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

[quarks]

Hello all,

probably as most of us, here in the forum, I am very interested in metrology (especially accuracy and calibration know how).

Therefore I studied the Keithley Low Level Measurement Handbook and read a lot of the very interesting posts and articles from Conrad Hoffmann (also member of this forum) about metrology, KVDs, Hamon Divider and bridges.

Inspired by robrenz (and with his permission) I used his low resistance spreadsheet, expanded it, added some of my better gear and The Reference Meter Fluke 8508A.

I was quite surprised to see, how my meters compare and that even my good 6.5 digit (Tek DMM 4050 and Gossen 30M) meters are quite bad (below 1 Ohm and above 1 MOhm).

To be able to get better results, the best seems to be working with Bridges and resistance references. But unfortunately I do not have a Null-Meter. Is there anything newer/better to use (maybe like the Null-Detector for DVMs from Conrad Hoffman)? And has anyone good readings about how resistance transfer standards work?

Another way could be adding a Micro-Ohmmeter (in the range 0.01% to 0.1% down to 1 mOhm ) and for high Ohms an Electrometer.
But so far I have not found a nice meter like robrenz's LOM-510A in Germany (or at least Europe). If you have suggestions, what could be worth to have a closer look at, please let me know.

My almost perfect all in one dream could be the Fluke 8508A, but besides the expected price tag, I was not even able to find a source for it. If there is one (or maybe a Datron/Wavetek 1281 / Fluke 1281A) please let me know.

thx
quarks

Edit: refined the chart and added the Burster 2304 Resitomat, which so far is the best Micro-Ohm Meter I could find
Edit: finally got the Fluke Reference Multimeter all in one solution, perfect for mOhm to GOhm

[robrenz]

Look at the Ratio mode on you tek4050 it should be the same as my 8846A. That will funtion as a null meter at 1ppm resolution except it will read 1.000000 when nulled instead of zero.

The % error on the chart is % of value.

[quarks]

That's a good hint. Have you used this function, is it usefull and with stable reading? When I read the German manual I did not understand how to use it. It seems to be badly translated, so I will check the english version and give it a try.
Thx

[robrenz]

It is the ratio of the input voltage which is input on the left hand "INPUT" terminals divided by the reference voltage which is input on the right hand "SENSE" terminals. You also need to short the commons together at the meter and have analog filter off. I assume 100 NPLC will give more accurate and stable readings.

[quarks]

It is the ratio of the input voltage which is input on the left hand "INPUT" terminals divided by the reference voltage which is input on the right hand "SENSE" terminals. You also need to short the commons together at the meter and have analog filter off. I assume 100 NPLC will give more accurate and stable readings.

Many thanks, I really overlooked and never used this indeed very usefull function, because of the bad translation in my German manual. Now I tested it and checked the Self Calibration procedure on my Fluke 720A and I am very pleased that all steps appear to be just perfect.

PS: I edited and refined the chart and added the Burster 2304 Resitomat, which so far is the best Micro-Ohm Meter I could find

[quarks]

Too bad nobody (except robrenz) had a suggestions for me so far.

I kept on testing the suggested ratio method with my KVD 720A ("2-39. Measuring Unknown Resistance" as described in the manual). For best accuracy it is suggested to use a Reference Resistor in the same order of magnitude, low emf leads and a Lead Compensator (the last two I do not have, therefore I used just ordinary unshielded test leads (but at least good gold plated banana with 2.5mm² cable)).

For this test I used my Burster Decade (0.01% and TK<1ppm/°C) as reference (Rstd) and a 1k resistor (as unknown Rx, because I wanted to be able to go up and down in value with Rstd) and dialed in 10/100/1k/10k and 100k Ohm. The calculated result were then 994.73/995.61/995.73/995.65/995.69 Ohm.

Even I did not have an ideal setup, the results seem to be quite accurate. I wonder if anyone has experience with this kind of measurement and likes to share it. If so, please let us know what you think. Is it worth keep on going this way? What improvement can be expected with low emf leads (maybe POMONA shielded cable with Tellurium/Copper gold plated bananas) and a Lead Compensator?

thx
quarks

[lewis]

I should be demoing one of these over Christmas: http://www.home.agilent.com/en/pd-1000001296%3Aepsg%3Apro-pn-34420A/micro-ohm-meter?&cc=GB&lc=eng

I'll let you know how I get on.

[robrenz]

That was originaly on my chart.  @ quarks, did you remove it on purpose?   Very nice specs

[quarks]

Hello lewis,
thanks for the reply. Please let us know, when you got it.

That was originaly on my chart.  @ quarks, did you remove it on purpose?   Very nice specs

Hello robrenz,
I have not removed it from the sheet, but when I expanded your low resistance sheet up to a wider range (from 1µ to 200G Ohm) I did not update the specs of the 34420A above the 200 Ohm your data was, so it looked funny for me not to have the rest of the data in the chart. I now have included it again with your original data. All other graphs show the whole range from every meter (except the two that cross the y axis, they go further down, see screenshot). So where the individual line starts it is the max. resolution (least significant digit) and where it ends it is the max. of its range. 

Right now I have 15 meters in the sheet. Only for the Burster 2303 I have not found a datasheet with the needed specs. If anyone has this, it would be great to have a copy. If there is another meter worth having a closer look if it performs even better than the ones I already checked, please let me know.

regards
quarks

edit: during my lunch break I edited the sheet, now all graphs go to the max. range

[quarks]

I do not know, if anyone else is interested, but while experimenting with resistors in the range from 100mOhm - 1mOhm (see attached picture), I found out that my Hameg 8118 seems to have missleading accuracy information in the documentation (see attached picture).
Instead of the claimed 10µOhm to 100MOhm, it goes down to 1µOhm and up to 1GOhm. I wonder if any one else has this meter and this could be Firmware related (my Version is 1.31 and the latest is 1.52). Any feedback would be helpfull.

Actually I do not know why this is, but this meter is measuring 1mOhm with resolution of 1µOhm quite well (last digtit is not very stable, but I repeated the measuremet many times, because I could not believe it).
Here are my findings (see picture), with meter set to SLOW, average 20 and auto. Before every measurement I shorted the Kelvin leads for zero adjustment.
Accuracy seems to be at least in the range of the test resistors (0.1% to 0.5%).  So I am really surprised, because I was prepared to see errors in the range of  100% for 1mOhm and 10% for 10mOhm. .

[robrenz]

I am interested but I dont have any input on this question

[quarks]

Because of the promissing results, I did measure a few more 100mOhm Resistors (Dale LVR-10 see picture) to select a possible candidate for a DIY Reference Resistors project. And I achieved a very stable 100.00 mOhm reading. What ever the real value might be, this looks really good. 

[Thor-Arne]

I've been thinking about a resistor reference to, something like the one Dave made.

I was thinking on using a couple of  Vishay Z-Foil resistors (Z201T and Z201T),
RS stock# 763-7309 and 763-7303.
These are 0.005% tolerance and 2 ppm/C.

The plan was to solder these directly to a gold plated banana jack, perhaps something that fits directly into the DMM ports.

Still thinking tho', these resistors is expensive.

[SeanB]

Do not solder direct, rather use a set of standoffs and solder the resistor to that ( so that leads have no strain)  or make a small PCB to hold the resistor, then from the pads make a 4 wire connection to have the sense and force wiring separated as close to the resistor as possible. Then short thick wires to the 4 panel mounted sockets, or another 2 wires to the pins on a small box, so that ant flexing os the case on insertion will not be transferred to the resistor. With a large enough resolution you can actually see the resistor changing value as you apply stress to it by pressing the body, and with these high accuracy ones it will show up thermal stress as well.

[Thor-Arne]

Thanks for the ideas, SeanB.

[quarks]

Yes, the Vishay Z-Foil are the best candidates for these DIY references I know of.
Besides the Z201 I also had a closer look at the HZ Series with Zero TCR (these go even down to 0.001% and 0.2 ppm/°C).
But I have not yet tried to buy some, so I don't know if I really can get them.

Meanwhile I have some very nice BURSTER Type 115 with 0.004% and around 1ppm/°C (this data is individually printed on each resistor, very nice; see picture).
But these are no longer available and the successor Types 1140/1150/1160/1178 are not quite as good (spec wise). 

[cwalex]

If it's ok to ask in your thread. Does anyone know of any good candidates for salvaging these kinds of resistors from old unwanted/faulty equipment?

[quarks]

If it's ok to ask in your thread. Does anyone know of any good candidates for salvaging these kinds of resistors from old unwanted/faulty equipment?

That is exactly what I did. I bought a defective Burster Digistant DC Standard 6406 as for spare parts for my working one.

But if you also want to do that, you should know what is inside and what you are willing to pay for it.

The downsite is, at least for the reference purpose I am looking for, if cannot meassure/compare the parts to known/good values, you may end up not to be confident enough to rely on these salvaged parts as your only reference.

[Thor-Arne]

Besides the Z201 I also had a closer look at the HZ Series with Zero TCR (these go even down to 0.001% and 0.2 ppm/°C).
But I have not yet tried to buy some, so I don't know if I really can get them.

That's really interesting, wonder what the prices of those would be.

I'm in no rush with this as I don't have a need for that precision just yet.
I don't even have a mOhm meter yet, but that is certainly on the wishlist. 

[quarks]

Do not solder direct, rather use a set of standoffs and solder the resistor to that ( so that leads have no strain)  or make a small PCB to hold the resistor, then from the pads make a 4 wire connection to have the sense and force wiring separated as close to the resistor as possible. Then short thick wires to the 4 panel mounted sockets, or another 2 wires to the pins on a small box, so that ant flexing os the case on insertion will not be transferred to the resistor. With a large enough resolution you can actually see the resistor changing value as you apply stress to it by pressing the body, and with these high accuracy ones it will show up thermal stress as well.

That is good input and I would do it the same way, you suggest. But what Du you think about the Banana posts and the solder? Do you know if it is worth thinking about Tellur Copper (like Pomona 3770) and if there is low emf solder (like the one with Cadmium)? I still cannot not really understand why this could make a big difference compared to very good gold plated posts (like from Hirschmann or maybe MC). The thermo couple / contact material would be Gold to Gold anyway. Maybe you or someone else can help me understand.

[robrenz]

IMO for only this application of 4 wire measurement of resistance, all concerns about material combination EMF's is mute. You have to use some form of thermal emf cancellation anyway at low ohm levels. It also only applies to the sense terminals not the source. So use offset compensated ohms or current reversal to eliminate the thermal EMF.  The type of testing we are talking about here is static so we don't need to worry about rapidly changing thermal emf voltages.

[SeanB]

I had them ad constants into a computer, where you could see them drift if you flexed the board, and where they were used as gain setting resistors around the input amplifiers. Solder on one and it changes value for hours afterwards.

[quarks]

That's really interesting, wonder what the prices of those would be.

If I remember correctly, I saw a price of 50$ or 60$ for one "HZ Series with Zero TCR". But because I do not even remember where that was, please do not rely on this.
Maybe someone else has a source and likes to share with us.

[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.

[quarks]

thx robrenz,
that is what I thought and with AC measurement like with HAMEG 8118 it is anyways not relevant. So I will keep on going with the Hirschmann posts/plugs I already have.

But I still wonder, what the impact of gold plated Tellur Copper could possibly be, compared to other gold plated materials. I would think only the contact metal (here Gold to Gold) itself should be responsible for the resulting Seebeck-Effekt (if I do not have an error in the way I think of it).

For what I know so far, Tellur seems to be used mainly because it makes copper easier to machine work with.
Also in the LT app note AN86, Tellur seems to be not even mentioned. Nevertheless the connection type comparison on page 49 (see screenshot) is quite interesting and I think and hope if the contact materials are the same (I use only Gold plated Hirschmann banana plugs and posts, with identical base material), everything should be fine. 

Edit: Just found a good app note from Fluke http://support.fluke.com/find-sales/download/asset/2548277_6001_eng_a_w.pdf