Null meters still in production? How to replace a Keithley 155 null meter? - Page 1
Forum section: electronics / general

author: ttttrigg3r
author_rank: Contributor
timestamp: May 18, 2022, 06:00:26 pm
content: We use the Fluke 752A Reference Divider which needs balancing every now and again. So far, the only thing I know capable of doing the balancing is a null meter. We use Keithley 155 which is still working...for now. Fluke says their 8588A DMM does not have capability to be a null meter. When I try to balance the 752A using that DMM, it's no where as good as a Keithley 155.So now we're using a product that's no longer supported and repaired to support something critical. As far as I know, no one is making null meters anymore. So what's replacing the null meter? What other options can I use to balance the Fluke 752A divider?Reference:https://us.flukecal.com/literature/articles-and-education/general-calibration-metrology-topics/application-notes/replacing-a

author: bdunham7
author_rank: Super Contributor
timestamp: May 18, 2022, 06:06:37 pm
content: Quote from: ttttrigg3r on May 18, 2022, 06:00:26 pmYou've linked to a paper from Fluke actually telling you that certain DMMs (including the 8588A) are workable substitutes for a null meter and explaining how to do it.What issues are you having using a DMM and what bias current are you measuring?If you prefer the somewhat deceptive smoothness and stability of the analog null meter, you're not alone. But you'll have to obtain and maintain one yourself, they're obsolete as far as the OEMs are concerned.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 18, 2022, 07:26:53 pm
content: Look for TEGAM AVM-2000 Null Meter

author: TimFox
author_rank: Super Contributor
timestamp: May 18, 2022, 07:44:03 pm
content: Interesting picture of the Tegam. Current price about $6000 USD, and more sensitive than the Fluke 845.When did manufacturers of such instruments delete the front-panel Ground post?My classic Fluke 845AB has Input High, Input Low, and Guard on the left, but Output High and Ground on the right.I prefer connecting my shield braids and boxes to Ground, while connecting Guard as required.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 18, 2022, 08:06:39 pm
content: All the old null meters are subject to problems. I've got two HP 419 and the Fluke 845 and all of them need service... when I get around to it. The paper referenced isn't very helpful because we can't all afford an 8.5 digit reference meter. Makes me want to go back to the old L&N "top hat" galvanometers! Point a laser pointer at it and put a paper target a few hundred feet away.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 18, 2022, 08:34:09 pm
content: The adjustment and use of the 752A definitely requires a null meter with leakage/bias current of<1pA, which these DMMs do not specify, nor provide. My 845A measures < 200fA at Null, whereas my 3458A has about -10..-20pA, creating big errors. Therefore you need to characterise any null meter for bias, before using itFrankPS I herewith promise to finally publish my findings on the 752A and the Null problem soon.

author: bdunham7
author_rank: Super Contributor
timestamp: May 18, 2022, 08:36:49 pm
content: Quote from: Conrad Hoffman on May 18, 2022, 08:06:39 pmThe link within the link goes over the details of using a meter with a bias current high enough that it can't be ignored. There are many readily available DMMs with bias currents under 50pA. The only issue would be making sure it was stable in the short-to-medium term--and I'm aware of quite a few that are that as well. The old Fluke 8500 series has trimpot adjustable bias and with some care it can be zeroed out to <1pA, although the medium term stability is more like +/- 5pA or so. I have an old 8500A in good working condition that I use for the occasional nulling task, although it is limited to 1uV resolution. I don't have anything to null at the level of the 720A or 752A.

author: 2N3055
author_rank: Super Contributor
timestamp: May 18, 2022, 09:07:10 pm
content: Quote from: TimFox on May 18, 2022, 07:44:03 pmYou get a front cover that goes over input terminals . Screws for cover are grounded. There is a system grounding point on the back...

author: TimFox
author_rank: Super Contributor
timestamp: May 18, 2022, 09:19:18 pm
content: Quote from: 2N3055 on May 18, 2022, 09:07:10 pmYes, I know. A properly-labeled post on the front is much more convenient, and used to be normal. I also understand the difference between guard and ground.

author: 2N3055
author_rank: Super Contributor
timestamp: May 18, 2022, 10:31:28 pm
content: Quote from: TimFox on May 18, 2022, 09:19:18 pmI never said otherwise.. I do too. What did I say wrong?I agree front is better place for the post.

author: TimFox
author_rank: Super Contributor
timestamp: May 18, 2022, 10:58:01 pm
content: I was commenting on how DVMs and analog null meters used to have a binding post on the front for chassis ground, but no longer.Is this some kind of safety thing? I don't understand why it is now omitted.

author: bdunham7
author_rank: Super Contributor
timestamp: May 18, 2022, 11:13:56 pm
content: Quote from: TimFox on May 18, 2022, 10:58:01 pmI think it may be considered no longer necessary because of the improved in-guard isolation of modern meters. I have one older DMM that has the front-panel ground connection and a link so that you can ground the negative terminal. I've observed that if you are measuring higher frequency AC that is ground referenced--like from a signal generator--that you need to make sure that the side that is ground referenced is attached to the negative terminal whether or not the ground link is installed. There is enough capacitive coupling between the in-guard AC input board and ground that there will be a significant error if you connect the other way around. That doesn't happen with more modern devices, or if it does the effect is much, much smaller.Some very high-end meters still have ground, guard or both on the front panel. AFAIK it is a low-side guard, the driven high-side guards only exist internally these days, at least on DMMs.

author: manganin
author_rank: Regular Contributor
timestamp: May 20, 2022, 02:37:34 pm
content: Quote from: ttttrigg3r on May 18, 2022, 06:00:26 pm...and keeps working, unless you do something stupid.If you are happy with the specifications, the 155 is hard to beat.There are no parts that age or wear. Unlike the older 419A and 845A.And the circuit is very simple and elegant, making it reliable. Unlike the new AVM-2000.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 20, 2022, 05:02:21 pm
content: The 155 seems to use those big 9V batteries like some of their electrometers. I was surprised to find that if you carefully open the box, from the bottom I think, there's just two regular 9V batteries plugged into a little terminal board. They're in parallel. Easy to replace for far less than they get for the genuine big one.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 20, 2022, 05:06:06 pm
content: Quote from: Conrad Hoffman on May 20, 2022, 05:02:21 pmI removed all the old nasty batteries and went with 9V block batteriesEven since October 2016, they are still working well.https://www.eevblog.com/forum/repair/keithley-155-null-detector-repair-and-restauration-attempt/

author: ttttrigg3r
author_rank: Contributor
timestamp: May 20, 2022, 06:00:46 pm
content: The issue when I try to use the 8588DMM as a null meter is that I'm not getting a stable settled reading. With the 155, I can have it on the 3µV range and the needle would be stable, so there's confidence there. For example if I'm balancing the 10:1 portion between positive and negative, its actually doable because I can read it. It's stable to 0.5µV.When I tried doing the same thing with the 8588DMM, and I'm following the exact steps as written by Fluke, I cannot get a stable reading. The reading jumps around; I've tried averaging of all sorts to no avail.When the Fluke representatives and their 8588DMM expert made a visit to our lab, they confirmed that the 8588 cannot replace a null meter like we've been led to believe. They have no answer when we asked them how to use that 8588 to balance this 752 divider. That was 1 year ago; I haven't heard any successful follow up since.

author: ttttrigg3r
author_rank: Contributor
timestamp: May 20, 2022, 06:03:13 pm
content: Thank you. I look forward to your publication. How do I subscribe?

author: ttttrigg3r
author_rank: Contributor
timestamp: May 20, 2022, 06:08:30 pm
content: This may be a tangent, but is there an alternative to the 752A divider? One that doesn't need to use a null detector to balance?We use it to transfer values from a 10Vdc reference standard to our Fluke 5730A calibrators. It's involved directly with our accreditation.

author: alm
author_rank: Super Contributor
timestamp: May 20, 2022, 07:34:00 pm
content: There used to be the Datron/Wavetek 4902S, but I don't think it's still available. Maybe look at theMeasurement Internationalofferings?

author: bdunham7
author_rank: Super Contributor
timestamp: May 20, 2022, 11:26:26 pm
content: Quote from: ttttrigg3r on May 20, 2022, 06:00:46 pmThe analog null meter can make a stable reading of a noisy signal because at those low levels it is very damped--the response times are typically glacial. If Fluke was unable to solve your issue.....0.5uV would be 500 counts on your meter. Does it exhibit that same level of noise when making a non-nulling measurement? And I don't know if this is an issue or not, but is autozero on or off when you are doing this? I've no equipment anywhere near your level, but in a day or so when my bench clears up, I'll set up the way I nulled something the last time to see if the results match my memory of them. I reread the Fluke paper on this and I think there's a missing element somewhere. Are you setting the meter to 10M or high impedance mode? I think perhaps I shunted the meter with a 1M resistor in parallel with a film capacitor, the same way you measure the bias current. The 1012ohms or so that the null meters refer to is the resistance between the inputs and ground. The resistance between the two inputs doesn't have to be that high because at null, there's almost no voltage and thus very little current, so even higher source impedance devices won't be loaded.Meanwhile, take care of your old null meter!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2022, 07:37:58 am
content: A capacitor parallel to the DMM input can help, ideally a low DA type like PP, not to add additional slow creep.An extra parallel resistor should not help - to signal source should be less than 1 M ohms.When using a normal meter, there are 2 possible problems: one is the isolation from ground / main, with possible injected current and just leakage resistance. For the isolation problem it may help to have also a capacitor from ground / the reference ground towards the meters low side. This way one could at least filter the higher frequency common mode problem.The other is the input current of the meter and it's current noise. Some 50 pA of input bias current may be acceptable if used with care (use both polarities and average), but the current noise can be a problem. In addition some meters have switching pulses in the input current and these can depend on the load / capacitance at the input. So this not only adds noise, but can also add systematic errors. The extra capacitor at the input can at least swamp the capacitive load problem, though worst case parasitic inductance can still cause resonances. So the filtering should be part of the DMM and more than just 1 capacitor.An input with low voltage noise tends to have more current noise - so a very low noise meter may not be the best choice. The best input is a compromise and depends on the source impedance.One could check if the problem is more with the isolation from ground or the meter not working well with a high impedance source by using the meter to test to read a zero on a resistor in the 100 Kohms (or what ever the relevant impedance). This way the seprate resistor has good isolation and would only show the current noise problem.

author: martinr33
author_rank: Frequent Contributor
timestamp: May 21, 2022, 04:43:04 pm
content: A 34420A nanovolt meter is a decent alternative to an analog null meter. Obviously, there are caveats but it is the best alternative to a null meter. Your grounding and shielding all have to be correct to get good readings.Regular DVMs are just way too noisy at sub microvolt readings.(there's also the Keithley 2182A, but the 34420A is a more versatile device becasue of its micro ohmeter).

author: bdunham7
author_rank: Super Contributor
timestamp: May 22, 2022, 01:12:44 am
content: Quote from: ttttrigg3r on May 20, 2022, 06:00:46 pmSo I set up and tried some nulling with mixed results. I'm using a Fluke 8505A DMM and two 10V standards and no super-special leads in a relatively calm room, but not a temp controlled lab. I haven't used the meter in some time so I was interested to see how well the zero offset and bias currents have held up, and the answer is pretty good, with the zero at +/- 1 count (100nV) and the bias current at ~-5pA after 4 hour warmup. I then measured each standard and got 10.000018 and 10.000024V respectively, with some 5 counts or so maximum noise, but generally pretty calm and within a 2-3 counts in the short term. The meter and one of the standards do have noticeable tempco so they do drift around a bit more than that over the medium term, but I'm more looking at your issue of not being able to get a stable enough measurement.I then set the two standards up to null them at the meter with a short between their negative sides. I didn't use any shunt across the inputs and I used the meters F2 filter and averaging. The result was a reading of about 6µV as expected, fairly calm in the short term but drifting around +/- 5-10 counts (0.5-1µV) or so in the medium term (say 5 minutes). This would be completely workable for nulling my standards as 1µV is 0.1ppm and that is 10-20X as good as even the most optimistic expectation for these standards (Fluke 731B and the 10V 24ppm model fromthevoltagestandard.com. Then I tried to sort of simulate your high-impedance source by putting an 82K resistor between the negative sides of the voltage sources and a 6.8nF capacitor across the inputs. Short term again was calm, the medium term was a little noiser, perhaps 8-12 counts. Then I used the same 1M/2.2nF shunt that I use for setting bias current, and the result was about the same reading and even calmer, back to 5-10 counts. So a high-impedance source isn't really a problem.As a nulling instrument for things at my level, the DMM is OK albeit quite inconvenient. I can probably null two standards more accurately than I can transfer them directly with the ratio function or by just manually switching them. And by the specs of the 752A (once you get past the obvious errors in the online version) what I'm doing would be almost good enough, so presumably with the newer meter and a controlled laboratory you should be able to improve on this. Settling times are quite long and any disturbance at all will send things wildly off--such as the cat walking in, the front door of the house opening, etc. But all-in-all, the errors and instability that I observed with the nulling process don't seem to be any greater than those observed with any other sensitive measurement.So, I'd ask--how much noise do you actually see when you try to do this? Can you make similar measurements at those levels with significantly less noise? With your assortment of equipment, I can't imagine Fluke wouldn't respond to your inquiry--ask them how they do it in their lab. The 752A is a current product, it would be amusing if they had some decades-old null meter doing this job during production.

author: Echo88
author_rank: Frequent Contributor
timestamp: May 22, 2022, 11:50:16 am
content: @ ttttrigg3r: The Guildline 7520 should be capable of replacing the Fluke 752A. Havent yet compared the uncertainties for both devices though.https://www.guildline.com/new-products/released/7520-automated-precision-voltage-divider

author: RoadDog
author_rank: Regular Contributor
timestamp: May 22, 2022, 02:53:32 pm
content: Quote from: bdunham7 on May 22, 2022, 01:12:44 amThey use the 845 for the 752 and 720.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2022, 04:53:18 pm
content: There is rather limited need for a null-meter and there are still some of the old null meters around. So the market for new meters is pretty small as most owners of a 752 likely have a suitable old null meter. It is still a bid odd not to have new ones, as it does not have to be expensive and may be usefull for a few other tasks too. Compared to the old ones a new one could offer some 4 digit resolution and better than 1 % accuracy to also use it as a more general battery powered voltmeter for small voltages (though not a full nV meter). The main point would be to have low bias and maybe a trim / test circuit for the bias. For good isolation battery operation is an easy solution as modern meters can be low power.With the 752 the source impedance (bath sides combined) should be at some 100 - 400 K ohms. So there is no need to have super low noise, as there is always quite some resistor noise. It is more like a need for very low input current noise. So when using an AZ OP to build a null meter it would be more like looking for a type like AD8628 or even max4238 and not LTC2057 or OPA189.The old null meters are also not really low noise, but good enough for the job.

author: TimFox
author_rank: Super Contributor
timestamp: May 22, 2022, 05:30:00 pm
content: When actually adjusting for a null, I find an old-fashioned d"Arsonval meter easier to use.For a nanovolt meter, a reasonable digital readout may be better.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2022, 06:27:37 pm
content: Anlog and digital read out have there advantages. With a digital signal one can use a more FIR type filter and this way get relatively fast settling, which can be a big plus.

author: e61_phil
author_rank: Frequent Contributor
timestamp: May 22, 2022, 06:31:23 pm
content: The Keithley 182 has a DAC output which gives an analog signal scaled to FS of the range. That DAC is updated with the display. Therefore, it applies the same filtering and you can connect a meter with a needle.

author: guenthert
author_rank: Frequent Contributor
timestamp: May 22, 2022, 10:41:48 pm
content: Null meters are comparatively (say against a longs scale DMM) simple devices (self-calibrating, no need for high linearity or speed). I think one with the performance of a HP419A is today very well within the reach of a dedicated amateur. The Keithley performs quite a bit better (low enough bias current that a bucking voltage supply was not deemed necessary), but here people rose to greater challenges.HP419A can be considered obsolete and many of them are meanwhile old enough that their neon bulbs / LDR based choppers don't work within specification or at all anymore and hence occasionally available for reasonable prices on the secondary market. I'd think the (double-walled!) chassis, voltage divider, large MCM and low thermal EMV input connectors would make a good starting point for a self-made one with modern amplifier.

author: syau
author_rank: Frequent Contributor
timestamp: May 22, 2022, 11:57:33 pm
content: Wonder if a nanovoltmeter be used as null meter

author: RoadDog
author_rank: Regular Contributor
timestamp: May 23, 2022, 12:57:17 am
content: I’m surprised no one is making a quality analog null meter anymore. There are other options yet for sub PPM measurements I don’t think anything compares. I’ve used a few different ones and the best was an 845ab that was in great condition. It was fast and extremely accurate. When the capacitors get old they get slow to null and start giving bad readings. The chopper amps get old as mentioned and you lose linearity through the ranges.The battery powered 845 is much better than the rack mount 845ar in my experience.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 23, 2022, 10:04:21 am
content: @ martinr33, e61_phil, Kleinstein, syau:The null-meter requirement for 720a, 752a is to be uncertain to <= 500nV or <= 200nV respectively, (see manuals), which in practice includes resolution and noise.For sure, nV-DMM fulfill both requirements, but they may be used only, if their bias current is < 1pA.This is also not specified for those under consideration (34420, 182, 2182, etc), as low noise and low bias are always contradicting parameters.Newer, low noise Chopper OpAmps also have high bias on the order of 20pA and more, and therefore are not suitable at all. 2057: 30pA typ, up to 300pA!Only older ones like the LTC 1052 might do the job, but must be selected to below their typical bias, i.e. 1pA typ., 30pA max.Here again, lower bias means higher noise.Those old optical chopper null meters are the appropriate instruments for this job.FrankEdit: I just had a look into my recent tests on the 752A. From that I have to emphasize, that a proper shielding configuration, following the 752A manual addendum, is much more important for noise figures, and in turn for zeroing correctly, than having an ultra-low noise null-meter.

author: branadic
author_rank: Super Contributor
timestamp: May 23, 2022, 10:55:58 am
content: Fortunately, it was shown thatK155F845 can also work with modern (H11F1) component based optical chopper, no need for neons and LDRs or mechanical choppers.https://xdevs.com/fix/f845ab/#chopi-branadic-

author: syau
author_rank: Frequent Contributor
timestamp: May 24, 2022, 12:22:08 pm
content: Quote from: branadic on May 23, 2022, 10:55:58 amAny body have the schematic fir the H11F1 chopper ? Want to upgrade my 845.

author: alm
author_rank: Super Contributor
timestamp: May 24, 2022, 09:25:01 pm
content: Dave Wise's posts ans responses by othershere and furthermight help.

author: branadic
author_rank: Super Contributor
timestamp: May 25, 2022, 10:15:15 am
content: The idea that a Fluke 8588A can serve as a null detector might come from the marketing claim of the 1281:QuoteSource:https://xdevs.com/doc/Datron/1281/doc/1281_spex.pdfAs the Fluke meters (8508A, 8558A and 8588A) are kind of successors of the 1281 this claim might have sustained without further proove?-branadic-

author: 1audio
author_rank: Frequent Contributor
timestamp: May 26, 2022, 03:36:38 pm
content: I just saw thishttps://www.analog.com/media/en/technical-documentation/data-sheets/adgm1001-1002-1003.pdfMEMS spdt switch that could be in effect a mechanical chopper. Could this be the basis of an improved null meter? Its not cheap at $25 but its good for 100 million cycles. . . 3.5 Ohms on resistance.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 26, 2022, 04:06:21 pm
content: Quote from: 1audio on May 26, 2022, 03:36:38 pmThey seem to have some high resistance to ground (likely to allow capacitve actuation of the switches. This may be acceptable, but would need some extra care. Otherwise a leakage current in the 0.5 µA range is a big no go.I see not real problem in using one of the lower bias AZ OPs (e.g. maxa4238, ltc2055, AD8551, LMP2011, LTC1052, ... maybe up to AD8628) combined with a trim / compensation for the bias current. There is a limited need for a discrete build chopper and if so, I would consider CMOS switches or JFETs (e.g. 2N4117). The zero voltage check could be with a latching relay or mechanical switch if needed.The LDR choppers are not magic and the H11F1 photo fets are more like poor performance (quite some offset and still possible charge injection) by todays standard.

author: ttttrigg3r
author_rank: Contributor
timestamp: May 26, 2022, 05:35:40 pm
content: Thanks for doing that. I'm not that savvy with electrical circuits, but I'll analyze it further. I will take your recommendation and follow up with Fluke to see how they're doing it on their end with the zeroing method.

author: ttttrigg3r
author_rank: Contributor
timestamp: May 26, 2022, 05:40:15 pm
content: Quote from: RoadDog on May 22, 2022, 02:53:32 pmYou're saying Fluke uses the 845 null meter to zero the 752? In that case, we're doing exactly what they're doing...

author: wutieru
author_rank: Contributor
timestamp: May 27, 2022, 12:38:13 am
content: I think nullmeter can be replaced by KEYSIGHT 34420A or KEITHLEY 2182A nanovolt meter if using in Voltage measuerment.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 27, 2022, 07:19:52 am
content: Quote from: wutieru on May 27, 2022, 12:38:13 amIt depends on the application:In some cases with a low source impedance those low noise DMMs are a good solution, way better than the old Fluke 845 or similar. In this case other DMMs can also work as good or better than the old null-meters.In the case in question at the start of the thread (use with a Fluke745) those nV meter are however a poor choice, as they are not suitable for a high source impedance.The old Nullmeters like Fluke 845 are not especially low noise - noise wise a Keithley2000 or HP34401 are likely better. The point where the Fluke 845 is better than most DMMs is with the input bias and maybe isolation from mains (though battery operation would be the obvious better solution). They are more like inbetween a DMM and a electrometer: low bias in the <10 pA range, low current noise and a moderately low noise to resolve some 100 nV with enough time (e.g. 10 second average).The market seems to be too small (and maybe enough of the old ones still alive) to offer a new improved alternative, which would not be very difficult or expensive (BOM wise, but the small volume may add). The AVM2000 is a more modern try in that direction, though in my oppinion not a very good one.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 01, 2022, 07:52:10 am
content: I have published my findings on the 845AR vs. 3458A using them for the 752A.https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/msg4207087/#msg4207087Frank