Author Topic: Null meters still in production? How to replace a Keithley 155 null meter?  (Read 5141 times)

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Offline ttttrigg3rTopic starter

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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
 

Offline bdunham7

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Fluke says their 8588A DMM does not have capability to be a null meter.

You'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.
« Last Edit: May 18, 2022, 06:09:05 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline HighVoltage

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Look for TEGAM AVM-2000 Null Meter



There are 3 kinds of people in this world, those who can count and those who can not.
 
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Offline TimFox

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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.
« Last Edit: May 18, 2022, 07:45:55 pm by TimFox »
 

Offline Conrad Hoffman

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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.
 
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Offline Dr. Frank

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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 it
Frank
PS I herewith promise to finally publish my findings on the 752A and the Null problem soon.
« Last Edit: May 18, 2022, 08:37:09 pm by Dr. Frank »
 
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Offline bdunham7

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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.

The 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.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline 2N3055

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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.

You get a front cover that goes over input terminals . Screws for cover are grounded. There is a system grounding point on the back...
 

Offline TimFox

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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.

You get a front cover that goes over input terminals . Screws for cover are grounded. There is a system grounding point on the back...

Yes, 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.
 

Offline 2N3055

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I also understand the difference between guard and ground.

I never said otherwise.. I do too. What did I say wrong?
I agree front is better place for the post.
« Last Edit: May 18, 2022, 10:35:08 pm by 2N3055 »
 

Offline TimFox

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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.
 
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Offline bdunham7

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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.

I 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.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline manganin

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We use Keithley 155 which is still working...for now.

...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.

 

Offline Conrad Hoffman

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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.
 

Offline HighVoltage

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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.

I removed all the old nasty batteries and went with 9V block batteries
Even since October 2016, they are still working well.

https://www.eevblog.com/forum/repair/keithley-155-null-detector-repair-and-restauration-attempt/


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Offline ttttrigg3rTopic starter

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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.
 

Offline ttttrigg3rTopic starter

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Thank you. I look forward to your publication. How do I subscribe?
 

Offline ttttrigg3rTopic starter

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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.
 

Offline alm

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There used to be the Datron/Wavetek 4902S, but I don't think it's still available. Maybe look at the Measurement International offerings?

Offline bdunham7

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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.

The 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..... :scared:

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 1012 ohms 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!
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline Kleinstein

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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.
 

Offline martinr33

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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).
 
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Offline bdunham7

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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.

So 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 from thevoltagestandard.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.

« Last Edit: May 22, 2022, 01:43:03 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline Echo88

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@ 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
 

Offline RoadDog

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The 752A is a current product, it would be amusing if they had some decades-old null meter doing this job during production.

They use the 845 for the 752 and 720.
“Every machine is a smoke machine if you operate it wrong enough.” ~ Ben Franklin (maybe)
 
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