Looking for infos/suggestions about accurate Low- and High-Ohm measurement

[quarks]

Checked RS prices, they are about 54 USD + tax and shipping for the Z201T.

If someone finds better prices, preferably the HZ series, I'd be very interested.

that seems a bit to high. I looked at mouser and it was about 20€ in Germany

[Conrad Hoffman]

My secret trick for low thermal emf wiring is a length of multi-conductor phone wire I salvaged from a junk pile somewhere. The stuff is solid unplated copper wire and has proved excellent for interconnecting voltage sources, 6 1/2 digit meters and KVDs.

Plated bananas, lugs and similar are a disaster. Even if you have to cut your own lugs from copper sheet you'll be better off. I've yet to have to resort to low emf cadmimum solder, but if you see solder joints marked with (I think) green paint in L&N equipment, those use the special solder.

Any place you have dissimilar metals that create thermal emfs, put the outgoing and return joints near each other so they track in temperature and cancel out.

[quarks]

Hello Conrad, 
thanks a lot for your reply. As stated in my initial post, you are also one who inspired me with your articles and knowledge to do all this interesting stuff.

So my gold to gold contact to be the best for no trouble is busted.
With your input I do even remember that I have read this before, on a very informative site (see link, unfortunately mainly in german, so anyone intersted here should look for translation)

http://www.amplifier.cd/Test_Equipment/other/other_Test_Equipment.htm

Also there was a suggestion like yours, to use telephone (massive) copper wire. But unfortunately I never tried it and even forgot about it.

About using pure copper I have in mind that oxidation is to be the worst for emf. How  do you take care of this?

Also all of my meters only have female banana plugs and therefore no post to screw the massive copper wire or a lug on. Can you suggest what to do on the meter site?

About the gold plating on the tellur copper posts (like the Pomona 3770 and IET BP-1000, I think these are also on my ESI resistors), what do you think about them? And would you still suggest to use pure copper instead of a gold plated lug or male banana with them?

Thx
quarks

[Thor-Arne]

Checked RS prices, they are about 54 USD + tax and shipping for the Z201T.

If someone finds better prices, preferably the HZ series, I'd be very interested.

that seems a bit to high. I looked at mouser and it was about 20€ in Germany

Yes, that's more like it. Thanks. 

[quarks]

with the input of Conrad, I did some testing around cables/connectors/adapters.

Please be aware this is not scientific at all and therefore maybe even useless, but here is what I did and found:

- I let my Valhalla 2701C (set to 100mV) and TEK DMM 4050 (set to 100mV DC, D-Filter, 100 PLC, High Input Z) warm up for > 3h
- I switched beetween different cable and connector types (see picture) several times
- I noted the readings every time after a settling time of > 30s to 60s
- after that I calculated the average of each cable set and the average of all cables and calculated the individual deviation compared to this average
- ambient temp was stable 20.4 °C

There are several uncertainties during the measurement readings, like drift of 2701C and DM4050 during the test, individual contact contamination of each connector, me sitting in front of it and so on.

And out of all of that, the max. calculated difference was 5µV (and this was between the BNC RG58 and the HCK 1m cables).
Is this reasonable or does any one have a suggestion to do it better?

thx
quarks

[Conrad Hoffman]

My voltage sources and KVDs have the usual tall banana jacks, as does my HP 3455, so using the wire directly is no problem. Even if you have to use bad banana plugs it should be no problem if they're at the same temperature- the thermal emf of each one will cancel out. It's usually the other end I have trouble with because that's the end I handle with my hot fingers. Everybody gold plates their tellurium copper parts so I have to believe it's no big deal. Copper oxide is supposed to be really bad, so I fresh strip my wires and or clean them by pulling through a piece of Scotchbrite. If you're going to work down at the PPM level, thermal emfs are just a way of life and you learn to identify them and either fix them or compensate by waiting for things to stabilize.

Did you know that long ago somebody made a power source by connecting eight zillion thermocouples together so you could put the assembly over a fire, stove or heat source?

[quarks]

Hello Conrad,

with the telephone cable, I think of using male bananas, where I can screw it directly in, instead of soldering it. 

About the gold plating, I still imagine it is best,  especially when both contact materials are gold.  At least one can be sure it is the best against corrosion.

With the Tellur Copper parts, have you any clue how Tellur can do anything for low emf?

About the thermocouple powersource, I have never heard of that.

Bye
quarks

[qno]

I came across this one:

http://www.edn.com/design/systems-design/4402904/Complete--low-cost--software-programmable-ohmmeter-measures-micro-ohms

[quarks]

thanks for the link qno, I will have a look at it.

[quarks]

I found an interesting article at a German university (unfortunately it is written in German)
http://www.uni-magdeburg.de/exph/messtechnik1/Parameter_Thermoelemente.pdf
In there is a nice table with "Seebeck-Koeffizienten" (see picture). As I read/understand it, Gold should not do any harm as it has the same coefficient as Copper (6.5 µV/K).
But Tellur has 500µV/K, in my understanding this should make Tellur Copper connectors bad for low EMF.

I hope some here knows more than I do and can either confirm what I think or can explain why Copper/Tellur connectors are still the best for low emf.

[BravoV]

Another source of great info and knowledge on high precision stuff is Leslie Green's book : Analog SEEKrets, and its free and you can get it here -> http://eevblog.com/files/seekPDF.pdf , this book is highly praised around here in this forum, personally I think its even better than Art of Electronic. 

Here a snippet to spark your interest 

[quarks]

wow 588 sites to read, now I am busy for quite some time

thx a lot BravoV, that's a nice present for Christmas

and merry Christmas to all
bye quarks

Edit: just a short update, I am impressed, this is really worth reading and sure is a great source to learn from!!!

[robrenz]

I hope some here knows more than I do and can either confirm what I think or can explain why Copper/Tellur connectors are still the best for low emf.

I don't know more than you do but here are my thoughts (not necessarily facts) on gold plated tellurium copper connections relative to thermal EMF.

Tellurium copper C145 is only used because of it's machinablility and mechanical properties, not because of its electrical properties. It's conductivity is only 93% of  IACS. It only has 0.5% tellurium so I don't think its Seebeck coefficient is going to be radically different than straight copper. The Seebeck coefficient of Copper and Gold only differ by 20µV at 100 degC relative to Platinum with a reference of 0 degC. I know It's not linear but that would be approximately 0.2µV/degC so pretty small. IMO Gold plated Tellurium copper has gotten a unjustified super low thermal EMF status that is totally unjustified. Gold plated C101 oxygen free copper would be a better material but not practical for the typical machining required for connectors.

The seebeck charts I have seen do not correlate the metals (Cu) with semiconductors (Te) so I am not sure the 500 for TE and the 6.5 for Cu are on the same scale

BUT thermal EMF's are generated by a thermal gradient in a material. The actual junction of the two dissimilar materials has nothing to do with the voltage generated other than, 1 establishing an electrical connection and, 2 having the two materials at the same temperature at the junction.
The gold plating is extremely thin (typ 1/4 to 1/2 micron) and should have an almost insignificant thermal gradient relative to the tellurium copper it is plated on.  In my mind this is no different than a isothermal block situation where secondary junctions cancel.  I think that in a thermally static situation any plating material will not generate a significant thermal EMF. Lets say this statement is blatantly wrong and it does generate a thermal EMF. Lets assume unplated copper wire leads that are perfectly deoxidized and swaged (not soldered) to the gold plated Tellurium copper Spade connector which is connected to a gold plated tellurium copper binding post which has a internal swaged (not soldered) unplated deoxidized copper wire connection.  We now have a Cu>Au>Cu>Au>Au>Cu>Au>Cu connection and it is its own isothermal block so all the Cu-Au thermocouples cancel each other. It still cancels if one is not gold plated Cu>Au>Cu>Au>Cu>Au>Cu.  Notice I stated swaged not soldered connections because the solder will be another thermocouple junction allthough these can also cancel if they are symmetrical and close to isothermal conditions with the main connection. 

Banana plugs that are gold plated are almost always brass that is gold plated and the actual spring contact is gold plated beryllium copper. This adds several more thermocouple junctions in the mix. and the banana spring connection is not a thermally efficient situation so we cant consider it a isothermal situation. So this is obviously inferior to spade lug connections.

IMO using short duration or single pulse offset compensated 4 wire ohms measurements on a thermally stable setup eliminates all these uncertainties.

[Conrad Hoffman]

Generally agree but I don't think the thickness enters into it. Nickel (or whatever) plated alligator clips are so bad as to be worthless, and they're probably a copper or brass underneath.

[quarks]

I don't know more than you do but here are my thoughts (not necessarily facts) on gold plated tellurium copper connections relative to thermal EMF.

Thanks a lot that you share your "thoughts". I really appreciate it and wish I would only have a little bit of your knowledge and skills. I already learned a lot from you, Conrad and others here. But I realize that I have to learn far more to get where I want to. I am still at the Low Ohms only, but really enjoyed the learning path.

Here are some more details I hope are helpful to go on with. 
(data is from various sources I already referred to and where possible double checked against diff. Application Notes)



If there are any errors, please let me know.

One thing I still hope to get deeper knowledge of, is how Resistance Transfer Standards work. Because I am desperate to find out about it, I will go ahead with Kelvin Double Bridge and Lead Compensation topics (and just ordered some Fluke and ESI gear). I hope this way I will learn a lot and If I am able to understand it, I should also get very good accuracy.

Bye
quarks

[robrenz]

There is no need to avoid bare copper to copper connections.  The Agilent 34420A nanovolt meter/micro ohm meter uses "almost pure copper connections" (translated Tellurium copper) in its connector. They recomend Caig Deoxit G100L on the  connector and even supply it with the meter. So use Deoxit G100L and if you have heavily oxidized copper just use a metal polish to clean the copper and then use the Deoxit.

[quarks]

here is a short update.

Today I received my long awaited kelvin ratio bridge and I just used my lunch brake for a first quick/dirty test. Attached is the setup. I know it looks very messy and not ideal at all, but I am happy with the very first result. I compared my Burster 1424 to my SR104, used the Valhalla 2701C as generator and the TEK DMM4050 in Ratio Mode (100PLC) as Nulldetector (thx again to robrenz for this great tip). The Deviation on the bridge shows exactly 0.01% compared to SR104 which happens to be exactly within the tolerance of the Burster. Now I still do not know the absolute exact value but I am confident that all my involved gear is within its own specs. When I have more time I will also do comparisons with my SR1010s.

[robrenz]

Wow some nice gear there and interesting test.

[quarks]

here is a short update of the comparison chart.

Now with Wavetek 1271 and the ESI 240C Kelvin Ratio Bridge (which is suppose to be 10ppm accurate in direct readout and has an amazing 0.1ppm resolution)

edit: error correction in chart

[robrenz]

Nice! This chart makes you realize why you can't find any .001% .001 ohm resistors. My LOM-510A is pretty good but I want a Burster

[PA4TIM]

To bad I missed this topic until now. Good to know I'm not alone in the quest for the holy grale of precision ( but sometimes it feels like chasing windmills)

I had the same troubles about what cables. I tried massive copper ( VD wire, you use for mains in the walls), coax, triax, shieded telephone wire. With normal bannas, goldplated ones, bnc-banana adapters, home made copper forks to screw under terminals ect,

Problem is you get different results but what is the right one ( the man with to many clocks story...)

I am very interested do some tests too when my lab is finnished. I sold a bunch of Racals that were in a 19 " rack. Now all my calibrators and KV deviders are in there and the Guildline standard cell cabinet is on top.

I have also the 1010 resistors and a GR decade. I can meaure G for high Ohms
I can measure using a GR1608, Boonton 63H, Prema and solartron 7,5 digit, Keithley 2000 and 196.
Also a HP sourcemeter that goes into the PetaOhm region but it is analog.
And the two KV deviders ( Fluke 720 and ESI decavider) but I have to figure out how to do this. There is a brief description in the Fluke manual as I recall.

Maybe there is a way to measure the one KV with the other using a calibration source and a null detector and so calculating some parmeters ?

Problems with seebeck I only experienced while calibrating, but if you wait long enough the contacts have the same temperature so no seebeck ( I assume, Roberts seems to tell the same if I understand it correct)
I had problems with dirt ( for very high Ohm) , pressure ( the grip of the kelvin clips, but even tiurning a banana a bit can give different reading in the mOhm or lower end) and current that heats up the resistor.

I found out that cleaning a 10 M resistor with ipa and steelwool for the legs made the difference between unstable and stable readings, but the writer of my Bible ( analog SEEKrets) seems to have other experiences. I measured leakage of my Fluke 720. I am very bad in remembering numbers but it was a factor 100 to much leakage. Putting the KV on 2 cleaned teflon blocks made leakage less, so the table conducted to much. i then cleaned the whole inside and outside with TRI ( at that time it looked like a good idea, and IPA after that) that improved leakage about 150X. Still not what the factory described but the best I could do. ( my 720 was total loss when I got it, had to make all swtich isolation myself, because all the studs and wiper adapters were broken in many peaces. Photos are on my site)

Now I stop, I can talk about this stuff for hours ;-)

Should i use some format for testing. You talk about a spreadsheet but I have to rest now so I'm typing this on my ipad and I can not open it right now, but should i use that ?

The Keithley book is for me too the source of this madness :-)

[quarks]

To bad I missed this topic until now. Good to know I'm not alone in the quest for the holy grale of precision ( but sometimes it feels like chasing windmills)

As we both already found out, we are chasing for ppm/precision. But you have so much great gear I feel like a beginner/hobbiest. Nevertheless I do have fun with it and today I received a Fluke 721A Lead Compensator (see picture) for my next test together with my 720A to see if Lead Compensation makes a noticable difference. But right now unfortunately I do not have enough spare time to do that very soon. BTW the KVD method is a quite easy setup.

With my actual gear I have good results in the range 1Ohm to about 10MOhm. Values below that and at the really High Ohms I have not yet touched. But I am looking forward to get there.
About the cables I am still experimenting (see my latest collection: Coax Aircell 5 and 7, special ESI and Fluke cables) and Conrad Hoffmann's secret tip with the twisted pair/telefon cable is really good, when you have binding posts on your meter. Triax cables come to my list, when I go >100MOhm. It would be nice I you could provide cable types and connectors you use.

I will post an update when I have results.

[quarks]

Nice! This chart makes you realize why you can't find any .001% .001 ohm resistors. My LOM-510A is pretty good but I want a Burster

luckily the chart only is showing the datasheet specs.
As I know from your posts, especially your LOM-510A is almost perfect accurate. To bad I cannot get one here in Germany/Europe.
Also my TEK DMM4050 seems to be far more accurate than it should be. According to my Wavetek 1271 and 10k Standard Resistor SR104, it seems to be around 10x better than the specs. 

[quarks]

Just in case there is still some interest, I finalized my tests/research with good repeatable results in the ranges I was most interested in (from mOhm to GOhm)

[acbern]

am interested in this, missed it originally unfortunatelly.
what were your findings here? what I am specifically interested in is:

-what accuracies did you achieve with the 720A, and how does that compare to the 3458A being used for voltage divider measurements?
-did the 721 lead compensator turn out to be required? the 720 manual says beyond 100 ohms total chain resistance it should (not shall/must) be used, but it is not shown in the schematics part.
My error propagation calculations show that between 10 ohms and 1megohm, the 3458a can be used reasonably well, with only a few ppm of accuracy error added, beyond, the 720a needs to be used.