Fluke 87-V HighRes values are different...? - Page 1
Forum section: electronics / testgear

author: Amaruk
author_rank: Contributor
timestamp: October 14, 2021, 08:33:52 pm
content: Hi,The other day when I was checking my 5uA DC current standard with a Fluke 87-V I noticed something really strange. When I compared the high-resolution values with the standard resolution values things didn't add up. I know that going into HighRes mode on this DMM does not make it more accurate, it just gives you more resolution which is fine with me. But what I did not expect to see was that the values did not agree when rounded. I then ran an experiment where I connected some of my DMM's in series with a Keithley 236 (set to a current source). The table in the attached image file shows my results. As you can see the high-resolution values have an offset of about 0.15uA when compared to the Keysight 34461A readings. However, in standard mode (one decimal less) the Fluke 87-V values are spot on...What could be causing this discrepancy? Are the Fluke 87-V calibrated separately in the standard mode and high-resolution mode? Thanks in advance!

author: AVGresponding
author_rank: Super Contributor
timestamp: October 15, 2021, 01:28:54 pm
content: The calibration steps don't include any for the Hi-Res modes, and there is no change in the input resistance/burden voltage, so it must be an artefact of the ADC implementation. Mine behaves similarly when sourcing 5-199uA from my Agilent U1401B.Worth pointing out that it is in spec, and that the LSD uncertainty count is x10 in Hi-Res mode, and that the nA that the LSD represent are well down into the noise for this meter.User manual:https://dam-assets.fluke.com/s3fs-public/80v_____umeng0200.pdf?tKDGTic.KN0dP9_UJVtSyLsuYWEUp3SYCalibration manual:https://dam-assets.fluke.com/s3fs-public/8xv_____cmeng0100.pdf

author: Amaruk
author_rank: Contributor
timestamp: October 15, 2021, 01:38:24 pm
content: Thank you! Yes, I am not concerned about being in or out of spec. It was just so confusing to see that while it is not more accurate in HighRes mode, it is actuallyLESSaccurate than in normal mode... Good to know! Still love this meter!From a user's perspective it would seem logical that the HighRes mode are the internal values and that they get properly rounded before being shown in normal mode. But that does not seem to be the case here which caught me by surprise. Thanks again for sharing your insights!!

author: AVGresponding
author_rank: Super Contributor
timestamp: October 15, 2021, 01:57:43 pm
content: Interestingly, if you manually set the 6000uA range, then go to Hi-Res mode, it gives a different reading to the 600uA range to the tune of 200-300nA for a 10uA nominal reading, when technically it should be identical.It does it in both polarities.I really think it must be how the firmware configures/uses the ADC.

author: Amaruk
author_rank: Contributor
timestamp: October 15, 2021, 03:00:53 pm
content: Interesting! Will have to try that too. As you can see in my images, I used auto-range on all DMM's. Thanks for sharing!

author: Fungus
author_rank: Super Contributor
timestamp: October 15, 2021, 07:22:34 pm
content: I like the way it comes up with a negative value in hi-res mode where it had a positive value in normal mode.

author: bdunham7
author_rank: Super Contributor
timestamp: October 15, 2021, 08:10:10 pm
content: Quote from: Fungus on October 15, 2021, 07:22:34 pmThe 20K/6K thing has always seemed weird to me and seeing the behavior here is even weirder, in spec or not. Clearly this is an offset issue, but I've no idea exactly how the 6K mode and 20K mode differ overall. The 6K mode was able to read spot-on, so in this case at least, the 20K mode doesn't seem to be worth paying extra for!

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 15, 2021, 10:50:20 pm
content: I use several 87-5 but I don't think I ever switched it to 20K mode.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: October 16, 2021, 12:17:16 am
content: Quote from: Fungus on October 15, 2021, 07:22:34 pmNot sure how exactly the hi-res mode works, but keep in mind that 0.1uA on the 600uA range represents an absolute value of ~0.0167% of that range. On the lowest range of this meter...I'm actually quite impressed that it really reads 0.1 in standard mode in the first place.

author: J-R
author_rank: Super Contributor
timestamp: October 16, 2021, 06:11:27 am
content: I recently snagged a faulty Keithley 197A that just needed some TLC, and as luck would have it was calibrated only about 1 year ago. This thread seemed like a good way to take it for a spin. Results attached.

author: TMM
author_rank: Frequent Contributor
timestamp: October 16, 2021, 06:16:57 am
content: I thought this was a pretty well known issue. All the 87V I've come across have a slightly different offset in low res vs high res and it can drift over time, you just don't notice the drift as much on 6000 count mode as it only effects the LSD.With nothing connected, one of my 87V reads 0.3uA in low res and 0.17uA in high res, so it's more accurate in high res... If you want LSD accuracy, high res and use of the rel function is your best bet.Quote from: Caliaxy on October 16, 2021, 12:17:16 am0.0083% of the ADC range because the range is from -600uA to +600uA

author: Amaruk
author_rank: Contributor
timestamp: October 16, 2021, 01:25:35 pm
content: Quote from: J-R on October 16, 2021, 06:11:27 amVery interesting results! Thanks for sharing! Your results confirm what I am seeing on my end with the 87-V: the standard resolution value is not just a rounded value of the high-resolution value for small currents. In your case, though, the accuracy is actually higher in the high-resolution mode. Perfect!To dig a bit deeper into this I also added my Fluke 87-III to the mix. As you can see in the results that are added to this post, the 87-iii does not seem to have this issue. Its values match nicely!I should also say that what I am observing here is related tosmall currents on the uA range. I also tested for small voltages on the mV range and neither my 87-V nor my 87-iii show rounding issues with those measurements. Attached is a picture that shows my results when all the DMM's are connected in parallell to a 5mV voltage source (87-iii and 87-V are in HighRes mode).

author: Amaruk
author_rank: Contributor
timestamp: October 16, 2021, 01:31:45 pm
content: Quote from: TMM on October 16, 2021, 06:16:57 amInteresting. I was not aware of this. Great idea about using the relative function in situations like this! Thanks for sharing!!

author: J-R
author_rank: Super Contributor
timestamp: October 17, 2021, 12:53:51 am
content: To clarify, I didn't use relative mode on the Fluke 87-V in any of my tests. Using relative mode on my specific sample yields about a -0.20uA offset throughout the readings so isn't usable.I obtained this 87-V as a new in box return and it came with a calibration certificate which stated 330.0uA for an "as left" value.Same story on the 287, but with a value of 500.00uA.No data for the Keithley 197A but after some additional tests I would say it is between 0.001uA and 0.003uA low from 0 to 5uA. Running on up to 200uA, it ultimately is about 0.025uA low, which is still well within spec: 199.785uA to 200.215uA.Was surprised to find my Keysight U1461A has a "hidden" 0.0001uA display and the digits appear to be meaningful. Just discovered this later today so no extensive tests yet.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 17, 2021, 07:30:27 pm
content: Someone in the TEA thread mentioned it might be thermal drift, so I decided to see if that is so. TLDR; it isn't.I took two sets of measurements at a tAof 19°C and 26°C. I measured at 1uA, 2uA, 4uA, 10uA, 100uA, and 199uA.Aside from the readings drifting up by the best part of 1uA and getting a bit noisier, the offset remains the same.Then I discovered something which might start to shed a bit of light on the matter, and I'm not ashamed to say, entirely by accident.I was taking the leads out and switching the meter and calibrator off, and purely by chance I left the F87 on until after I removed the leads... and the offset almost disappeared!A quick fiddle about reveals that without the banana plug (once I saw there was an effect I used a plug on its own rather than a lead) the meter reads 0.2uA in 600uA range, 0.07uA with HR, and 0uA in 6000uA range, -0.1uA with HR.With the plug in, the readings are 0.0, -0.14, 0, -0.3 respectively.So it looks a bit like the lead detection system is causing at least a part of the issue, maybe they thought to compensate for it in normal res mode but forgot to do so for HR? Here's the circuit diagram, see what you think.

author: Amaruk
author_rank: Contributor
timestamp: October 18, 2021, 01:19:11 pm
content: Wow - that is quite the discovery! I will have to take a closer look at that too. I'll let you know what I find out on my end. Good find!

author: Amaruk
author_rank: Contributor
timestamp: October 19, 2021, 05:35:10 pm
content: An update: I can now confirm the finding first reported by AVGresponding. Great job figuring this out!!!Here is a short video that shows my Fluke 87V with and without leads connected in both standard and high-resolution mode on the uA range. As you can see in the standard resolution mode the meter goes to 0.0uA as soon as you connect the leads as one would expect (they are not connected to anything in the other end). However, in the high-resolution mode that is not the case. My meter goes to about -0.15uA which is the offset I observed when I ran test measurements in the 0.1-100uA as reported above.Link to video:

author: J-R
author_rank: Super Contributor
timestamp: October 20, 2021, 05:33:20 am
content: Connect the leads through a decade resistor box instead of a short/DUT and run through the tests again. Try 100-500 Ohms. I got some interesting results.

author: EEVblog
author_rank: Unknown
timestamp: October 20, 2021, 05:57:36 am
content: Quote from: Amaruk on October 19, 2021, 05:35:10 pmMy 87V shows 0.3uA on 6000 count mode, and 0.14uA on 20000 count. No change when input is shorted.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 20, 2021, 12:50:35 pm
content: Quote from: TMM on October 16, 2021, 06:16:57 amI have 3 87 V here and they seem to agree with yours.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 20, 2021, 04:48:18 pm
content: Quote from: J-R on October 20, 2021, 05:33:20 amQuote from: EEVblog on October 20, 2021, 05:57:36 amYou misunderstand me; I am not using a shorting link, I am just inserting a bare plug with no lead in order to connect the two halves of the uA socket that make up the lead detection system, I didn't connect anything to the common socket at all.

author: Amaruk
author_rank: Contributor
timestamp: October 20, 2021, 05:07:32 pm
content: Good point regarding the shorting ports. I also did not short anything in my video as my two leads are not connected in the other end so the meter is seeing an open - with and without leads connected to the ports. But as you say, the magic happens when you insert the banana plug in the uA port. The COM port has nothing to do with this at all.

author: EEVblog
author_rank: Unknown
timestamp: October 20, 2021, 10:34:51 pm
content: Quote from: AVGresponding on October 20, 2021, 04:48:18 pmAh, got it.Yeah, confirmed:

author: EEVblog
author_rank: Unknown
timestamp: October 20, 2021, 10:56:21 pm
content: FYI, on the BM786 it's zero with no lead, and then when the lead is plugged in it just up for a second or two but then settles back down to zero.On the 121GW with no lead there is an offset, but it drops to zero with the lead.

author: J-R
author_rank: Super Contributor
timestamp: October 21, 2021, 04:42:41 am
content: The offset in Hi Res mode on my 87-V jumps a bit between -0.02uA and -0.05uA. (Naked plug in mAuA jack only.)My somewhat cryptic comment about connecting a decade resistor box is pointing out that there is probably current flowing between the COM and mAuA jacks. You can see this in the 100-500Ohm range, as the readings change quite a bit.The best I could do to measure this at the moment was 1kOhms through the Keithley 197A which resulted in about -0.020uA and 3kOhms through the U1461A was -0.0007uA. I did what I could by using shielded leads, swapping polarities at both ends, etc. but maybe someone with some metrology-grade equipment could jump in.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 21, 2021, 05:06:54 am
content: Quote from: J-R on October 21, 2021, 04:42:41 amSince there are several resistors in series and parallel between the two jacks internally, adding up to around 99.918Ω, this is a given.The point I am trying to make is that this seems to be compensated for in firmware normally, but not in the Hi-Res uA ranges.EDIT: I have also looked for this effect in my other meters but they either show no effect, or the lead detection system prevents any reading until a plug is inserted (F289), and none of them have a Hi-Res mode in any case (500000 count mode not available on uA on the BM869S).

author: J-R
author_rank: Super Contributor
timestamp: October 21, 2021, 08:26:04 am
content: I think we are still not on the same page. I am saying the Fluke 87-V is generating a small voltage/current between the mAuA jack and the COM jack when in uA mode. The voltage is approximately -0.025mV. (The current measurements are in my previous post.)You can verify this by connecting the mAuA and COM jacks to another DMM's V and COM and disconnecting only the battery in the 87-V. You will see the voltage slowly change from -0.025mV to zero.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 21, 2021, 05:13:49 pm
content: Quote from: J-R on October 21, 2021, 08:26:04 amThere's a current flowing, so by definition there's a voltage across the resistors that the current is flowing through. You'll find the same on the A jack.The voltage is the drop across the current shunt due to the current supplied by the lead detection circuit, which is derived from the -2.5V rail, reaches the m/uA jack via R48 and R7, and the A jack via R11 and R10.The voltage you are talking about is the drop across the resistors that make up the m/uA measuring shunt, R5 and R6 for mA, R4, R5, and R6 for uA. The current flow and voltage drop is the same for normal and Hi-Res mode, it's just compensated for in the firmware in normal mode, but not in Hi-Res mode.

author: Amaruk
author_rank: Contributor
timestamp: October 21, 2021, 06:58:37 pm
content: Thanks Dave for your short video on the eevblog2 channel that clearly highlights the issue!I always knew your 121GW and BM786 are two very good multimeters and with this in mind think I might put them high up on my list for Santa this year. The BM786 just seems to fly off the Amazon shelves here in the US. I can see why…

author: EEVblog
author_rank: Unknown
timestamp: October 23, 2021, 03:01:49 am
content: Quote from: Amaruk on October 21, 2021, 06:58:37 pmJust sent three more boxes to amazon, but that will probably only last a month or so.

author: Amaruk
author_rank: Contributor
timestamp: October 28, 2021, 03:00:19 pm
content: Quote from: EEVblog on October 23, 2021, 03:01:49 amThat is awesome!I was hoping to update this thread with an official response from Fluke. But after a few weeks of waiting I have still to hear back from them unfortunately. Yes, this silly observation is not a big thing at all as the DMM is still in spec.What I take out of this is that it is probably best to view the Fluke 87-V as a 6,000 count multimeter as that is how it is calibrated. The 20,000 count mode is good for observing small drifts but the accuracy in that mode is not really calibrated as it can be less accurate as we have seen here. If I need a Fluke DMM with a higher count than 6,000 and/or a DMM with higher accuracy, I should be looking into getting the Fluke 289. I just wish that model had a 7-segment LCD, longer battery life and built-in Bluetooth....

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2021, 03:41:53 pm
content: Quote from: Amaruk on October 28, 2021, 03:00:19 pmOr try a Brymen.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 28, 2021, 03:57:38 pm
content: Quote from: Amaruk on October 28, 2021, 03:00:19 pmI use the 87V a lot at work and I think I always use it in the 6000 count mode. I have the 289 and 287 available but I don't like it display just like you and the battery life if short too. I have the 189 that I don't use often at all but recently I use it and found that I like it better than both the 289 and 87V but I think it's battery life is also short.

author: J-R
author_rank: Super Contributor
timestamp: October 28, 2021, 06:49:38 pm
content: It's simply not true as a general statement that the 87-V 20,000 count mode is not calibrated or is only good for watching changes. I showed in my previous uA tests that it was more accurate and provided more usable digits than standard mode. I've also previously run multiple DCV tests with the PDVS2mini and it's basically within 1-2 digit in 20,000 count mode from 0-10V. My Fluke 87-V did come with a valid calibration certificate plus data. But I also have the equipment now to verify it going forward.20,000 vs. 6,000 is a big difference for some voltage ranges that you may encounter, such as 1.5V, 12V, and 120V. Conversely, it makes no difference in the case of 4.2V, 24V and 240V, which is where a 50,000/60,000 count DMM might be better suited.There is also the perhaps esoteric question of whether the fact that a specific 87-V might display 9.85uA instead of 10.00uA for example is actually of any concern to someone other than a volt-nut.In my opinion, the bigger issues for the 87-V as an electronics/bench meter are that it has no interfaces, uses a 9V battery, defaults to AC for some modes and resets some settings when changing the selector.

author: Amaruk
author_rank: Contributor
timestamp: October 28, 2021, 07:06:24 pm
content: Good points!My point here is simply that it was said that this meter is calibrated in its 6,000 count mode only. Assuming that is indeed correct, and, as we have seen here, there can be differences (rounded) between the 6,000 count and 20,000 count data, it seems logical that the 6,000 count data should be more reliable than the 20,000 count data as it is not checked during calibrations. Granted, some DMMs might be faulty or is this quirk related to a firmware bug as was suggested here?In my case, my 87-V has a valid calibration still and its value in the 6,000 count mode is spot on as you can see in the data.Again, it would have been nice to hear what Fluke had to say about this and if I do hear back, I will update this thread. But yes, this is a volt-nut type of thing....

author: J-R
author_rank: Super Contributor
timestamp: October 28, 2021, 07:54:18 pm
content: I'm pretty confident that there is only one set of calibration data stored. For my 87-V, the calibration data printout was only for the standard mode, so it is unlikely that it would be that accurate in Hi Res mode if they just left it uncalibrated. As we've seen with other DMMs, internally it is common for a device to have more digits of resolution but it is filtered/averaged before being presented to the user.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: October 28, 2021, 10:35:33 pm
content: Quote from: Amaruk on October 28, 2021, 03:00:19 pmTo be on the safe side, yes.My experience with the hi-res mode in my relatively new Fluke 87V (less than 1 year old) is different from yours. Contrary to what’s written in the data sheet and to the word on the street, the hi-res mode does increase accuracy, bringing my F87V on par (i.e. within 1-2 counts in hi-res mode) with any better spec’d 60,000 count Brymen (I have two of those).10 uA measured in 6000 count (top) and hi-res (bottom)1 uA measured in 6000 count (top) and hi-res (bottom)0.1 uA measured in 6000 count (top) and hi-res (bottom)I also happen to have an older F87V’s, bought about 7 years ago on eBay (with unknown history). In general, my both F87V agree with each other within a count (in 6000 count mode) and within less than 10 counts (in 20,000 count mode). The new one is always more accurate in hi-res mode (than in 6000 count mode), the old one is sometimes slightly less accurate in hi-res - but we are talking about 5-6 counts off (accuracy judged by comparing the readings against a 6½ digit Keithley).At 0.1uA (and under), the old one behaves somehow like yours (much worse accuracy in hi-res):0.1 uA measured in 6000 count (top) and hi-res (bottom). Old F87V on the left:At 1uA it's a tough call (6000 count top, hi-res bottom). Old F87V on the left:It might be that my newer 87V is optimized for better accuracy in hi-res mode, whereas the older one is optimized for better accuracy in 6000 count mode. But we are talking about differences of 1-2 counts in 6000 count mode, so all bets are off. Maybe the older one is just slightly off overall and this is only visible in hi-res mode (but it’s still within its published specs). Or maybe it's just random variance between meters not spec’d at such a high accuracy in the first place.In relative mode, both F87Vs show higher accuracy in hi-res at the lower end of the scale (600.0uA). I was able to measure with both down to 0.05uA. Old F87V bottom left, new F87V bottom right, DMM6500 top:Having said that, the accuracy of most meters is generally reported at full scale and guaranteed over a reasonable range at the upper side of the scale (e.g. for measured values larger than 10% of a given scale). The lower you are on the scale, the higher the relative noise and the lower the precision of the measurement. You can’t simply measure 0.1uA on a scale of 600.0uA and expect the same precision you would get if you measured, say, 500uA on the same scale.I wouldn't use an F87V to measure 0.1uA in the first place (not even an F289 or a Brymen 689S…). There are other meters for that.About my tests: I don’t have a precision current source, so I used a Datel voltage reference in series with a 1MOhm resistor. All meters were in series. For the relative measurement, I disconnected the voltage reference, shorted the terminals and use the displayed values on each meter as offset.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 29, 2021, 05:04:34 am
content: A more meaningful experiment would be to obtain readings of the same current in 600uA mode with Hi-Res OFF, and then 6000uA mode with Hi-Res ON.The question of calibration is moot; there are explicitly no calibration steps for Hi-Res modes in the calibration manual, however the uncertainty figures quoted, and the way the ADC input gain is set, clearly imply that the ADC will receive and transmit the same data in the scenario I specify, and that the difference in the readings is purely down to the current from the lead detection circuit being accounted for in normal modes, but not in Hi-Res mode.Is this a bug, or merely an omission? I suspect the latter.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 29, 2021, 06:52:48 pm
content: Here you go.Pics of plus and minus 10uADC in 600uA normal range, and 6000uA Hi-Res.

author: Amaruk
author_rank: Contributor
timestamp: October 31, 2021, 04:00:12 pm
content: Quote from: AVGresponding on October 29, 2021, 05:04:34 amYes, I think so too.As you pointed out earlier, the meter is calibrated in 6000 mode which would mean thatonly if there is no offset between the two modesthe high-res mode will have thesame accuracyas the standard mode (and not better). Any offset between the modes, and many have observed offsets in this thread, will impact the accuracy in high-res mode if the meter is fully calibrated. Now, if a meter is more accurate in the high-res mode and shows an offset in the standard mode, that can probably be adjusted with a calibration.Again, we are all on the same page here that this is just a silly quirk in the 87V's behavior and nothing that matters when it comes to real world measurements.