Fluke 17B+ / RMS

[Electro Fan]

I know there have been a ton of threads on DMMs but I'm homing in on purchasing a 17B+ for a college student.  It's reasonably priced, compact, a Fluke, and generally seems to have good functions and ok specs. The one thing I'm not too sure about though is that it seems to do RMS "averaging" vs. "true" RMS.   

This link is for the 17B (not the 17B+); under Basic features it says "True-rms readings Avg AC" ??
http://www.mantech.co.za/Datasheets/Products/FLUKE%2015B-17B.pdf

This link is for the 17B+; it doesn't reference rms at all ??
http://www.kipia.info/upload/DOCU_PDF_RAZN/Fluke_doc/Fluke15B_Plus.PDF

Any thoughts on the significance and practical results of true-rms vs avg rms? 

I fully get that a non-RMS DMM will read 5.5 volts AC where a true-RMS DMM will read 5.0 volts AC (with a 5 volt AC source), but I'm trying to understand why Fluke would list the 17B as "True-rms readings Avg AC" (and also why Fluke's spec sheet for the 17B+ doesn't reference rms at all).

Thanks for any help/clarifications.

[Fsck]

can I just suggest the bm257 as an alternative? it even has a logging option which is a handy function to have. google says they should be around the same price range (without logging kit)

personally, I don't find myself doing ac measurements often. I actually don't remember the last time I had to do one, but I don't pay attention to such details.

I believe the true-rms box means whether or not the meter is true-rms or average ac, and in this case the meter is only averaging.
I suspect that in the B version, they recognized the confusion from the above statement in the datasheet and simply omitted it so it's probably just averaging.

[Lightages]

The 17B+ is not TRMS. It is still a pretty good meter and built very well. As has already been said, the BM257s has some more features and is TRMS, AC only..

[Electro Fan]

I appreciate all the enthusiasm for Brymen - there are may Brymen fans here, no doubt - but I'm really more interested for this particular situation to go with the Fluke. 

What I'm trying to understand is what is the practical limitation or disadvantage of "average rms"  vs "true rms", and why would Fluke list the 17B with a row header that says "True-rms" and then across from the row header list "Avg AC"?  And why does Fluke not cite the same wording/terminology on the spec sheet for the 17B+?  Perhaps it is as Fsck says, Fluke decided it was confusing - so maybe the 17B+, like the 17B does averaging rms - in which case I'm just down to figuring out what the practical limitation of averaging rms will be vs. true rms.  For example, on an AC signal that reads 5 volts on a True rms DMM and 5.5 volts on a non rms DMM, what would it read on an averaging rms DMM?

Thanks again for any help with these questions.

[Muxr]

Normal averaging works just as well as True RMS if you're dealing with your standard sinusoidal AC signals. Where True RMS gives you an advantage is if you're trying to measure non sinusoidal signals, square wave for instance.

However many True RMS meters are AC coupled so they can't deal well with DC offsets in AC measurement mode. So it's another thing to watch out for.

w2aew does a really nice explanation in this video:

[Electro Fan]

Normal averaging works just as well as True RMS if you're dealing with your standard sinusoidal AC signals. Where True RMS gives you an advantage is if you're trying to measure non sinusoidal signals.

However many True RMS meters are AC coupled so they can't deal well with DC offsets. So it's another thing to watch out for.

w2aew does a really nice explanation in this video:

Ok, that's very helpful.  So, is a standard U.S. 120V AC signal likely to be sinusoidal (seems like the answer is yes).  So now we're down to use cases where the AC signal is something else - in which case the reading could be off by 10% (or more?)?  And then to your point, even if a meter is True-rms it still might not get the reading right if there is a DC offset, so True-rms alone doesn't assure an accurate AC reading.  An accurate reading would require AC + DC rms (or is that AC-rms + DC-rms?), correct.  So when a meter is listed as AC and DC rms that doesn't mean there is an application for DC rms by itself (because any DMM meter can figure out the DC level), what AC+DC rms means is that the meter will calculate a non-sinusoidal AC signal value correctly and it will do so while factoring in a DC offset. 

So.... for most common uses (like checking wall voltage) an AC rms avg DMM will do the job; the next step up would be true AC rms, and the cat's meow would be a AC+DC rms DMM.  Yes?

Thanks

[Muxr]

You got it!

[Electro Fan]

Thanks Muxr, but I have a few more questions

At about 1:40 in the video w2aew (who is consistently magnificent with his videos) says that the average of his pure sine wave is simply 0, which is not the same as the rms value.

What I think all this means - please correct me if I have it wrong - is that a true rms AC DMM calculation must somehow calculate the shape of the signal (probably via sampling) and then determine the value.  So even if the waveform is not a pure sinewave the DMM will calculate the correct value described by the waveform.  This will result in the right answer as long as there is no DC offset, but if there is a DC offset this "true rms AC" DMM will arrive at the wrong value.

Next (as per the point made at 1:40 in the video), if a meter labeled as an "Average rms AC" DMM computes the value of a true sinusoidal waveform it will get the right value - but NOT because it calculated an AVERAGE value.  It will get the right value only because it simply calculates the peak (or (peak to peak)/2?) value and then applies the .707 multiplier.  If this is correct, while it is called "average AC rms" it is not really computing the average value of the waveform, rather it is assuming that the waveform is an "average" (ie, "typical") sinusoidal waveform which allows it to get the right answer with less sampling and calculating sophistication than a true rms AC DMM.

Next, we have the non-rms DMM which for some reason is so brain dead (probably to save a few logic gates and therefore a few $) that it just measures the peak value for AC and doesn't even have the ability to multiply by .707.  Hard to believe this saves enough money to warrant leaving it out, but I guess these products exist because designers (and marketers) have decided that if they include it and call it an "average" AC rms DMM then people might expect an accurate result on AC (because they saw "rms" on the label) and then later become disappointed (if measuring a non pure sinusoidal AC signal).  So both to save a few $ and also to avoid disappointment DMM manufacturers generally don't offer "average' AC rms.  In the case of Fluke, they figured they had every other combination offered so they should serve the market for "average rms AC" with the 17B but they got tired of addressing all these questions and just left it out of the 17B+ literature (but presumably left it in the 17B+ product - which is just a guess since it's not confirmed anywhere?).

Finally, as mentioned earlier, for excellent results in all situations a true AC+DC rms meter is required, which interestingly is not something included even in the highly regarded (by some/many) Fluke 87V.

Yes, No?  Thanks again

PS, if these understandings are not correct please let me know.  On the other hand, if these understandings are correct I guess I'm inclined to still go with the 17B+ as a gift but the better understanding of these distinctions might have taken just a bit off the 87V's rose bloom; I might have to wait for the 87VI to see if it offers true AC+DC rms

PSS, is there anyone out there with a Fluke 17B+ who can measure a pure AC sinewave and determine if the 17B+ gets the same answer as a known good true RMS DMM?

[Muxr]

Thanks Muxr, but I have a few more questions

At about 1:40 in the video w2aew (who is consistently magnificent with his videos) says that the average of his pure sine wave is simply 0, which is not the same as the rms value.

What I think all this means - please correct me if I have it wrong - is that a true rms AC DMM calculation must somehow calculate the shape of the signal (probably via sampling) and then determine the value.

So even if the waveform is not a pure sinewave the DMM will calculate the correct value described by the waveform.  This will result in the right answer as long as there is no DC offset, but if there is a DC offset this "true rms AC" DMM will arrive at the wrong value.

Wikipedia describes three different analog methods on how True RMS measurements are accomplished:

an analog multiplier in a specific configuration which multiplies the input signal by itself (squares it), averages the result with a capacitor, and then calculates the square root of the value (via a multiplier/squarer circuit in the feedback loop of an operational amplifier), or

a full-wave precision rectifier circuit to create the absolute value of the input signal, which is fed into an operational amplifier arranged to give an exponential transfer function, then doubled in voltage and fed to a log amplifier as a means of deriving the square-law transfer function, before time-averaging and calculating the square root of the voltage, similar to above, or

a field-effect transistor may be used to directly create the square-law transfer function, before time-averaging.

I think one of these is probably how your typical DMM does the calculation/sampling.

Next (as per the point made at 1:40 in the video), if a meter labeled as an "Average rms AC" DMM computes the value of a true sinusoidal waveform it will get the right value - but NOT because it calculated an AVERAGE value.  It will get the right value only because it simply calculates the peak (or (peak to peak)/2?) value and then applies the .707 multiplier.  If this is correct, while it is called "average AC rms" it is not really computing the average value of the waveform, rather it is assuming that the waveform is an "average" (ie, "typical") sinusoidal waveform which allows it to get the right answer with less sampling and calculating sophistication than a true rms AC DMM.

Correct it's assuming the sinusoidal waveform hence why it's applying the .707 multiplier. Different waveforms would have different formulas, "Non true average RMS" just assumes the signal is a sine wave. Here are the formulas for other common waveforms:

But since DMMs can't detect which type of waveform is used, you're stuck with the measurement only being applicable to sine waves.

Next, we have the non-rms DMM which for some reason is so brain dead (probably to save a few logic gates and therefore a few $) that it just measures the peak value for AC and doesn't even have the ability to multiply by .707.  Hard to believe this saves enough money to warrant leaving it out, but I guess these products exist because designers (and marketers) have decided that if they include it and call it an "average" AC rms DMM then people might expect an accurate result on AC (because they saw "rms" on the label) and then later become disappointed (if measuring a non pure sinusoidal AC signal).  So both to save a few $ and also to avoid disappointment DMM manufacturers generally don't offer "average' AC rms.  In the case of Fluke, they figured they had every other combination offered so they should serve the market for "average rms AC" with the 17B but they got tired of addressing all these questions and just left it out of the 17B+ literature (but presumably left it in the 17B+ product - which is just a guess since it's not confirmed anywhere?).

My guess is that 17B+ has RMS averaging (using the .707 method above). I have a few Fluke non true RMS meters, 27II and the 83V and they all behave consistent with the method of [non-true] RMS averaging. I've tested this using a signal generator.

Finally, as mentioned earlier, for excellent results in all situations a true AC+DC rms meter is required, which interestingly is not something included even in the highly regarded (by some/many) Fluke 87V.

Yes, No?  Thanks again

This is correct. However even with the Fluke 87V you can measure AC+DC. You just have to measure the components separately and use the following formula to derive your AC+DC value.

Fluke 87V specifically includes a low pass filter to help you make low frequency AC  measurements.

Fluke has an article on this: http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/true-rmsfacts

PS, if these understandings are not correct please let me know.  On the other hand, if these understandings are correct I guess I'm inclined to still go with the 17B+ as a gift but the better understanding of these distinctions might have taken just a bit off the 87V's rose bloom; I might have to wait for the 87VI to see if it offers true AC+DC rms

I think you got it.

[Electro Fan]

Thanks again Muxr,

So even the 87III uses/used the (non-true rms) averaging method?  I guess when the 87III was introduced it just cost-too much do add semiconductors that could do true-RMS?

Maybe the same or similar method used in the 87III is used in the 17B+?

What do you think, does it make sense to get a student a an averaging RMS Fluke, or should I cave to the Brymen camp?   I love my Fluke 179

Thanks again, EF

[Muxr]

Thanks again Muxr,

So even the 87III uses/used the (non-true rms) averaging method?  I guess when the 87III was introduced it just cost-too much do add semiconductors that could do true-RMS?

Maybe the same or similar method used in the 87III is used in the 17B+?

What do you think, does make sense to get a student a an averaging RMS Fluke, should I or cave to the Brymen camp?   I love my Fluke 179

Thanks again, EF

Fluke makes non-True RMS higher end meters for some reason. I just don't know what that is. For instance I find a lot of the ex military sales of meters on ebay tend to be non true RMS (it's what my 27II is, military issue). I think there is something military measures that requires it. Perhaps only to be consistent with military procedures written for RMS averaging or something.. but I have no clue.

It's a tough call on which one to get. I haven't used a 17B+ to say with certainty. But I trust a Fluke. If I was going to get a Brymen it would perhaps be the BM257s. Because the price is right and I think it's one of the better meters they make.

The biggest thing that turns me off from 17B+ is the lack of the bar graph. I find the bar graph pretty useful in electronics. Let's you notice flutter, and can give you an insight in what you're looking at beyond what just the numbers tell you.

I don't find the True RMS, non-True RMS to be a big drawback, as long as you know the limitations. I use my 83V and my 87Vs interchangeably on a day to day basis and I don't feel restricted by the lack of True RMS at all.

edit: Yes I am pretty sure 17B+ has a [non true] RMS averaging. I just checked my little 101. And if they bothered to implement it on their cheapest meter don't think they would skimp on the 17B+ and put a non RMS averaging in.

[PedroDaGr8]

Thanks again Muxr,

So even the 87III uses/used the (non-true rms) averaging method?  I guess when the 87III was introduced it just cost-too much do add semiconductors that could do true-RMS?

Maybe the same or similar method used in the 87III is used in the 17B+?

What do you think, does it make sense to get a student a an averaging RMS Fluke, or should I cave to the Brymen camp?   I love my Fluke 179

Thanks again, EF

Just a quick note. He said 83V, not 87III. Every version of the 87 has had True RMS

[Muxr]

Oh right, good catch, didn't notice that. Yes 87s have always had true RMS. Their cheaper 83 cousins didn't have it.

[Electro Fan]

Thanks again Muxr,

So even the 87III uses/used the (non-true rms) averaging method?  I guess when the 87III was introduced it just cost-too much do add semiconductors that could do true-RMS?

Maybe the same or similar method used in the 87III is used in the 17B+?

What do you think, does it make sense to get a student a an averaging RMS Fluke, or should I cave to the Brymen camp?   I love my Fluke 179

Thanks again, EF

Just a quick note. He said 83V, not 87III. Every version of the 87 has had True RMS

Thanks for spotting that and mentioning it.

[Electro Fan]

Ok, after all that it's a thinker.  17B+, or maybe a Fluke 115...?
(And I can see why the Brymen 257s has found a good price/feature niche)

[BravoV]

Finally, as mentioned earlier, for excellent results in all situations a true AC+DC rms meter is required, which interestingly is not something included even in the highly regarded (by some/many) Fluke 87V.

Yes, No?  Thanks again

Yep, for TRMS AC + DC and displaying "both" at the same time, only at 287/289 series if we're talking only Fluke brand.

[Muxr]

Fluke really makes it hard in that price category. 115 doesn't have uA, and 17B+ doesn't have a bar graph. 17B+ doesn't have true RMS, but 115 doesn't have temperature. All of these are useful features. They really make it hard to chose.

Really for a meter over $100 you'd expect the full gambit of the essential features.

Brymen BM257 would cover all these features. I like Fluke but I think I would pick the Brymen in this price bracket honestly. Normally I would suggest perhaps a good Fluke used deal on Ebay, but you're buying a gift, so perhaps Brymen is a good option.

[Lightages]

@electrofan

I would like to know why you a fixed on the Fluke 17B+?

There are many option for good meters with TRMS at lower and slightly higher prices. If you are open to other brands then:

Uni-Trend UT139C $50 with TRMS
Amprobe AM-270 $115 with TRMS and higher accuracy

Just some other options.

[oldway]

RMS measurements are especially important for current.
Indeed, the heating of the electrical conductors and windings of the transformers is dependent of the RMS current value.

In electricity, we generally work with sinusoidal voltages but the currents are often non-sinusoidal.

It is recommended never to use the "current inputs" of a multimeter in a high voltage circuit and high power circuit.
Use a current probe, a transducer, a current transformer or a shunt.

In electronics, the simple measure of the rms value of a current or voltage is not enough, you also have to know the waveform.
Then you must use an oscilloscope.

For all these reasons, the right choice is, in my opinion, regardless a true RMS or not multimeter  + a true rms current clamp.
The current clamp MUST BE TRUE RMS

[Lightages]

In the case stated by oldaway, TRMS AC only is not enough. If you need to now real TRMS you need AC+DC measurement.

[BravoV]

RMS measurements are especially important for current.
Indeed, the heating of the electrical conductors and windings of the transformers is dependent of the RMS current value.

In electricity, we generally work with sinusoidal voltages but the currents are often non-sinusoidal.

It is recommended never to use the "current inputs" of a multimeter in a high voltage circuit and high power circuit.
Use a current probe, a transducer, a current transformer or a shunt.

In electronics, the simple measure of the rms value of a current or voltage is not enough, you also have to know the waveform.
Then you must use an oscilloscope.

... and, when it comes to work with mains and probing it's voltage n current with a scope, an isolated scope is a must and it's expensive accessories like special isolated probe, current probe and etc, which is at the end, the price league is now way beyond an ordinary cheap bench top scope anymore. 

... damn, this never end isn't it ? 

[oldway]

For power electronics, you need an AC + DC rms current clamp with output for oscilloscope.
(For safety, current clamp only, without voltage and resistance measurements...I used such a Metrix current clamp, but I don't remember the model )

EDIT: it was an Itt Métrix MX 1200s...

The best option is , for example, a Fluke i410 ac+dc current clamp + a True rms multimeter.
I have an old Fluke 80i-410 ac + dc current probe, but it has a serious shortcoming: it has no offset adjust.

... and, when it comes to work with mains and probing it's voltage n current with a scope, an isolated scope is a must and it's expensive accessories like special isolated probe, current probe and etc, which is at the end, the price league is now way beyond an ordinary cheap bench top scope anymore. 

  You can choose a cheaper solution: use an isolation transformer to probe mains voltage (is generaly sinusoidal) and ac or ac+dc clamp with output for oscilloscope to probe mains current.

[Electro Fan]

@electrofan

I would like to know why you a fixed on the Fluke 17B+?

There are many option for good meters with TRMS at lower and slightly higher prices. If you are open to other brands then:

Uni-Trend UT139C $50 with TRMS
Amprobe AM-270 $115 with TRMS and higher accuracy

Just some other options.

I wouldn't say I'm fixed on the 17B+ but what I like about the it is that except for true-RMS it has almost everything I am looking for in a DMM.  It would also be nice to have the bar graph and better specs (higher count, more resolution, better accuracy), but except for the lack of true-RMS (for AC) I could live with the the features and specs.  I like the fact that it's somewhat compact and I definitely like the fact that it is a Fluke.

I've had a Fluke 179 for about 10 years and it is rock solid durable, a joy to use, and I trust the readings (I've measured it against other DMMs and standards and I'm very confident in it).  It would be nice if it was tad more compact but that's a relatively small issue.

I've purchased other DMMs and I think I've read the reviews on almost every popular meter mentioned on EEV.  I had (still have) an Amprobe.  On paper it was a great meter, in reality the display was/is almost unusable.  What I've found with not only DMMs but test equipment and almost any category of product is that when you try to save money you can occasionally do so, but too often you wind up spending more because when products don't meet expectations you can sometimes wind up buying twice.  I'm sure Amprobe and UniT and especially Brymen make some A-OK products but without being able to experience them in person it's hard to know what you are really going to get until you open the box; if you could try products before buying them or return them for a refund it would be easier to take a chance on products that offer more value (more features and/or performance for a lower price), but saving 10-20% is only good if the product satisfies you, otherwise you might end up spending 80-90% extra.  Sure you can tough it out and accept the lesser product if it doesn't meet your expectations but why do that with products that you might keep for 5-10 years or longer?

Net, net:  I've come to trust Fluke.  At $50 or so I'm guessing that the UniT 139C or maybe one of their other models is a best choice, and in the $100-150 range my best guess is that the Brymen 257s could be the winner, but for about the same I'm starting to think the Fluke 115 is probably what I'm looking for.  I don't like the fact that the knob on the 11X series isn't recessed (but I generally don't lay my 179 face down) and I could find some other nits about the 115 too.  For example, it doesn't have mA setting, however it will show milliamps in the 0.001 format.  I think for a gift for a student it might almost be desirable to focus more on milliamps and millivolts in learning Ohm's law as opposed to thinking about microamps and having to think about replugging to avoid fuse blows.  Long story short, long after the satisfaction of having saved $15 is the satisfaction or dissatisfaction of using a product for years - so Fluke is my first choice until I discover something better. 

Again, I'm not saying other DMM manufacturers don't make some good products but in my experience Fluke has gained my trust where some others haven't.  If I could try before buying it would be easier to get me to switch.

Just some thoughts.  EF   

[Muxr]

I can definitely understand it. If this is going to be someone's only meter I'd go with a 17B+ then. If it's a second meter then I would go with a 115, since you can use the uA and temp measurements on another meter.

edit: bah, I don't know. The bargraph is really useful. And uA just increases the chance they will blow the fuse while learning. Temp is handy but a rarely used feature. So I think you're right 115 does seem like a better choice.

[Electro Fan]

I can definitely understand it. If this is going to be someone's only meter I'd go with a 17B+ then. If it's a second meter then I would go with a 115, since you can use the uA and temp measurements on another meter.

Muxr, does that mean you would value the microamps feature of the 17B+ over the true RMS of the 115?  I would think that going to a full digit milliamp would suffice but possibly coming up with a wrong AC voltage would potentially be a problem - or maybe I'm not looking at this properly?   This is going to be a first and only (for some time) meter.  EF