Fluke 87-V HighRes values are different...?

[AVGresponding]

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.

Since 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).

[J-R]

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.

[AVGresponding]

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.

There'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.

[Amaruk]

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…

[EEVblog]

The BM786 just seems to fly off the Amazon shelves here in the US. I can see why…

Just sent three more boxes to amazon, but that will probably only last a month or so.

[Amaruk]

Just sent three more boxes to amazon, but that will probably only last a month or so.

That 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....

[Fungus]

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....

Or try a Brymen.

[BeBuLamar]

Just sent three more boxes to amazon, but that will probably only last a month or so.

That 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....

I 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.

[J-R]

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.

[Amaruk]

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....

[J-R]

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.

[Caliaxy]

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.

To 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.

[AVGresponding]

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.

[AVGresponding]

Here you go.

Pics of plus and minus 10uADC in 600uA normal range, and 6000uA Hi-Res.

[Amaruk]

Is this a bug, or merely an omission? I suspect the latter.

Yes, I think so too.

As you pointed out earlier, the meter is calibrated in 6000 mode which would mean that only if there is no offset between the two modes the high-res mode will have the same accuracy as 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.