T.C. measurements on precision resistors

[TiN]

VPG on their case study paper state it's VHA518-7, with having Dipl.-Ing Guido Weckwerth, CEO, wekomm engineering GmbH as author

[Dr. Frank]

VPG on their case study paper state it's VHA518-7, with having Dipl.-Ing Guido Weckwerth, CEO, wekomm engineering GmbH as author

If original VHA518 would have these stability figures (in practice, not on paper), then there would not have been any reason for Vishay to get involved with Wekomm (or vice versa).
If you read the study more carefully, these VHA518 were the starting point / basis of the WEKOMM standard only.

Guido Weckwerth really told me on phone, that he made many special improvements together with Vishay on their MBF technology to overcome the known drawbacks.
Our German standards institute - PTB - already acquired several WEKOMM standards, as these are presumably more stable than the usual ones. On a 10k resistor @ 23.0°C, short and mid term stabilities on the order of 1E-8 were demonstrated (versus QHE standard).

Especially 1A shunts (1 Ohm , 0.1Ohm) showed much better retrace behaviour after power burden, on the order of 1ppm, which was not possible with former standards, or required much higher effort.

Frank

[ltz2000]

PTFE insulation is very important for low leakage

Actually no, because the insulation resistance of the binding posts dominates. And there is no real need for the the on board PTFE insulators either. The printed circuit board could have been simply divided into two pieces and mounted directly to the binding posts. Air is an excellent insulator. And that way you also get rid of the wire inductance and a number metal-metal connections (source of thermal EMF).

I found my notes of that talk, so silver coated wire should be better than pure copper because of thermal voltages.

Copper-Copper <0.3 µV/°C, Copper-Silver 0.5 µV/°C. But most important is to minimize the number of joints and make them thermally equal.

I think in this case using exotic materials has been more important than thinking the whole picture and doing the math.

[Dr. Frank]

Actually no, because the insulation resistance of the binding posts dominates. And there is no real need for the the on board PTFE insulators either. The printed circuit board could have been simply divided into two pieces and mounted directly to the binding posts. Air is an excellent insulator. And that way you also get rid of the wire inductance and a number metal-metal connections (source of thermal EMF).


Copper-Copper <0.3 µV/°C, Copper-Silver 0.5 µV/°C. But most important is to minimize the number of joints and make them thermally equal.

I think in this case using exotic materials has been more important than thinking the whole picture and doing the math.

You are right, but these binding posts are also highly insulating, and extremely expensive, as Guido told me.

If you chose a solution with cables which might touch each other, you must use PTFE, or maybe Kapton which is also highly isolating, but may have better dielectric behaviour.
In my lab, I use relatively cheap PVC mantled cables, which have a resistance of 1E10..1E11 "only". This already creates 1ppm error on 10k, if you drill plus and minus cables to reduce noise pickup.

A yes, one important feature of these cables was the thermal resistance between the resistor element and the binding posts.
That also balances the temperature distribution.
This is important, if you use these devices as working standards, instead of leaving them for days in an oven.

Also don't forget, that Dave has got a prototype, only WEKOMM knows how the devices were built nowadays.


Wire inductance and capacitance may be another aspect, but these are generally so low that they do not play an important role during measurements of the resistor. The time constants involved with these parasitics usually are mostly in the µs range.
AC resistance bridges usually are also capable to measure the ohmic part only.

There are other, more disturbing effects, similar category as L,C parasitic, which I will describe later on.. I'm still collecting information & measurements.

Copper easily oxidizes, giving many 100µV/K thermo offset instead.
The junctions were crimped, not soldered, so this problem applies.
Don't know, how silver coating and its oxides behave, maybe better.

But anyhow, if the whole assembly is well thermally stabilized, these offsets vanish, and will also be eliminated by Offset Compensation techniques.

Frank 

[TiN]

Mounting directly at binding posts would also introduce all mechanical stresses directly to resistor as well. That is what they seem trying to avoid with all that springy crimping and spongy holder.
And they make different value resistors, up to 100Meg in same case, so perhaps PTFE wire is due to that (save cost and unify BOM for all resistors).

Silver surface in air form sulfide layer as well, by turning black, but slower than copper oxidation if I remember correctly.

[ltz2000]

You are right, but these binding posts are also highly insulating, and extremely expensive, as Guido told me.

Also don't forget, that Dave has got a prototype, only WEKOMM knows how the devices were built nowadays.

I'm still collecting information & measurements.

Sorry to say, but when we talk about this specific product you sound more like a business partner rather than the scientist familiar to all of us.

So far this is just a $50 resistor in a box with a $5000 price tag. No data of any kind, just sellers vague promises of the exceptional performance. Somehow reminds me of the high-end audio business.

BMF resistors have been used as transfer standards in primary metrology for decades. Especially in AC/DC metrology because of their excellent AC behaviour compared with wire wound resistors. The 10^-8 short term stability mentioned is achievable using off the self BMF resistors without any special treatment or black magic.

My point is that at the moment I see nothing to justify the astronomical price tag. The metrology business is very conservative and for a reason. The equipment are expensive not because of the material cost, but because someone has done the hard, expensive and time consuming work for you. The scientific proof of performance is what you pay for, not some technology or fancy look. From that perspective I think that Wekomm with their marketing first approach has started from the wrong end.

[Theboel]

I really don't know if the resistor cost only 50 USD but I will not judge it before put it side by side to some proven standard,
I understand they have calibrated with with a very good standard but the main concern are reliability how long they will kept it.

[Dr. Frank]

Sorry to say, but when we talk about this specific product you sound more like a business partner rather than the scientist familiar to all of us.

So far this is just a $50 resistor in a box with a $5000 price tag. No data of any kind, just sellers vague promises of the exceptional performance. Somehow reminds me of the high-end audio business.

BMF resistors have been used as transfer standards in primary metrology for decades. Especially in AC/DC metrology because of their excellent AC behaviour compared with wire wound resistors. The 10^-8 short term stability mentioned is achievable using off the self BMF resistors without any special treatment or black magic.

My point is that at the moment I see nothing to justify the astronomical price tag. The metrology business is very conservative and for a reason. The equipment are expensive not because of the material cost, but because someone has done the hard, expensive and time consuming work for you. The scientific proof of performance is what you pay for, not some technology or fancy look. From that perspective I think that Wekomm with their marketing first approach has started from the wrong end.

Your statement is very insulting!
You even impute to me that I have a business interest towards WEKOMM.

I don't tolerate that at all, and expect your apologies!


In the beginning, I have also been very sceptical about their claims.
At this instance though, I'm only reproducing information, I got from WEKOMM directly, and these sounded very reasonable and serious to me, also the document from PTB.
This in fact made me think different.

You obviously make some wrong assumptions about WEKOMM and their resistors, even so imputing dubious business behaviour to them.
Instead of also bringing them in discredit w/o good reasons, I propose that you contact Guido Weckwerth directly for details.


I also think, that your claims are wrong, that it is really that easy to create and even measure 1E-8 stability, short term and especially also mid term, i.e. over several days,  on 'analogue' resistors (in contrast to cryogenic QHE ones).
You also have to accept, that 1E-8.. 1E-9 is on the absolute edge of precision measurements when comparing to a QHE standard, due to thermal voltages, noise , and so on.

Standard MBF resistors have many drawbacks, as we as 'amateurs' already discussed in this thread, like e.g. hysteresis effects, which normally limit their stability figures to 1e-6 .. 1e-7.

Please give a valid reference of the MBF standard resistor you mentioned, which is assumed to have 1E-8 stability, on what scale ever.

Maybe some very good SR104 or similar PWW based standards may have this capability, on the order of < 1e-7 maybe, but these are also extremely costly, although their obvious BOM cost may be a fraction of that only.


Frank

[engiadina]

@Itz2000:
No claims. Just facts.

I attached some documents from our german National Standard Institute PTB. They qualify our resistors as primary transfer standards.


@Theobel:
Long term drift if measured by comparing a resistor directly against the Cryogenic Hall Standard for some days. That tells us the drift with a resolution of 10^-10
Dr. Schumacher from PTB developed main parts of the Cryogenic Hall Standard and some mathematical methods for deriving some quite reliable long term drift from these measurements. We specify a max yearly drift of 1ppm, but up to now all resistors were way better than that.

We did not yet reach the good behaviour of an old SR104, they are extremely good. But we are working on that.

At the time being our resistor is better than most of those of our competitors.
Oh, and for example try to load a Fluke 421A-1? with 1A (makes 1Watt power) and do the same with our resistor. Ours drifts max 10ppm and comes back to it's original value better than 10^-8. That lead to the construction of a second resistor type, being capable of heavy loads. That one drifts about 0.8ppm at this load.

Guido

[engiadina]

Sorry, PDF files are big ....

[engiadina]

Second Cal Certificate ...

[engiadina]

And this is the "high load" resistor, being tortured in the PTB lab ....

[ltz2000]

I don't tolerate that at all, and expect your apologies!

Sorry, no intention to be impolite. Not at all.

You obviously make some wrong assumptions about WEKOMM and their resistors, even so imputing dubious business behaviour to them. Instead of also bringing them in discredit w/o good reasons, I propose that you contact Guido Weckwerth directly for details.

They voluntarily came out at the very early stage by sending a free sample unit to Dave. Obviously the idea was to get free publicity for their product among the possible customers. You must admit that the claims were quite exceptional, so it was not a surprise that they were not accepted straight away just because the manufacturer says so. From the publicity management point of view it would have been a good idea to be prepared to show at least some proof for their claims.

Please give a valid reference of the MBF standard resistor you mentioned, which is assumed to have 1E-8 stability, on what scale ever.

Almost any type will be work. The main benefit of the BMF resistor is the small size, which allows it to be temperature stabilized extremely accurately. A common solution is an oil filled solid machined aluminium block. The block can be made large compared with the size of the resistor cavity providing excellent temperature uniformity. The small size also makes the insulation and heater arrangements much much easier (compare with the LTZ1000!). But because of the small size you also need to be very carefull with the power coefficient, no free lunch.

In case of the Wekomm resistor the size benefit is lost. It is a large box mostly empty space inside, which brings in the air convection problems. The only way to temperature stabilize is an air bath providing quite limited performance. 10^-8 is not practical.

Maybe some very good SR104 or similar PWW based standards may have this capability, on the order of < 1e-7 maybe, but these are also extremely costly, although their obvious BOM cost may be a fraction of that only.

A new SR104 costs about the same as the Wekomm. A second-hand unit much less.

Again the limiting factor of the short term stability with the SR104 is the temperature stabilization arrangements. It can and has been modified for oil bath though.

[splin]

@Itz2000:
No claims. Just facts.

I attached some documents from our german National Standard Institute PTB. They qualify our resistors as primary transfer standards.

Perhaps I'm missing something but as far as I can tell, all those documents tell us is the resistor values as measured by the PTB  at 23C +/- 20mC which is of no interest whatsoever. Might as well have provided a certificate telling us what colour it is to within +/-10^-8 nm. There is nothing about stability with temperature or time which is what is interesting. What is your point?

@Theobel:
Long term drift if measured by comparing a resistor directly against the Cryogenic Hall Standard for some days. That tells us the drift with a resolution of 10^-10
Dr. Schumacher from PTB developed main parts of the Cryogenic Hall Standard and some mathematical methods for deriving some quite reliable long term drift from these measurements. We specify a max yearly drift of 1ppm, but up to now all resistors were way better than that.

We did not yet reach the good behaviour of an old SR104, they are extremely good. But we are working on that.

I'm sure that your products are very good quality, but where is the evidence that it performs any better than an off the shelf Vishay hermetic foil resistor which is specified to drift less than 2ppm in 6 years, or .3ppm/year for <100 euros?

At the time being our resistor is better than most of those of our competitors.
Oh, and for example try to load a Fluke 421A-1? with 1A (makes 1Watt power) and do the same with our resistor. Ours drifts max 10ppm and comes back to it's original value better than 10^-8. That lead to the construction of a second resistor type, being capable of heavy loads. That one drifts about 0.8ppm at this load.

Guido

For 3700 euros I reckon I could put a lot of Vishay foil resistors in parallel and get much, much less than 10ppm shift under load.

[ltz2000]

No claims. Just facts. I attached some documents from our german National Standard Institute PTB.

Still no data of the stability, which is probably the most important parameter of a standard resistor.

They qualify our resistors as primary transfer standards.

?

EDIT: splin was faster...

[engiadina]

... off the shelf Vishay hermetic foil resistor which is specified to drift less than 2ppm in 6 years, or .3ppm/year for <100 euros?

Have you ever checked that

Why then would Fluke sell their 742A resistors if those off the shelf resistors are way better in all respect ?

[engiadina]

A new SR104 costs about the same as the Wekomm.

Yes ... and is specified with a drift of 2ppm/year. Have you ever bought one the last five years and verified that drift ?


I respect, that most of you are extremely critical, especially towards a new manufacturer.

Well, in public we are rather new in that business. In fact we are working on that subject for more than three years now. And I know very well that the metrology business is a business of trust. Not claims or buzz marketing.

Do you really think you could start a business in this field just by claiming?
How many resistors were you going to sell before the first customer starts complaining? Those guys from cal labs meet regulary with the PTB guys and talk. I know that two month ago many cal lab engineers met at METAS for a special measuement course. They talked as well.

Actually I believe just ONE bad product sold will kick you out of business in metrology if you are in a newcomer position like us.

[ltz2000]

Why then would Fluke sell their 421A resistors if those off the shelf resistors are way better in all respect ?

Because they can and customers buy.

The Fluke 742A is far from perfect. Especially the long term stability is poor. But on the other other hand, there is no remarkable hysteresis, the temparature coefficient is very low (matched resistors of opposite tempco) and the power coefficient is acceptable.

[ManateeMafia]

The IET SR-104 datasheet is here ...

http://www.ietlabs.com/pdf/Manuals/SR102-104_im.pdf

Drift rate 1ppm first yr with 0.5ppm thereafter.

[HighVoltage]

And this is the "high load" resistor, being tortured in the PTB lab ....

Do you also have some pictures of the PTB lab equipment, measuring this reference resistor?
What equipment are they using?

[ltz2000]

A new SR104 costs about the same as the Wekomm.

Yes ... and is specified with a drift of 2ppm/year. Have you ever bought one the last five years and verified that drift ?

The specification is 0.5 ppm/year and it is very conservative. The actual drifts of the four SR104s in the lab I worked for were between 0.06 and 0.08 ppm/year.

Do you really think you could start a business in this field just by claiming?

No, absolutely not. But based on the limited and possibly inaccurate information I have got from this thread, that is what you are doing.

[splin]

... off the shelf Vishay hermetic foil resistor which is specified to drift less than 2ppm in 6 years, or .3ppm/year for <100 euros?

Have you ever checked that

Ooh now your teasing! No I haven't but it would hardly be unusual to find out that Vishay, once again, are being economical with the truth. I'm sure we're all on the edge of our seats waiting for you to tell us some of your findings if it's possible?

Dr Frank was certainly convinced that his own 5 VHP202Zs were drifting much less than Vishay's long term stability spec and I have no reason not to believe him. https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/360/

On the other hand, rather bizarrely, Vishay actually publish this http://www.vishaypg.com/docs/63620/63620.pdf showing a 6K45 VHA518-11 drifting 11ppm over 5.5 years, directly contradicting their 2ppm/6 year claim.

Even the 12k9 in that test didn't meet the spec at just over 2ppm in 5.5 years - not far off but the graph shows the drift accelerating over time to .58ppm/year after 2000 days (3.5ppm in 6 years). They even have the cheek to use a truncated version of that 12K9 graph in the H-series datasheet - truncated no doubt because the slope of the drift at 1000 days is only .27ppm/year (1.62ppm in 6 years), or less than half that at 2000 days.

So perhaps Dr Frank got lucky and the 2ppm/6 year claim is actually worthless - if Vishay had any better test results than the above I've no doubt they would have used them in the datasheet rather than one showing that it didn't meet spec!

[engiadina]

And this is the "high load" resistor, being tortured in the PTB lab ....

Do you also have some pictures of the PTB lab equipment, measuring this reference resistor?
What equipment are they using?

Sure ... basically the PTB are using a Fluke Transconductance Amplifier controlled by a Wavetek calibrator to generate the power. Then they have a range extender from MI (that blue box you can spot) to measure the current via a reference resistor (in the silver box). That is compared to the current through the bigger resistor.
The smaller 1Ohm resistor is used as a short to keep the current flow constant, when the bigger resistor is not loaded. That just helps stabilize the transconductance amplifier.

[Theboel]

I Thought the 2 ppm / 6 years are the claim for shelf life and for load life are 20 ppm / 2000 hours at rated power ?
http://www.vishaypg.com/docs/63120/hzseries.pdf  (Pages 3)

so the claim from vishay and the information from DR Frank is Valid for me.

Honestly I don't interested in this area of discussion but I think its far better for WEEKOM to ask PTB to do a test like this to support their claim. 
http://www.vishaypg.com/docs/63620/63620.pdf

[ltz2000]

Off-topic, but the first Transmille calibrator that I have seen in a primary lab.

The details are not visible but could be
3010 Calibrator and
EA3012 Transconductance Amplifier