace my trusty 78.
I am a Senior Engineer at the cal lab that did the test. I got an engineer to do it and then did it myself.
We use a Fluke 5522A calibrator.
My work contract prohibits me using the name here.
Hi Kerlin, I feel bad for anyone experiencing test equipment issues. Testing has enough rabbit holes even when equipment is working properly. If all resistance measurements below 30 ohms are not inspiring confidence that doesn't seem right for any DMM, much less a Fluke.
Maybe Fluke would consider this to be a warranty issue? Has Fluke said anything about your meter?
I used to think Fluke had some type of "lifetime warranty" but the 117 web site page indicates it's three years - but even if it's out of warranty maybe Fluke would consider this to be a latent defect that they could address. I would think if they had any customers to keep happy it might be people who work at cal labs.
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Is it has to be a handheld multimeter? The Vici VC8145 is a 80000 counts bench meter with a true RMS converter (AD737J) and Peak Hold function and costs about 150usd.
There's a review of it here: https://lygte-info.dk/review/DMMVici%20VC8145%20UK.html
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How accurate were you hoping to be? The UT61D is a 1%-ish meter with a 3kHz rated bandwidth. It may be very inaccurate on non-sine signals above 3kHz no matter what some YouTube shootout shows. Meters in that price range are unlikely to have an actual TRMS converter that works at any decent bandwidth--they have a cheaper sampling system that works fairly well up to a few kHz.
1% accuracy is probably sufficient for my purposes.
The shootout I mentioned showing the UT61D reading accurately on triangle waves up to 30khz was Dave's EEVblog video #99 part 2, comparing against his Fluke 87, so I have no reason to doubt the result. https://youtu.be/3-_dUB8vy8U?t=2436 (https://youtu.be/3-_dUB8vy8U?t=2436) He does mention that it's only "rated" to 3khz. It was the only DOT that gave an accurate reading at 30khz to match the Fluke.
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Is it has to be a handheld multimeter? The Vici VC8145 is a 80000 counts bench meter with a true RMS converter (AD737J) and Peak Hold function and costs about 150usd.
Preferably handheld, and hoping to find something half that cost. I'd be fine with it having no other features besides TRMS voltage.
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Electro Fan I will contact Fluke again on the basis of your last point, it should provide some result.
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To clarify my frequency resolution requirements for TRMS voltage measurements with band limited pink noise, the highest band I might need to measure might be 5khz - 20khz, and the content above 10khz would be fairly insignificant.
I did eventually manage to find the true RMS test in one of the meter shootouts, showing the UT61D (presumably much the same as the UT61E?) performing well at higher frequencies. It's a little more than I was hoping to spend, but definitely on my short list.
The UT61E is quite different than the UT61D although, as you saw, independent reviews (myself included) found out it can reach quite a wide frequency range.
Part of the problem I'm having is that few, if any of these meters seem to list any specs pertaining to the resolution / computation of their RMS measurements, so determining if a meter has an "actual TRMS converter chip (or a sufficiently fast digitizing system)" is not obvious.
An idea of the specification you are looking for is translated in the form of a parameter called crest factor (https://en.wikipedia.org/wiki/Crest_factor). Simple waveforms such as triangle, square, sinewave, tend to be well behaved and well within the crest factor range of any TRMS converter but, when peaks and fast transitions start to be introduced such as in pulsed or audio waveforms, that is where the discrepancies start to happen.
In a meter such as the UT61E, for example, the AC TRMS specs mention a crest factor of 3 and the accuracy of each range and the spread with frequency. You can tell how the manufacturer guarantees much less accuracy beyond 1kHz, for example.
(https://www.eevblog.com/forum/beginners/true-rms-multimeter-recommendations/?action=dlattach;attach=1150128)
The Thsinde meter I linked to mentions having a "double integral A/D conversion for core". Is this relevant? A web search only revealed maths that was way over my head.
This is a topology of the A/D converter and double conversion is usually more stable across the temperature range. However, the A/D is only one piece of the entire device and tolerances of external components tend to be predominant, especially in cheap meters.
As bdunham7 mentioned, the accuracy for complex waveforms (a higher crest factor) is something found only on more expensive multimeters - I have a BM857 that is specified up to 20kHz and it works well above that range in my experience.
If used is an option, the Fluke 8060A is a very decent meter still to this day and is specified up to 100kHz.
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I question the need for true rms. If you are working with large crest factors, it's likely that your interest should be in peak value, not rms. If the peak is high enough it will be limited by the device under test, regardless of the rms, and thus undergo distortion. Presumably you are trying to avoid distortion.
So the question is, if the rms is too small to allow proper clipping level, then it won't be loud enough. It's not easy to measure loudness but rms is not the way.
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Yes, the issue starts to become much more suitable for an oscilloscope and/or audio analyzer than a simple multimeter.
By the way, in my tests up to 5V I was able to get a consistent and accurate peak reading on the UT61E when I applied square pulses of 700ns. The other meters I tested (87V, 189, BM857, U1282A and U1273A) were between 200us and 800us.
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I question the need for true rms. If you are working with large crest factors, it's likely that your interest should be in peak value, not rms. If the peak is high enough it will be limited by the device under test, regardless of the rms, and thus undergo distortion. Presumably you are trying to avoid distortion.
So the question is, if the rms is too small to allow proper clipping level, then it won't be loud enough. It's not easy to measure loudness but rms is not the way.
My interest is in both peak, and RMS, the goal being speaker protection at high output, rather than measuring "loudness".
Speakers have two primary failure modes - excessive peak voltage causing over-excursion, and excessive RMS voltage leading to thermal failure.
I'll spare you the long winded explanation of what I'm doing, suffice to say it does necessitate measuring true RMS with the aforementioned test signal.
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Short duration peaks will not cause excessive cone excursion. But then, they don't contribute much to rms either. Further, cone excursion is very frequency sensitive so rms is only a rough approximation. It needs to be weighted based on a profile of the speaker.