My First Fluke: The 87V Crash Course

[jedidove]

Hey all,

I'm a Physics/Math undergrad soon to be Computer Engineering/EE major at an engineering school (5-6 year joint degree program). My main research and interest is in Quantum Computation, and I plan to purse a EECS PhD as well as possible a Physics PhD. In my spare time, I have a lot of hobbies, some of which include electronics and mentoring a high school robotics team.

When I bought my first personal electronics gear, I was smart enough to dish out for a good Weller soldering station. However, I was stingy with the meter and couldn't afford much so I got a cheapo "MASTECH" DMM. I guess that actually turned out to be a good thing because if gave me a good excuse to get a good meter now. The other day at my robotics team I saw they were working with a similar DMM to mine (same casing). I had just finished teaching some kids how to solder so I was going to show them how to use a DMM. I grab the DMM and go to measure the voltage on our 12v battery......~0.02V unstable.....same thing same thing...I was embarrassed at first because I said "watch this" and then nothing worked lol. I get the instructor...same thing........then it jumps to 12.75V all of a sudden. Needless to say the DMM was garbage and the instructor wasn't too happy either. They had grabbed the meter from a Physics lab.

So I took the opportunity to explain the difference in quality and price between cheapo's and a good meter like a Fluke. Fortunately, my advisor in Physics at my uni was a EE so he loaded ALL the labs (including freshman) with Flukes. The circuits lab has a dozen 187's. As I discussed this with them I started to worry about my meter too. Especially since I had even used it for some quick reference stuff when working for a defense contractor over the summer. So as we continue to talk about things, I decide that it's time for me to get a decent meter.

I managed to find some school book/supplies/scholarship money to use for it so I was ready to jump at the grand poobah of Flukes and snag a 289. However, it was a bit out of my price range. After watching Dave's review, I realized that it would have been a stupid choice anyway since it is much bulkier than an 87V, takes time to "boot up", makes you dig in menus to do things, etc. I carry my meter in my school messenger bag so bulkiness makes a difference. Also, for the general purpose stuff I need it for the 289 seemed like it would be more of a disadvantage than advantage. However, I still wanted a top of the line Fluke so I decided on the 87V. The price was inline with that of a textbook, the size isn't awful, its fast, probably more rugged, and it has the added plus of being such an iconic meter that it sort of seemed like a right of passage into the Fluke world and a great first Fluke. Not to mention it had the main features I wanted (temp probe, relative, max min avg, etc.).

I am very happy with my 87V. Though I'm a little surprised they killed the 187 line since it seems like there is a gap between the 87 and the oversized 289 firmware updatable computer. That said, I think it was good that I had to get a 87V because of the history and iconic status.

The only thing that I'm a little annoyed about is that there a few things that neither the included "manual" and the real full online manual cover in descriptive detail. So I hoped to make this thread (in addition to my introduction) a potpourri of 87V related questions I and perhaps other beginners have, as well as a place for the veterans to give there tips and tricks.

Firstly, the manual shows that the display has support for both "display hold" and "auto hold" but I can only figure out how to enter auto hold. Granted, I think auto hold will probably completely replace the functionality I would get from regular hold and add some, I just wish I knew whether or not the regular one was available. I took an educated guess which turned out to be right that the regular hold lcd icon and functionality is triggered when pressing AutoHOLD in min max avg mode in the current measurement submode. Is this the only place you can access non-auto hold?

Next, I'm a little confused about the difference between using the frequency feature while in DC vs AC. The full manual gave some situations when you would use which, but it isn't super clear what the real difference is, and it's a bit confusing that there is a frequency measurement in the DC mode.

I've always been a little irked that DMM's only give you rms for AC. Granted rms is the most useful, but peak voltages can be useful and some cheap meters actually calculate rms by taking peak/?2 so clearly a peak measurement isn't a hard thing to implement along side rms. On this note, I had an idea. I figured that I could use my 87V to measure the peak voltage of an AC signal by measuring it in DC volts min max avg mode using the faster 250 microsecond time scale. I was under the impression that min max avg just did the corresponding measurement once every 100ms or 250µs and then extracted the data and DC would take literal voltage measurements. So with the faster time scale, the closer the signal was to 4kHz, the longer you would have to wait before you got a good peak sample due to a beat note type analysis. I was most interested in the peak voltage so I could calculate the crest factor of signals which would be a rough way of estimating the wave shape without a scope (sharp vs flat top vs sine). Along with the frequency and duty cycle features this would make a potent combo in a handheld meter.

So I try it out on a wall socket. I get ~118-119 rms and a max of about 162 (the min was negative and a little smaller magnitude). This was pretty close, but not quite the ?2 factor I expected. So either the signal isn't quite a sine or this idea isn't working as a peak measurement. At 60Hz it definitely shouldn't be a sampling issue. I don't have a scope but I tried using the function generator and scope in circuits lab after class and it seemed like I was seeing the same slight undershooting of my attempted peak measurement. Though I can't tell for sure since scopes don't give super accurate voltage measurements anyway. I did some reading...and I ran into some information that suggested that the PEAK min max was actually meant for just this, measuring PEAKS (duh!). I had thought peak was just a new word for fast, as in peak acquisition speed. To test this, I tried the same thing but using min max avg in AC mode. Sure enough, regular min max avg gave the rms min max avg, but when I went into peak mode it jumped up to ~162. Again this is confusing since I would expect the AC peak min max avg mode to just do faster data on the rms value, though maybe the term rms doesn't make sense at those timescales (granted I don't see how it would make sense at 100ms then either).

So what exactly is peak min max avg and how does it differ between DC and AC?

On a similar note, I'm also a little confused as to what the DC and AC measurements actually measure. At first I had assumed AC measured the true rms of the signal and DC literally measured voltage, so an AC signal would read quickly varying in DC mode and I could use the avg feature to get the DC offset. After reading some, it appears that the AC feature measures the true rms of the AC component only (no offset). They say you can get a true total rms by taking the AC and DC measurements and doing the orthogonal square square root sum up, but this leads me to believe then that the DC function actually measure the DC offset of the signal. So if you measure an AC signal using the DC mode does it actually give you the time average of the signal? In the manual they say to use a range at or higher than the AC rms measured value, but they say nothing more. If anyone would like to elaborate on this I would appreciate it.

Also, I couldn't find much on the smoothing feature. I assume it smooths the signals in some way, like a watered down version of the low pass filter function, but it's functionality isn't entirely clear to me.

Since I'm going to have this meter on me pretty often, I'd like to know all the ins and outs of it and how to use it to its greatest capacity. Unfortunately, I feel like the manual has lead me to ask more questions than it answered. I know this is a doozy of a post and I appreciate anyone who took the time to read any of it. I'd love to hear thoughts regarding any aspect of my questions or any general tips and tricks of 87V use.

Thanks!

[Kiriakos-GR]

Interesting story at list .... ( that's the first thing that comes in my mouth first )

There lots of questions in the table .

So lets start from somewhere like for example the TRMS on AC ..

When ever I get an new tool, its because I had realize that I need it on the field ..
Example : I had to repair an uninterruptible power supply , and I did ,
at the end I measured the output,  my non-TRMS  multimeter measured an output of 160V  ( mains at 220 default ),
the same measurement with an capable - TRMS  will show 220 V  3% + / _  .

I am very happy that I got an capable - TRMS multimeter , and now its impossible to be fooled again, by measuring AC no matter the source.

About TRMS on DC , I have no idea where some one will find such an source and I can only speculate that is for DC/DC  converters .
I do not touch such electronics ,  i have no experience with those.


About the " Hold " my opinion are that practically, its an non worthy function .
My vote goes for the " min-max-average "  and mostly for measuring current , it can detect spikes.

About the" smoothing feature"  , this is about the bar-graph .
Recently  I was wondering how many refreshes per second at the bar-graph, are detectable by the human eye ?
And I started wonder about it due the fact, that one multimeter had in the specs 120 per second ..
Or at list this is what it was reported.

In any case , I had characterize this function , as an simple assist for the naked eye, if the measured value are too fast.
The simplified description would be "  preview in slow motion ".
Where this can be shown as useful ?   I do not know, but I can again speculate that one hand drill with damaged carbon brushes,
it could cause an strong variation if I try to measure the Amperes of it !!
And the slow motion could be turn as useful effect.

  
 
 
 
  
 
 

[alm]

Granted, I think auto hold will probably completely replace the functionality I would get from regular hold and add some, I just wish I knew whether or not the regular one was available.

Not sure if display hold is available outside of the min/max mode, but I've never seen the point of display hold. If I had my hands and eyes available to press a button, I wouldn't need it.

Next, I'm a little confused about the difference between using the frequency feature while in DC vs AC. The full manual gave some situations when you would use which, but it isn't super clear what the real difference is, and it's a bit confusing that there is a frequency measurement in the DC mode.

Table 6 of the manual sums it up pretty well: AC uses a trigger level of about 0V (slightly higher since RMS converters are non-linear close to 0V), DC uses a positive trigger level (10-30% of full scale). AC frequency measurements won't work for signals with a DC offset (eg. pulse train), DC may have issues with small AC signals.

I've always been a little irked that DMM's only give you rms for AC. Granted rms is the most useful, but peak voltages can be useful and some cheap meters actually calculate rms by taking peak/?2 so clearly a peak measurement isn't a hard thing to implement along side rms.

I believe this is incorrect, the cheap meters are average responding, which means they calculate the RMS value (for pure sinusoidal signals) from the average value of the rectified signal. This Fluke article (obviously completely unbiased) seems to confirm this.

On this note, I had an idea. I figured that I could use my 87V to measure the peak voltage of an AC signal by measuring it in DC volts min max avg mode using the faster 250 microsecond time scale. I was under the impression that min max avg just did the corresponding measurement once every 100ms or 250µs and then extracted the data and DC would take literal voltage measurements.

My impression is that the normal min/max mode takes a sample every 100ms, but although the peak mode can detect peaks of at least 250us, I don't think it samples continuously at that rate, which is why it can't calculate an average. The fact that the peak needs to be repetitive for it to capture 250us pulses hints that there's some sort of equivalent time sampling going on. They probably use something like an analog comparator set to trigger at the previous peak level to trigger the measurement, and need >> 250us to sample and re-arm.
 

So I try it out on a wall socket. I get ~118-119 rms and a max of about 162 (the min was negative and a little smaller magnitude). This was pretty close, but not quite the ?2 factor I expected. So either the signal isn't quite a sine or this idea isn't working as a peak measurement.

This could easily be distortion, loads like bridge rectifiers tend to draw the most current at the top. Look at the 'how clean is your mains' thread from some time ago for some examples.

I did some reading...and I ran into some information that suggested that the PEAK min max was actually meant for just this, measuring PEAKS (duh!). I had thought peak was just a new word for fast, as in peak acquisition speed. To test this, I tried the same thing but using min max avg in AC mode. Sure enough, regular min max avg gave the rms min max avg, but when I went into peak mode it jumped up to ~162.
Again this is confusing since I would expect the AC peak min max avg mode to just do faster data on the rms value, though maybe the term rms doesn't make sense at those timescales (granted I don't see how it would make sense at 100ms then either).

I believe you are correct, peak min/max is intended for instantaneous(ish) values, so no averaging (which RMS is) is taking place. Min/max samples at a much slower scale than most AC signals, so it has to use some sort of average. Averaging instantaneous values wouldn't make much sense, since that would be 0V by definition for a pure AC signal, so they have to use some sort of RMS conversion for that. But I haven't seen this documented, and I haven't done tests to verify this. You could use a function generator, and set the frequency proportional to the 'sample rate' (eg. 100Hz for peak min/max, 1Hz for normal min/max) to test this. If your hypothesis is correct, you would expect lower values (compared to the 'average' AC value) with min/max, about sqrt(2) lower.

So what exactly is peak min max avg and how does it differ between DC and AC?

No difference between AC and DC I think, although I would rather consider DC very low frequency AC, since there's no such thing as a peak DC value (that would be an AC component on top of the constant DC value).

On a similar note, I'm also a little confused as to what the DC and AC measurements actually measure. At first I had assumed AC measured the true rms of the signal and DC literally measured voltage, so an AC signal would read quickly varying in DC mode and I could use the avg feature to get the DC offset.

If the signal is slower than the sampling rate (say .4Hz), this would be the case.

After reading some, it appears that the AC feature measures the true rms of the AC component only (no offset).

Yes, it's usually AC coupled.

They say you can get a true total rms by taking the AC and DC measurements and doing the orthogonal square square root sum up, but this leads me to believe then that the DC function actually measure the DC offset of the signal.

It's intended to measure the DC component (although it also measures very low frequency AC, as above). For example, a 5V power supply with 1Vrms 120Hz ripple would measure 5V on DC, and 1V on AC.

So if you measure an AC signal using the DC mode does it actually give you the time average of the signal?

I believe so, which would be zero for pure AC. Not sure how they determine the average, it may just be a low-pass filter.

Also, I couldn't find much on the smoothing feature. I assume it smooths the signals in some way, like a watered down version of the low pass filter function, but it's functionality isn't entirely clear to me.

I don't think they give detailed specs, but I consider it a form of digital low-pass filtering, or something like a windowed average.

[PetrosA]

It sounds like you might be confused yet about a few things, just like me most days

When they talk about RMS values in AC voltage they're generally referring to a part of the sine wave that is below the peak and which corresponds to the DC voltage that would be needed to produce the same amount of heat in a resistor.  With a true sine wave, an averaging meter will use a standard equation to calculate that voltage which is something like RMS = 0.707 x peak. So if you have a 10V DC source powering a 2 Ohm resistor, there will be 50 watts of heat made. To achieve that with AC volts you would need a voltage with peaks of 10/0.707, or 14.14VAC. So for clean 118V mains you should see approximately 166V peaks - very close to what you saw and I suspect the discrepancy in the peak is because of dirty mains.  When the wave form gets distorted or other than sine waveforms are used, averaging a value for RMS doesn't work and you need to use a TrueRMS meter. This is especially important for electricians because wires, buses, switches etc. as well as overcurrent protection devices react to the RMS values in the way they heat up (and eventually trip or burn out) rather than to the peak values. If your meter gave you TRMS values in DC mode, that's probably just a convenience thing. Normally DC mode won't see any voltage from a mains source. Peak/min/max/avg in DC mode will be giving you readings based on DC voltage variations and the max and peak readings should be the same, while in AC, the peak and max will be different since the max will be a TRMS value and the peak will not.

Frequency in DC can happen for two reason (that I know of...). One is when you have a rectified DC voltage which is actually AC voltage converted to a DC voltage. Because it's derived from AC, it pulses to some degree along with the AC since wave. In my world the most common example of this would be sticking a diode inline with a doorbell switch to feed an electronic chime. The other type of DC with frequency will be from a PWM control which pulses a steady DC voltage to reduce the output (ex. VFD motor controls).

I'm sure others can fill in the holes I'm leaving behind

[jedidove]

Thanks for the responses! It's a little late so I'll give a full reply in the morning but I wanted to clear up a thing or two.

I agree that it's probably just dirty mains considering its a difference on the order of 6 volts. I had figured that but I also wasn't sure how valid my peak measuring idea was at the time. I really need to find a function generator that displays the output voltage. I'll try the one at school again with a hopefully clean sine was and calculate the crest factor for my peak measurement.

@PetrosA sorry if my post wasn't well written. I didn't have the time to organize as I would have like too  I'm familiar with rms and all that. I just finished Analog Circuits last semester so it's still all fresh in my head. As for the DC frequency thing, I just meant that you can use the frequency function while measuring "DC" volts. As alm pointed out, the manual specifies the only difference being triggering properties, but it just seemed a little counterintuitive to me at first.
 
Thanks again for the info and especially the clarification on the role of the DC function! Off to bed

[jedidove]

I agree that the display hold is a bit pointless. I think I'll get much more use out of AutoHOLD and I'm sure it will replace all the functionality that I would have gotten out of display hold. I was a little baffled at first mainly because of the two separate icons described in the manual but I guess they are there just to accomodate for the behavior in min max avg mode.

Thanks for the clarification of the frequency function in AC vs DC. I probably wasn't entirely focused when I skimmed that section of the manual. I kinda wish they included the full manual instead of the quick start guide in a million different languages.

On a related note. How does the duty cycle measurement vary between AC and DC modes? Moreover, I know you can select + and - triggers for duty cycle (and frequency) but does that trigger somewhere on the + and - slopes or does it trigger only when it crosses the 0V line? i.e. if I had a TTL pulse train would + and - give me the two complementary widths? or would that only work if I had a pulse train that dropped below 0V?

I didn't know that about averaging in cheap meters. That makes much more sense.

The non-continuous peak idea seems valid since it wouldn't make sense otherwise why they don't include avg and why there is such a huge difference between the two rates (100ms and 250µs). I took another look at some data I grabbed from the function generator in class and the "crest ratio" came out to be like 1.43, so it erred on the OTHER side of the ?2 value so I feel more confident that the peak function is working the way I hoped. I'm going to do some more testing when I get the chance. @Alm can you clarify what sort of testing you had in mind when you were talking about the sampling rates?

Has anyone used the smoothing feature in any practical situation?

EDIT: I just realized that for some reason when I post the sqrt character it comes out as ? so sorry for any confusion that may have caused.

[Kiriakos-GR]

Has anyone used the smoothing feature in any practical situation?

You had just gave to me an idea , about an voting , about its functionality , like , how much we use it. 
Or how useful we find it from 1-10 .

But the forum code does not help for such complicate voting .
Like, many options, and scaling about its one .

[tyblu]

Holy crap this is long. Will wait for abridged version to come out -- possibly feature film.

[Kiriakos-GR]

Holy crap this is long. Will wait for abridged version to come out -- possibly feature film.

There is an good explanation for it ..

Few years back FLUKE was supplying the big fat "User manuals" , with lots of info.
Today it gives 10 pages the most , and you have to become an private investigator,
so to get at the bottom of all the details.

But I will agree that jedidove  , does not do savings about using words .  


Even Dave had notice that Fluke had start to cut corners on the size of the manuals.  
  
The FLUKE 8050 an simple DMM , with one "User manual" six times the size of what you get with 87V or 28II .

[jedidove]

Holy crap this is long. Will wait for abridged version to come out -- possibly feature film.

LOL...Don't worry, I'm currently shopping a script and I think I may have Steven Spielberg interested.


When I get a chance to sit down in the lab for awhile I'll test what I can and make a (hopefully much shorter) post here summarizing the results and open questions.