Does your DMM range/autorange independently on its dual display? - Page 1
Forum section: electronics / testgear

author: ci11
author_rank: Regular Contributor
timestamp: March 03, 2021, 04:47:07 pm
content: Wytnucls introduced me to the Fluke 867B a few years back and there's been no turning back. It's a great meter and in fact, some features are so intuitive that I've taken them as how DMMs should all be like this - the auto-direction diode test is one - it tests regardless of how the leads are attached to the diode.The other is the topic of this post - a flexible autoranging link between the main and secondary readings on dual display/2nd meaurement-capable DMMs. I find this immensely useful, but went up against brick walls as I started huntung for upgrades to the 867B for a handheld as well as a 6.5 digit bench DMM, because I found more that do not than do this as a matter of course such as VDC in the main display and mVAC in the secondary display useful for seeing the AC ripple on a PSU, for example. Attached is a photo of the 867B showing the ripple of a 24VDC PSU in mVAC.My Fluke 189 cannot do this. Reading the manual for the 289, it appears it cannot do it either. I tried the Keysight 34411a and it cannot, nor does it appear that the DMM6500 can without chattering the relays incessantly. Wytnucls confirmed his Fluke 185 cannot but his Rigol DM3058E can - please see attached picture from him.It would be interesting to know what other DMMs can and cannot do. If you have a DMM has a dual display and can range independently, either through autoranging or manual ranging, please confirm with a post and a picture of it doing so.Thank you.

author: bdunham7
author_rank: Super Contributor
timestamp: March 03, 2021, 05:49:14 pm
content: I can confirm that neither the 289 nor any of the 6.5 digit Fluke bench meters with dual display have independent ranging. I'm not sure about the 8808A. PSU ripple is lost in the TRMS converter residual counts on those meters.The independent ranging seems like a nice feature, but it turns out to not be all that big a deal because you can read your PSU voltage and ripple by just selecting DCV, then ACV. Of course then you might then question what the point of dual-display is. I don't know whether this feature was abandoned due to lack of interest or some necessity regarding the input and protection design. I can see why this feature would be a problem with an AC+DC TRMS setup, but with an AC-only design with a blocking capacitor on the input of the AC section and separate scaling circuitry, I don't see why you can't have independent ranging.

author: ci11
author_rank: Regular Contributor
timestamp: March 03, 2021, 06:47:10 pm
content: Quote from: bdunham7 on March 03, 2021, 05:49:14 pmCharacterizing more than one attribute of at the exact same moment in time has many benefits, determining cause and effect is one that comes to mind quickly. Looking at dual displays that cannot independently range can be very frustrating as only part of the story is told.

author: bdunham7
author_rank: Super Contributor
timestamp: March 03, 2021, 08:30:29 pm
content: Quote from: ci11 on March 03, 2021, 06:47:10 pmThe problem with that is that your dual-ranging dual-display meter isn't able to characterize more than one attribute at the exact same moment in any meaningful way either unless it has multiple ADCs like a power meter would have. If the DC component of your reading changes at all during the process, your AC reading will take some time to settle again--so you can't really observe changes in both readings in real time.I'm not saying I wouldn't like to have the feature, but rather that I've found that I don't think it as important or useful as it might first appear. I find that being able to select DC aperture and AC filter settings to be more useful.

author: bd139
author_rank: Super Contributor
timestamp: March 03, 2021, 09:19:06 pm
content: It’s a stupid feature. Buy two meters. Problem solved! Both have independent ranging and a lot of the meters have to switch modes between alternate readings which makes it much slower to update anyway.I’ve never needed to see two at precisely the same time and when I have I am usually using an oscilloscope.

author: bdunham7
author_rank: Super Contributor
timestamp: March 03, 2021, 09:33:39 pm
content: Quote from: bd139 on March 03, 2021, 09:19:06 pmI wouldn't call it 'stupid' unless you are dismissing the idea of dual displays altogether. The way the OP wants to use the meter would probably be the most common way to use a dual display and losing the AC reading in the residual counts makes the dual display usefulness quite limited.

author: bd139
author_rank: Super Contributor
timestamp: March 03, 2021, 09:35:03 pm
content: Only use for dual display IMHO is displaying absolute and relative values at the same time.

author: ci11
author_rank: Regular Contributor
timestamp: March 03, 2021, 11:36:53 pm
content: Quote from: bdunham7 on March 03, 2021, 09:33:39 pmThe indpendently autoranging dual display on the 867B helps me do:1. ripple measurements on linear DC rails - dominant VDC and mVAC secondary readings2. DC offset measurements on AC rails - dominant VAC and mVDC secondary readings3. syncrhonzed AC level and duty cycle measurementsIn all these cases, not having independent ranging would require a second meter. Also, a scope can do these same measurements but typically not with the same accuracy as a good DMM like the 867B, and not as portable.Enough DMMs have dual displays that the question bears asking. Well-regarded suppliers like Brymen, Fluke, Gossen, Hioki and Keysight just to name a few cannot all be "stupid" to include them but rather did they think through the applications well enough to implement the in useful ways in their customers' hands.This is the answer I was hoping to find from those who own and use dual display DMMs, and whether they find them useful or wanting.

author: bd139
author_rank: Super Contributor
timestamp: March 04, 2021, 07:38:14 am
content: The BM867 is a fine example of why it’s rubbish. The meter is mostly unusably slow when it’s in dual reading mode. As for floating measurements I tend to do a whole DC measurement set for the system ie bias conditions etc, then apply signal and then do AC measurements.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 04, 2021, 09:43:43 am
content: The Fluke 867B autoranges to a minimum of 300mV in AC mode. Because of the unreliability of the true RMS converter below 5%, any measurement in that low value is pretty unreliable, so not really suitable to show the ripple of a PSU, which is usually below 10mV RMS.After some experimenting, it seems that anything above 25mV AC should be in the ballpark.

author: HKJ
author_rank: Super Contributor
timestamp: March 04, 2021, 10:05:47 am
content: Not many DMM's are fit to measure ripple on a DC voltage, as a start assume any DMM with a single mV range on the rotary switch cannot.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 04, 2021, 12:42:10 pm
content: I tested the DMM7510's dual measurement capabilities a bit without flicking the relay. Here are the results. Unfortunately, I haven't tested every possible combination. It is too long. Unfortunately, the manufacturer does not provide this information.

author: ci11
author_rank: Regular Contributor
timestamp: March 04, 2021, 03:23:18 pm
content: Wytnucls' point of TRMS converter acciuracy is well taken. That caution applies to picking the tool suited for the job at hand, and 4.5 digit DMMs like the 867B may lure us into a false sense of secuity in the measurements. There is only so much it can do.What an independently ranging DMM can do is to highlight possible faults, even if it cannot characterize them accurately. Competent linear PSUs exhibit ripple at well under <10mV, but not when there are part failures. The easiest way to determine the presence of failed or failng parts is to monitor both AC and DC.MegaVolt, it would be good to know if you think the DMM7510 would be a good way to measure linear PSU ripple accurately and reliably in the dual display mode.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 04, 2021, 04:00:41 pm
content: The Rigol 3058 copes pretty well with 10V DC and a ripple of 2mV RMS with frequency up to 10kHz.The 867B overreads by 25%.

author: HKJ
author_rank: Super Contributor
timestamp: March 04, 2021, 04:11:44 pm
content: Quote from: Wytnucls on March 04, 2021, 04:00:41 pmBench meters can often do it, but may need a relay change.I just checked: Keithley DMM7510 & DMM6500, Keysight 34465A & 34470A, all could handle 10VDC with 2mV AC @ 100kHz, a Fluke 8846A could not, there AC & DC needs to be same range.

author: bdunham7
author_rank: Super Contributor
timestamp: March 04, 2021, 04:39:26 pm
content: Quote from: ci11 on March 03, 2021, 11:36:53 pm1) You need to be sure that this works accurately. A meter with a high-quality blocking capacitor and separate scaling circuitry for the AC section will probably work well. Other setups may be iffy.2) You need to know what the normal mode AC rejection of the DC system is. In most high quality DMMs, it is pretty good as long as the peak AC value doesn't exceed the DC range, but beyond that I would start to wonder.3) I don't actually have any meters that display both those readings, but I don't think dual-autoranging is implicated in doing so. There's no 'range' for duty cycle.I've actually done both 1) and 2) somewhat routinely while repairing things. For 1) I usually have a device with multiple power rails and I want to do a quick check and see if I have a PSU problem. A failed filter cap or other problem usually shows up pretty plainly on a 6.5 digit DMM even without dual-ranging. For 2) I use the 289 and display both AC and DC. The DC doesn't have the residual count problem that the TRMS converter does so it was able to handle the most extreme example I can think of, which was adjusting a DC bias to +/- 5mV with the presence of 38VAC.Here are two photos of an 8846A measuring a PSU. The residual counts hide the very low ripple, but the meter would easily display a few mV or more from a noisier unit.

author: bdunham7
author_rank: Super Contributor
timestamp: March 04, 2021, 04:59:27 pm
content: Quote from: HKJ on March 04, 2021, 04:11:44 pmThey may 'do it', but what is the settling time when you engage the AC input in the presence of a large DC offset? And do those meters wait that long? My 8846A takes a good 15 seconds plus to settle on AC with a 10VDC input and standard filter settings. So 'needing a relay change' in this instance probably means it won't work very well.

author: HKJ
author_rank: Super Contributor
timestamp: March 04, 2021, 05:05:10 pm
content: Quote from: bdunham7 on March 04, 2021, 04:59:27 pmOne or two seconds between updates, the DMM6500 is slowest.

author: bdunham7
author_rank: Super Contributor
timestamp: March 04, 2021, 10:26:59 pm
content: Quote from: HKJ on March 04, 2021, 05:05:10 pmIf they can settle that fast when changing modes then something must be different about their internal scaling setup. Also, don't all of those meters have a digitizing AC measurement system rather than an analog TRMS chip? Anyway, if they can accurately read 2mVAC over 10VDC within a second or so, they've done something right.

author: David Hess
author_rank: Super Contributor
timestamp: March 04, 2021, 11:14:01 pm
content: Dual display meters usually make the measurements through the same input divider so ranging cannot be independent. Offhand I know of no exceptions.

author: HKJ
author_rank: Super Contributor
timestamp: March 05, 2021, 06:02:22 am
content: Quote from: bdunham7 on March 04, 2021, 10:26:59 pmFirst AC measurement is considerable slower, I believe that the input capacitor is charged to 10V at that point and do not get discharge when switching to DC, i.e. subsequent measurement can be done without any delay due to the DC offset.The Keithley uses a old fashion TRMS chip, the Keysight uses a digitizing TRMS circuit.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 05, 2021, 07:53:29 am
content: Quote from: ci11 on March 04, 2021, 03:23:18 pmI checked. It can measure AC and DC voltage. But every switch between measurements causes the relay to click.You cannot control double measurement remotely

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2021, 08:02:49 am
content: Quote from: MegaVolt on March 05, 2021, 07:53:29 amNormal for most bench DMM's.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 05, 2021, 07:10:11 pm
content: Didn't bother to check my Fluke 289 as someone already mentioned it doesn't but just checked my BM869S.It's a no. Causing either value to scale up or down in AC + DC mode makes both readings change scale.