Moin Frank!
On Thu, 21 Jul 2022 18:58:08 +0000
[email protected] wrote:
> first off, as you obviously descend from Northern Germany,
Actually.. I'm not :-P
> and are currently located @ MPI Saarbrücken, I propose that you
> afterwards send me your 10kOhm standard for characterization, in
> my cool lab near Frankfurt. Out there it was up to 38°C,
> downstairs, only stable 22.4°C.
Thanks a lot for the offer. I might take you up on this.
But I don't think it's a temperature issue. The 10kΩ resistor
takes ages to settle. I've been using APER 1, DELAY 1,
AZERO ON, OCOMP ON and varied DELAY from 1 to 10. Between DELAY 1
and DELAY 10 is almost 10ppm of difference. To get below 0.1ppm
I would need a DELAY value in the order of 20 to 30. While that
works for me, it's bascially a no-go if I want to get the
resistor calibrated. The two other resistors do not show such
a settling. Repsectively, if they do, it's below the uncertainties
of my measurement setup. Even using NPLC 50 with DELAY 0.5 instead
of APER 1 does not appreciably change the measured value.
> I have read the whole thread.. sorry that I respond very late as I
> was locked out due to a wrong address.
No worries. I'm greatful for all help!
> There are several pitfalls in your setup and with the basic
> principles of Offset Compensation methods in general for 10k and
> 100k Ohm range resistance measurements.\
> There are a few guys who already have explained some of these
> effects on the eevblog forum, I cite two threads here:
I'm aware of these issues and read quite a bit of the discussions
on the eevblog forums. I also tried to quantify the influence of
the differnt switches.
> Here are several topics I’d like to explain briefly.
>
> 1. emf voltages and cables\
> The Offset Compensation method (of 3458A or other long range DMMs) is
> intended to remove all emfs. \
> Therefore, it’s not necessary at all to use low emf cables, jacks, aor
> resistors. Manganine has higher emfs compared to others, so no problem.\
> The discussion here is misleading concerning the root cause of your noise
> and instability problems
Yes. I switched to AWG28 PTFE wires and saw no effect whatsoever.
So, neither the EMF nor the dielectric absorption was an issue.
> 2. PVC cables have too low an isolation resistance. For 10kOhm, these
> introduce errors on the order of ppms when you drill plus and minus, but
> 10ppms when measuring 100kOhm.
Intersting, I haven't thought about that. Though, as noted above,
the use of PTFE insulated wires didn't make a difference for me.
>
> 3. The 3458A, as well as other DMMs are quite susceptible to external noise
> in your lab. \
> Therefore, you MUST use drilled 4W cables, which implies using PTFE.\
> These MUST have a shield around, which is connected to GUARD of the DMM
> (not grounded), and best be connected to the case of your standard resistor.
While the room I'm in is not the quietest, the noise level I see
in my measurements are compatible with what the HP3458 datasheet
says. So I don't think I have an issue there. And on top of that,
I'm averaging over minutes to hours for each "reading". So most
of that noise should average out below the other uncertainties.
> 5. You need to do your measurements with the 3458A at a very
> constant room temperature. My basement lab is constant to +/-
> 0.2°C over a week or so. Therefore I can perform DCV measurements
> stable to 0.1ppm over 24h before I have to redo an ACAL. I did not
> yet try such Ohm stability measurements, but this mode is
> definitely much less stable.
Yes, that's one of the big problems I have. My "lab" is a spare
office nobody is using at the moment, in an 1990s building with
lots of glass and open spaces but no air conditioning. On top of
that I have multiple instruments running, using a total of probably
1kW in a small, unventilated room. That's why I have such high
temperatures. There is no basement room that I could use.
But I'm actually glad that I got even a separate room and could
cordon it off. I'm at an institute of theoretical computer science
after all. I'm one of the few people there, for whom a computer is
not just a black box on a piece of paper. Heck, it took me months
to get it into the heads of people that nobody should enter that room
not even the cleaning lady, because I can see that in my measurements.
To combat the temperature issue a bit (and for a handful other
reasons) I run a full ACAL every 3h. I see quite a bit of temperature
coefficient on the relevant values for resistance measurement, but
everything within what I would expect.
> 6. The OCOMP switches the measuring current on and off, as
> explained in those two threads. This leads to polarization
> currents, or dielectric absorption, every time when when the
> current is switched. That leads to a lowering of the measurement
> by at least 1ppm for 10k and 5ppm for 100k, depending on the
> cables / isolation used.
Even OCOMP off and AZERO on lead to the same effect here. And
not just 1ppm, but >10ppm for the 10kΩ. Independent of the cables
used.
> Therefore, a delay of 1 sec for 10k and
> 5sec for 100k is necessary. This has to be tested. Increase the
> delay from zero, until the measured value does not increase any
> more.\
> 1k and lower, as well as 1M and higher should be made with
> small delay of 30msec, as no such effect arises. These are
> universal parameters, also valid for all other DMMs with a similar
> OCOMP function.\
> The optimal setting of the 3458A can be found in the links. It’s
> basically APER 1, DELAY 1 in your case.\
> usage of NPLC 50, instead is counter productive, as this only increases
> the measurement time unnecessarily.
At least for the 1kΩ and the 100Ω resistors, the difference between
NPLC 50, DELAY 0.5 and APER 1, DELAY 1 is below measurement
uncertainty with multiple hours of averaging. The additional time
needed for NPLC 50 with DELAY 0.5 (about 30%) is well compensated
by the 6dB lower noise. For the 10kΩ resistor.. well..
> 7. use statistics, limited to 16 samples for one data point, see links.\
> The mean value will meet the transfer uncertainty of 0.1ppm and stability
> \~ Standard Deviation of 0.2ppm.\
I do apply statistics, not in the 3458, but outside. This
way I can more accurately evaluate things and compensate for
thermal effects and other things that would otherwise get hiden
in the statistics.
> If you use much more samples, medium term instabilities kick
> in, which have nothing to do with your test problem.\
> These are temperature and timely drifts of your DMM and of your
> DUT. Think of the Allan Deviation statistics, what it is about.
Yes, I'm well aware of these.
> 8. Characterize your 3458A for Time and T.C. stability of its
> resistance mode.\
> The 40k resistor of my 3458A has < 0.3ppm/K T.C., and is stable
> to a few ppm over several years. I already have a VHP101 inside,
> maybe your old instrument is not as stable.\
This I would like to do, but unfortunately, I lack the equipment
to do so. I would need to be able to put the 3458 into a temperature
chamber, while keeping a resistor standard in a different camber.
This is equiment I can't get here, where there is no budget for
anything beyond pen and paper and glorified typewriters.
> Anyhow, the 3458A is no reference standard, therefore, you
> should make long term stability measurements in relation to
> another external reference (Volt, Ohm)
That's how I started. I wanted to do some precision measurements,
for which I needed some standards. So I got some cheap resistance
standards off ebay. To see whether they are not some duds and drift
too much themselves, I wanted to use my 3458, that I know has been
treated well and shouldn't drift too much, as "the standard" and
see whether the resistance standards are at least within what's
plausible.
Well, the results so far are, that the 1kΩ and the 100Ω are
probably ok and the 10kΩ has some weird effect that I can't
pin down.
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
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