Measuring milliohm resistance - questions - Page 1
Forum section: electronics / testgear

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 04:13:42 am
content: Ok, never give a closet volt nut, time nut, ohm nut, etc. a device with too many digits.So I've got my new 34465A and I'm busy measuring stuff (what else is knew?).I saw a video someone did on 4 wire resistance measurements where the guy decides that a 45 cm length of 18 gauge wire should measure about 9.6 or so milliohms. So that's become my benchmark. I made a 45 cm length of 18 gauge wire and I measured using various test leads (in 4 wire configurations). In come cases (with some test leads) I could reproduce the results from the benchmark, sort of. I had some banana plug to plain old alligator clip leads that worked pretty well but the alligator clips are a bit wide so it's hard to get the clips at exactly 45 cm.... are we measuring from the inside, outside, or the middle of the alligator clips? (And by the way are we measuring from the sense leads (I think this is correct), or are we measuring the 45 cm from say the middle of the force leads (I'm guessing not.)So from the middle of the alligator clips on the sense leads is 45 cm in my baseline measurement. I also try another 4 leads using banana to minigrabbers - these allow for slightly more precise alignment on the copper wire.I also tried a pair of eBay Kevlin clips. So far, these seem to be slightly (a few tenths of a milliohm) the furthest from the benchmark 9.6 milliohm.The 4 test leads with minigrabbers happen to have slightly smaller diameter wire leads than the regular alligator clip leads. Seems like using 4 wires it shouldn't matter what gauge of the 4 leads are - or maybe the slightly thicker alligator leads offer less resistance?So, you can see I'm driving myself slightly crazy measuring stuff.... and that's before I get hooked on the trend chart (and the histogram).While watching the histogram slowly draw it's line at 100 PLC I notice that there is maybe about 0.9 milliohm variation (peak to peak over about a 30 minute period. Why is that? Why doesn't a piece of wire holding still (no bumps) not read a consistent resistance? Maybe it's at the limit of an ADC and the sampling capability in the 34465A?So, I figure I'll quit watching the line graph draw and go do something more exciting like watch grass grow or paint dry.As I walk away for a few minutes and come back I notice the resistance has dropped back from the low 10s to the mid to high 9s of milliohms. So I develop a theory that I am sitting so close to the copper wire that I'm giving off just enough body heat to warm the wire and slightly increase the resistance? Is this possible?Maybe the TEA police should take away the meter until I should attend the weekly therapy meetings.Seriously, 1) does lead gauge on a 4 wire setup have any impact on the readings? 2) All 4 leads within each test are the same gauge but do they need to be? 3) Why would some of my 4 lead tests be (seemingly) more accurate (closer to the benchmark) than my eBay Kelvin clips? 4) should 45 cm of 18 gauge copper wire really be properly benchmarked at ~9.6 milliohms? 5) Is it normal for a resistance trend chart to vary by about 10% (plus or minus 5%) over a 30 minute to 1 hour test? (It kind of oscillates up and down within a few minutes so I don't think it's reflecting the ambient temperature in the room, but maybe it's the effect of the air conditioning in the room - just seems like there is more fluctuation than would be caused by the air conditioning cycling on and off once in a while). 6) Is it possible that I'm heating the copper wire by sitting close to it and thereby increasing the resistance?Thanks

author: bdunham7
author_rank: Super Contributor
timestamp: September 03, 2020, 04:33:14 am
content: The meter is passing a known current through the wiring and the sense leads are measuring the resulting voltage drop. Figure out what that current is (you can just measure it directly with another meter) and then calculate that voltage. You'll see that the voltage is very small. You might get similar results if you tried to measure a voltage that small just due to noise from wherever. Anytime you go to the extremes, whether it is milliohms, gigaohms, nanovolts or picoamperes, you run into some sort of limitation and it usually boils down the the same issue--very small sense voltages.

author: Brumby
author_rank: Unknown
timestamp: September 03, 2020, 04:47:05 am
content: Quote from: Electro Fan on September 03, 2020, 04:13:42 amWhich is why I'm not venturing any further than my 3478AQuote4 wire pushes a known current through the force leads. These can be pretty much any length. The sense leads measure the voltage drop that occurs along the DUT between them, therefore the resistance measured will be that between the points of contact of the sense leads.Normally I would suggest the two closest points of contact on the sense lead connections would define the segment being measured - but with extra, extra, extra digits, the 3D analysis of the contact zone might be a tad more complex. I have no experience of this level of measurement ... and to preserve my sanity, I don't plan on gaining too much more than what I read here.....

author: Rasterist
author_rank: Contributor
timestamp: September 03, 2020, 05:00:38 am
content: 1) I don't know but I'd guess not. That's part of the reason they invented 4-wire measurements.2) Beats me but, again, I'd guess not.3) Beats me4) Why not take another 45 cm of 18 gauge copper wire and perform a 4-wire measurement on it?5) Beats me, but you may be able to eliminate temperature as a source of the oscillation by logging it along with your measurement.6) Almost certainly not, but what do you mean by "close?" If there's more than a couple of inches between you and the wire I'd be surprised if there were any effect. Of course, you could eliminate any doubt by measuring the difference in ambient temperature between your skin and the wire.I've had a 34461A for a couple of years now and am very happy with it. Got it primarily to perform 4-wire measurements.

author: bob91343
author_rank: Super Contributor
timestamp: September 03, 2020, 05:30:56 am
content: First you need to clip the drive current clips as far as possible from the sense leads so that the contact resistance doesn't generate heat and that the current distribution across the wire is uniform.Then you need to clip the sense leads in such a way that it encompasses as much of the circumference of the wire as possible.You are correct about temperature effects. The temperature coefficient of resistance of copper is about 0.00393 per degree, or 0.393% per degree Celsius.In addition there is contact potential. If the clips are not copper, the interface with the copper generates a thermal voltage which may or may not be canceled by the thermal voltage at the other clip. This is averaged out if you are using ac for measurement. But if using ac, you must account for skin effect. Looking at the clips, they are metal that is plated with a different metal and perhaps soldered with an alloy to the copper wire of the test lead. A multiplicity of interfaces.The temperature gradient along the test sample is important. As is the purity of the copper, since different alloys exhibit different resistance. Depending on the excitation current, there may be heating of the wire enough to affect the results.Your experiments generate more questions than they answer.

author: dcac
author_rank: Frequent Contributor
timestamp: September 03, 2020, 05:52:20 am
content: Fluke has a good paper about Thermal EMF here:http://download.flukecal.com/pub/literature/p18-21.pdf

author: Fungus
author_rank: Super Contributor
timestamp: September 03, 2020, 06:20:18 am
content: Quote from: Electro Fan on September 03, 2020, 04:13:42 amTemperature. Passing any current through a wire will heat it up.It should stabilize eventually.Quote from: Electro Fan on September 03, 2020, 04:13:42 amI'd go with "air movements".Quote from: Electro Fan on September 03, 2020, 04:13:42 amYou might be causing convection currents.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 06:29:22 am
content: Quote from: bob91343 on September 03, 2020, 05:30:56 amYour post and the others are very helpful and encouraging but this point above might be the most fundamental/biggest/immediate area where I think there could be improvement in my understanding/testing. I was under the (very wrong?) impression that having each force clip as close as possible without touching its companion sense clip was desirable. But if I understand your point above it indicates pretty much the opposite.So for example, rather than measuring the resistance of 45 cm of wire with a 50 cm wire (an extra 2.5 cm on each end) it would be better to have the force clips 1 meter apart with the sense leads 45 cm apart? If this is correct (to keep the force clips from generating heat), is there a practical limit or rule of thumb on how far the force clips should or need to be from the sense clips? Thx

author: 2N3055
author_rank: Super Contributor
timestamp: September 03, 2020, 06:37:06 am
content: Something to read for starters...

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 06:52:34 am
content: Fungus, when you say it should stabilize eventually (assuming I make “air improvements“), roughly how long is it until eventually? An hour, longer?Also, general question, any chance that as nifty as the 34465A is, maybe somewhere around 1 milliohm it is nearing it’s capabilities and then noticeable improvement requires a microohm meter? I’m sure my understanding and technique can get more from the 34465A but at some if you want more grapefruit juice you have to get another grapefruit rather than just squeezing harder. (I’m pretty sure I’m going to be happy with whatever the 34465A can do, just checking to see how I know when I’m getting close to what’s reasonably near the last few drops of juice in the grapefruit.)

author: Fungus
author_rank: Super Contributor
timestamp: September 03, 2020, 07:03:32 am
content: Quote from: Electro Fan on September 03, 2020, 06:52:34 amI'd say minimum ten minutes for the the air to stabilize and the wire to heat up to equilibrium. That's after the power supply and multimeter have been on and working for half an hour.

author: MosherIV
author_rank: Super Contributor
timestamp: September 03, 2020, 07:49:48 am
content: I am not sure your experiment is valid, you are measuring an unknown resistance and trying to compare it to a theoretical calculation.I suggest starting with a known resistance. Eg 0.1\$\Omega\$ 5% is easily available. Measure it with a known fixed current and ohms law.Be careful with applying the current for too long, the heat will change the resistance.

author: Brumby
author_rank: Unknown
timestamp: September 03, 2020, 08:57:40 am
content: Quote from: MosherIV on September 03, 2020, 07:49:48 amWhy would you call that invalid?I think it's entirely reasonable .... matching theory with practice seems a pretty fair thing to check out.

author: BravoV
author_rank: Super Contributor
timestamp: September 03, 2020, 09:02:59 am
content: Quote from: MosherIV on September 03, 2020, 07:49:48 am+1 , yep, play with a known resistance 1st, and start to experiment with the 4W variations to learn how it effects the result.Don't know about 34465A, does it excite/bias DUT with DC ?As once you move further to AC measurement, be prepared to face with a big can of worm, as things may get more complicated.Example below, the resistor was measured in AC excitation at 1 kHz, spot on at it's designated reactance in Ohm or resistance in DC, but once the test frequency changed say to 1 MHz, it may not be at 10 miliOhm anymore.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 09:16:32 am
content: Quote from: BravoV on September 03, 2020, 09:02:59 amThat is very cool! (very nice!)

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 09:42:07 am
content: Ok, kinda learning 4 wire testing, temp + resistance measurements, theory, and the 34465A all at the same time.After reading that I might need to run the test for a while to let it settle and then reading that I could run the test too long (cause over heating) I went to inspect - and I'm pretty sure as I got within a few inches there was a bit of a spike caused by my inspection (a disturbance?). Hard to say for sure - very preliminary experimenting, learning, sharing.

author: BravoV
author_rank: Super Contributor
timestamp: September 03, 2020, 10:03:57 am
content: Quote from: Electro Fan on September 03, 2020, 09:42:07 amAs before, get yourself a bunch of sub-Ohm resistors, instead of random wires as DUT, and start profiling them, and also measure the temperature. Use 2nd meter to measure the DUT temperature too. And use hot air solder, or hell your mom/wife/gf hair dryer to start heating up the DUT and see how they affect the result.I went very similar route years ago, gathered few resistors and start learning. They doesn't need to be metrological grade stuffs, but decent enough.Regarding the 4 wires measurement jig, sometimes you have live with the limitation of the DUT, for example, like below my resistors, they are 0.1% accuracy and temp.co at 50 ppm/K, see the black wires ? Those are the sense lines, which defy what ever 4 wires jig/clips we have, as we have to manually connect it to the meter.Better version has beefy sense terminals (thinner metal terminals) instead of just wimpy black wires as above.

author: Someone
author_rank: Super Contributor
timestamp: September 03, 2020, 10:10:28 am
content: Quote from: Electro Fan on September 03, 2020, 06:52:34 amYou're down in the noise, stability isn't great:

author: MadTux
author_rank: Frequent Contributor
timestamp: September 03, 2020, 10:18:39 am
content: Normal DMM sucks for measuring low resistance.Microohm measurement works much better, if you have something like a HP E3614A or similar to send a few Amps trough the DUT next to your meter.Fiirst use DMM to set the current to a nice number, like 1A or 6A and connect it to DUT.Use DMM or nanovoltmeter to measure voltage over DUT and use it to calculate resistance.

author: Fungus
author_rank: Super Contributor
timestamp: September 03, 2020, 10:22:57 am
content: Quote from: BravoV on September 03, 2020, 10:03:57 amThis.If you suspect temperature/air movement is causing problems then heat it up/blow on it. See what happens.

author: BravoV
author_rank: Super Contributor
timestamp: September 03, 2020, 10:27:04 am
content: Quote from: MadTux on September 03, 2020, 10:18:39 amLOL , I'm worry we are all slowly push the OP into the rabbit hole of metrology.As your suggestion, may pop out another can of worms,which are ... stable, accurate current source and "maybe" and 2nd comparable meter to monitor the current, while other meter measure the drop voltage, this things will never end.

author: Fungus
author_rank: Super Contributor
timestamp: September 03, 2020, 10:30:40 am
content: Quote from: BravoV on September 03, 2020, 10:27:04 amDon't worry, he's already there.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 03, 2020, 10:34:07 am
content: Just readind this make me crazy lolllInformative stuff in the pdf's

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 03, 2020, 10:38:54 am
content: Quote from: BravoV on September 03, 2020, 10:27:04 amThe current source can be any. The main thing is that the current meter and voltage meter work simultaneously.

author: BravoV
author_rank: Super Contributor
timestamp: September 03, 2020, 10:44:00 am
content: Quote from: MegaVolt on September 03, 2020, 10:38:54 amAre suggesting to the OP to buy 2nd meter like 34465A, or even better one ?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 03, 2020, 10:52:25 am
content: Quote from: BravoV on September 03, 2020, 10:44:00 am))))))) What else to do))

author: MadTux
author_rank: Frequent Contributor
timestamp: September 03, 2020, 10:59:00 am
content: Since the voltage drop over DUT usually is only in the mV to µV scale, an inexpensive DMM with like 0-5-0.1% current accuracy is probably enough. Even something like Keithely 2010 has only 5 relevant digits below 1mV and the last one is more or less noise, so something like AN8008 should be good for the job.In real world scenarios, a few % accuracy is fine most times, at least it's good enough for me to calculate power dissipation of some replacement power rails on a welder repair project, where the previous power rails had melted.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 03, 2020, 12:39:06 pm
content: Start with the right tools for the job.I typically LabView to automate the measurements. This not only allows me to record my results but also control the meter. Labview will read the meter some number of times and check the standard deviation. If it's above some threshold (measurement is not yet stable), Labview will repeat the measurement until it meets this criteria. I will typically plot the standard deviation to get some visual idea how things are going. I will typically serial number the parts I am wanting to measure and this information is fed into Labview as well. I will normally have some sort of part hold to make sure things don't get mixed up. I may also document things like the date, time, ambient temperature, humidity... I will typically scribble out some sort of procedure that I plan to follow. I may also include this into the software to walk me through each measurement (boring). This all gets documented as well.The meter is on a calibration schedule. Beyond being sent in for calibration, a second set of low cost standards are used as a sanity check for the calibration as well. (pic1-3)Nitrile gloves are worn at all times. Clean the workplace and Kelvin probes. (pic4)In my case, I may be looking at more than one component. Make sure to probe the same location every time. It's easy enough to calculate the amount of error this can cause.I've mentioned many times my use of beach towels to control the air currents.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 05:19:48 pm
content: Quote from: BravoV on September 03, 2020, 10:03:57 amThose look pretty and pretty useful for testing/benchmarking. I get it. Thanks!Better version with sense terminals looks very cool - never knew such a thing existed.If you or anyone has any links to U.S. suppliers that you recommend please post. Thx

author: dcac
author_rank: Frequent Contributor
timestamp: September 03, 2020, 05:26:50 pm
content: I couldn‘t find what test current 34465A uses in the 100ohms range, but i.e. on Fluke 8846A it is 1mA.So 9.5mohms x 1mA = 9.5uV over the DUT.I noticed you seem to be using Nickel plated banana plugs on you test leads. If so each copper-solder-nickel “junction” can create several uV per degree C according this table:It also shows Gold plated plugs/connections are much better or have a significantly lower tempco 0.3uV.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 05:59:49 pm
content: Quote from: Fungus on September 03, 2020, 10:30:40 amI think I'm on the surface at the very edge of the opening to the hole peering down and getting a glimpse and a small sense for what's in there - thanks to all the feedback here.I'm debating whether it makes sense to enter and explore further. I can almost taste a microohm meter. But I think my better judgement is telling me to back up and be happy to revisit Ohm's Law and get a better handle on the basics that these discussions keep pointing toward as the foundation for any rabbit hole exploration.Next time I pass along the wisdom to a new enthusiast that they should not just pick one DMM but be sure to get two I will better understand and better appreciate the slippery slope path I might be advocating. It's kind of hard to imagine that after about 6 years of playing with this stuff and then another year of contemplating which first bench DMM to purchase, and then getting the 34465A on the bench for a couple weeks that somehow what I need now is another meter, or more meter(s). I'm pretty sure I don't. But then if I skip a weekly TEA therapy meeting who knows what I might think I need the following week. Seriously, I think I'm good on meters for awhile but one (or a few?) of those high precision low resistance benchmark resistors might be nice.Most importantly, I can't thank everyone here enough for all the teaching and encouragement. The study of this field is fascinating and incredibly intrinsically rewarding - it is a source of great enjoyment. Go EEVblog and EEVblog forum members

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 06:01:46 pm
content: Quote from: coromonadalix on September 03, 2020, 10:34:07 amIt's contagious.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 06:07:11 pm
content: Quote from: joeqsmith on September 03, 2020, 12:39:06 pmAll goofy-ness on my part aside, this post is amazing. This is a bunch of very helpful advice that reflects a ton of experience at the intersection of theory and practice. Probably should be a sticky posted somewhere. Thanks Joe!

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 06:25:48 pm
content: Quote from: dcac on September 03, 2020, 05:26:50 pmThanks for this post and the notice about the banana plug material. Until I got the 34465A I had never heard of Mr. Seebeck and Mr. Peltier, and it was sort of surprising to learn that simply putting dissimilar metals together can create a small voltage. I probably should have figured that was possible since it doesn't seem too much different than battery chemistry but it's an example of what happens when you start measuring stuff, and it's why I have become addicted to test equipment. I have found that measuring (even if you don't have a purpose to measure other than measuring) causes you to test and then build new hypotheses (hypothesis plural), and the more you do this the more you learn, and the more you learn the more you realize how much more you don't know, which requires more reading/researching, questioning, and more measuring and testing.Even though I tripped across the Seebeck effect for the first time a few days ago it hadn't dawned on me until reading your post that the effect might be (is) occurring at the meter connections to the banana plugs; until just now I was entirely focused on the alligator clip end of the test leads. It really is a very deep and probably increasingly wide rabbit hole.

author: nfmax
author_rank: Super Contributor
timestamp: September 03, 2020, 08:23:39 pm
content: The Seebeck effect doesn't actually occur at the junction(s). A voltage is developed wherever there is a temperature gradient along a metal wire. Different metals have different voltages when subject to the same temperature gradient, so in a circuit comprising two metals there will be a net voltage difference, but it's not generated 'at' the junction.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 08:40:07 pm
content: Quote from: nfmax on September 03, 2020, 08:23:39 pmI'm sure I'm missing something subtle or not so subtle, but if different metals have different voltages at the same temperature gradient, wouldn't the net difference in the voltages appear at the junction? Or maybe the point is that the voltages are different throughout the two metals and the voltage difference can be measured at each of the two metals even if they are not touching (this is a little harder for me to envision)? Thx

author: Fungus
author_rank: Super Contributor
timestamp: September 03, 2020, 08:48:04 pm
content: Quote from: Electro Fan on September 03, 2020, 05:59:49 pmNot necessary. Ohm's law tells you that all you need is the same meter and more amps.(shouldn't be a problem with 18-guage wire)

author: Electro Fan
author_rank: Super Contributor
timestamp: September 03, 2020, 08:56:02 pm
content: Quote from: Fungus on September 03, 2020, 08:48:04 pmI'm pretty sure more amps are less expensive than a microohm meter

author: bob91343
author_rank: Super Contributor
timestamp: September 03, 2020, 09:46:41 pm
content: Don't get me started on rules of thumb. They are ignorance in disguise. If you want to know how much extra wire you need to avoid significant heat transfer, you need to calculate it and perhaps measure it. One way to tell is to move the source current clips farther away and see if your results change. Of course they will but you have to decide whether the difference is significant and whether the change is due to your efforts or something else, like noise or where you are standing or time of day or corrosion progress in the materials or any other of a large number of stimuli that affect readings. Or your measuring device limitations.We need to be physicists, chemists, metallurgists, and philosophers to understand much of this. And nobody understands it all. You can read 100 books on the subject (there are many more than that) and what you will learn mostly is that there is a lot to learn.I would like to own an HP 3458A but I will have to be content with my 3456A.

author: graybeard
author_rank: Frequent Contributor
timestamp: September 04, 2020, 08:25:31 pm
content: You should get yourself a set of Kelvin test clips. They work great for measuring low resistance values. I have two sets. The one shown here I use with my bench meters, and another set adapted to my Der EE DE-5000 LCR meter.Chris

author: Martin72
author_rank: Super Contributor
timestamp: September 04, 2020, 09:16:17 pm
content: Hi,Quote from: Electro Fan on September 03, 2020, 04:13:42 amI´ve don´t read the whole thread, so forgive me when I´m repeating already posted things...1) In my opinion yes, the bigger the gauge for thecurrentis, the "colder" it get´s when current is flowing through - The gauge depends on the the test current*2) No. The gauges for the voltage drop measure could be "ways" smaller on a 4-wire lead test cable - You only measure the voltage, not the current*.3-4) The own gauge/resistance is not important until it meets the needing for the testcurrent you use.5) Yes, you heating up the leads by letting current flowing through it, so wait until it will saturating ( values become stable) before notifying the measured values.Also the values can "oscillating" when contacts are not proper.6) Sure you do heating it with your body presence, but it shouldn´t have that much impact.*) We do milli-ohm measurings as following : The less the expected resistance value are, the more current we´re letting flowing through, to make sure to measure a stable drop-voltage.For example, it is no uncommon we using 25A and measure the drop-voltage as close as possible on the DUT - Kelvin clamps are more ideal for this, when the gauge for the current flow is big enough.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 04, 2020, 10:48:37 pm
content: Quote from: graybeard on September 04, 2020, 08:25:31 pmThxI have an eBay set of Kelvin leads that I purchased on eBay and I just ordered a second and third pair (missed the TEA therapy session this week). One of the new sets looks a lot like yours. I'm going to see if the various designs produce similar or noticeably dissimilar results.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 04, 2020, 10:53:16 pm
content: Quote from: Martin72 on September 04, 2020, 09:16:17 pmHi Martin,Thanks - that was helpful. I appreciate it.fwiw, some of my questions were misguided by some "research" and some "testing".... see my post following this.EF

author: joeqsmith
author_rank: Super Contributor
timestamp: September 05, 2020, 12:26:50 am
content: Most of the time when I have needed to measure low resistance, the components would never handle 25A. If you look at my 1 ohm low cost standard, it's only rated to 500mA @ 500mV.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 05, 2020, 12:39:59 am
content: Ok, another mea culpa.When I first received the 34465A and got going with some initial tests I figured I'd check to see if the meter seemed accurate. I know, that's a lot balls to question Keysight. And I didn't really have any good way to check most low level measurements but for voltage I had a few benchmarks I was familiar with (a Fluke 179, a PentaRef, etc). Nothing that should be equal to the 34465A but at least a few sanity checks. On voltage everything made sense or at least was explainable to the extent I could measure stuff.When it came time to check resistance I didn't have much except some pretty mundane resistors and my various 2 wire meters, plus a DE-5000. The DE-5000 is very useful for resistance but I decided I needed a better DUT for measurements at the milliohm level. Then I tripped across a video where the video author measured the resistance of a 45 cm length of 18 gauge copper wire. And to make it official he walked through the values he found somewhere, and he mentioned that he has a caliper to check the wire diameter, and he walked through the math. At the end he says a 45 cm copper wire should be about 9.6-9.9 milliohms.Well, silly me, I ran with that. At which point I became eager to find a way with various leads to get a measurement of about 9.75 on the 34465A. Why 9.75? Because it's halfway between what my new reference just told me.Then I posted here asking questions to help me understand why my measurements might be a little off. And some of you tell me "don't goof around with the theoretical measurements". Well, that turned out to be good advice. And I'd add some more good advice which would be "if you are going to goof around with theoretical measurements - first check the assumptions behind the theoretical measurements".I watched the video again and this time I heard the author say 18 gauge cooper wire is 1 mm in diameter. Turns out that various table and calculator sites indicate the diameter is 1.02362 or 1.02369 mm. Also the author came to the conclusion that the resistivity of copper is 1.68 - 1.72 x 10-8ohm.m at 20C but as best I can tell maybe the value should be 1.678 (pretty close to the low end of the author's scale, but not sure where he got the 1.72 figure). When you account for the extra diameter of the wire and you work off the lower end of the resistivity range and you do the math, maybe the answer should be that 45 cm of copper wire is more like 9.2 milliohms - but at least now I can see how some assumptions could have driven 9.6-9.9. For the last few days I've been looking for about 0.5 milliohms that went missing. I won't bore you with all the trend charts I could post to show my search for the roughly half milliohm that went MIA (Missing In Action).So, now I'm going to go back and chase my (theoretical) tail some more (I know, I was warned not to do this) to see if maybe the 34465A and my existing (both Kelvin and 4 separate) leads can produce results closer to the expected 9.2 milliohms. And in a week or so when the next two sets of Kelvin leads show up from eBay I can see what I learn about lead variability.At three sets of eBay Kelvin leads (including the two more that were ordered just before realizing why half a milliohm probably went missing) I'm still well ahead of the cost for one good set from Keysight or Keithley, but I'm chipping away at what could have been good set of Brand A leads. If it's not one thing (stressing the budget), it's another (stressing the space consumed) - but it's all for a good cause: measuring.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 05, 2020, 12:52:51 am
content: Wow, almost immediate success!Measurement is still stabilizing but it's looking pretty close to 9.3 vs the expected 9.2. However, my temp check (which is only so-so but maybe ok) says the temp is about 22.7 C vs 20 C and when I add the 0.393 percent per degree C (copper temp coefficient) I think that works out pretty close to 9.3 milliohms. Might be a testing mirage but happiness would be when measurements come in as expected.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 05, 2020, 01:15:23 am
content: Shifting gears and going a little bit off topic - here is a plug for the 34465A or more precisely the concept of putting trend charts and histograms into DMMs. From my last post you could see I was kind of excited about getting a measurement that came within about 0.1 milliohm of what I expected. I should have waited a little longer to watch the trend chart - now things have drifted up, and down, and up, etc. So while I was getting antsy to see where the measurement would settle I tried to eyeball what the trend chart was telling me about the trend (I guess that's why it's called a trend chart) and then I got the idea to check the histogram. I know this sounds stupider than elementary to everyone who is way down the path on this but it's pretty nifty to be able to hit a button and go from the trend chart to the histogram. Back to the theoretical resistance of 45 cm of 18 gauge copper wire it looks like there might still be about 250 microohms MIA.But I have a theory on where they went missing and I'm going to check that next.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 05, 2020, 03:34:27 am
content: Quote from: Electro Fan on September 05, 2020, 01:15:23 amHaving it in the meter for me wouldn't be as helpful as just having a way to pull the data out of the meter to where you can plot what ever you want.Here's my old 34401A looking at a 0.1 ohm shunt from a DMM. There's a histogram of the resistance, along with a graph showing the raw and mean data. LabView was designed to automate testing and makes short work of it. Now that National Instruments offers it for free, I recommend giving it a try.

author: dcac
author_rank: Frequent Contributor
timestamp: September 05, 2020, 01:50:31 pm
content: Quote from: Martin72 on September 04, 2020, 09:16:17 pmFluke actually specify max lead resistance in 4-wire mode for 8845A/8846A. But those figures seems to leave plenty of margin - as much as 10 ohms per lead is allowed in the 100 ohms range. You almost wonder if that's a misprint.But still the values probably are based on max output voltage from the 8846A current source - so even with 10+10 ohms lead resistance you still get a constant current through the DUT. And then there's the input impedance for the sense leads - and here 10+10 ohms probably will not cause much error due to the high input impedance.Of course no test leads are going to have 10 ohms resistance. And for very low ohms measurements i.e. below 0.1 ohms i think the type of metals the test leads are made from is (much) more important than the resistance.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2020, 07:36:52 pm
content: To get more exact sense lead placement, use blade edges or for thicker wire, sharpened steel probe tips; sewing needles work well.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 05, 2020, 09:01:16 pm
content: Quote from: David Hess on September 05, 2020, 07:36:52 pmThis reminds me to the setup I used to check "wire" for defects that show up as higher resistance:2 razor blades with a 2 cm spacer to measure the voltage and normal clips to apply a test current.

author: Martin72
author_rank: Super Contributor
timestamp: September 05, 2020, 10:15:53 pm
content: Quote from: joeqsmith on September 05, 2020, 12:26:50 amYou´re right, my Ascel Milliohm Meter fits better in this case, as it "only" got max. 100mA testcurrent.At work, we´re testing "hefty" shunts and resistance between handles to the case, therefore 25A won´t be too much for it.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2020, 10:32:42 pm
content: Quote from: Kleinstein on September 05, 2020, 09:01:16 pmOne of my sets of multimeter probes have sharpened piano wire tips. They are just the thing for puncturing insulation or conformal coatings, not sliding off of conductive surfaces, and drawing blood for proper rituals to the test equipment gods.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 06, 2020, 12:30:11 am
content: Quote from: joeqsmith on September 05, 2020, 03:34:27 amThanks for the recommendation.I found the link to the free version - they refer to it as the Community Edition - seems fully featured, just for non-commercial use.https://www.ni.com/en-us/shop/labview/select-edition/labview-community-edition.htmlThat's a very good looking computer interface screenshot you attached - did that come as an out the box DMM template with LabView or did you build that from scratch?

author: coppice
author_rank: Super Contributor
timestamp: September 06, 2020, 12:57:33 am
content: Quote from: Fungus on September 03, 2020, 06:20:18 amYou didn't mention the important figure that brings what you said into focus. If you are measuring a copper wire remember that the temperature coefficient of copper is 0.4% per degree Celcius. It takes only a very small change of temperature to see a big change in resistance. A little air movement doesn't cause minute changes in resistance, as it might with a high quality resistor. It produces very obvious changes in resistance.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 06, 2020, 04:12:31 pm
content: Quote from: Electro Fan on September 06, 2020, 12:30:11 amThe interesting thing with Labview is you can create a decent UI in a few minutes. It's just a matter of dragging, dropping, changing some fonts and splashing some color on it. But a UI is not a program. You still have to create the underlying code.They offer example code for the 34401A along with several other brands/models of test equipment. This can help speed up the development. There's nothing in this particular program that was out of the box. I use a GPIB controller that NI no longer supports, which required a bit of reverse engineering on my part and then rolling my own code to support it. That's been a bit of an on going effort.This particular program is normally something that would be integrated into a larger test. Here for example a source meter I was working on compared with my 34401A.I use the source meter for testing battery life of the meters. I was playing with a 32-bit ADC from TI about a year ago that looks a little better.What's interesting about the 32-bit part is I had looked at where someone else had looked at this same ADC1262EVM evaluation board who ended up using a small embedded controller to interface with it. Their claim was Labview was not stable and would crash after a few minutes. Labview will not magically turn a poor programmer into a good one. Many years of use as shown it is possible to write very stable programs with it.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 06, 2020, 06:14:40 pm
content: Quote from: joeqsmith on September 06, 2020, 04:12:31 pmTo be square, while I would like to become more proficient (ie, proficient at least a little bit) with remote/computer processing of test equipment data, it's just on the cusp of being over the hill and beyond the horizon for most of what I do - which is to try to figure out cause and effect starting with Ohm's Law and then get as deep and wide as I can go from there - but every time I get anywhere I realize I still don't have a good grasp even on Ohm's law.So, I'm not adverse to post-processing data and I see the learning opportunities in it - but I kind of like the easy snapshot views on the graphical interface of the 34465A front panel. Nothing like a set of measurements and some trending to help confirm or disprove a hypothesis made in the process of learning. For what I do, I don't need a ton of data points to see if I'm on the path or off the path. Most of the time I'm looking for the trends and the underlying reasons for the trends more than the occasional outlier - but I realize there can be a lot of meaning in outliers. Truth be told, I'm just a beginner who is intrigued by and enthusiastic about the more intermediate and advanced uses of test equipment.FWIW, I did make a run at figuring out LabVIEW 2-3 years ago and below are some tutorials I found useful in case anyone might find them interesting or useful as a starting point.Having said all that, I love the LabVIEW program you made for looking at your 0.1 ohm shunt with the 34401A. If I can find the baseline 34401A example in LabVIEW I'll give it a whirl to see if I can enhance it using your program as a design goal. Thx!LabVIEW TutorialsWriting Your First LabVIEW ProgramData Flow Programming BasicsLabVIEW Data TypesCreating Sub Vis in NI LabviewUsing Charts and Graphs in NI LabViewUsing Debugging in NI LabViewUsing the Tools Palette in NI LabVIEWUsing Arrays in NI LabViewUsing LabVIEW Case Structures (if then, etc.)Using Loops in LabViewTaking a Measurement with Your Computer

author: joeqsmith
author_rank: Super Contributor
timestamp: September 06, 2020, 09:27:41 pm
content: Quote from: Electro Fan on September 06, 2020, 06:14:40 pmI have to say, you are way further ahead compared to my early days learning the basics with my Radioshack analog meter. I damaged that old Radioshack meter more times than I care to admit, which improved my soldering skills.You may want to consider seeing if they offer any sort of classes at a local college. Another possible option is you may have a local amateur radio club that offers classes to help study for the entry exams. I assume they still have to know some level of theory. This may actually be a better resource as you may find they are more willing to help you one on one.The attached ZIP file contains the original 34401A examples ported to LabView 2014. It should support both GPIB and RS-232. I can't speak to how well they perform or how well they were written. I tried them with LabView 2011 running under an XP virtualbox using GPIB and it did communicate with my meter. I can't test it with the newer LabView or with Windows 10 because of the lack of GPIB support. I suspect it's fine as a starting point. You may find better examples on-line.****The following link provides some information about the drivers and uses the 34401A as an example.https://d1.amobbs.com/bbs_upload782111/files_17/ourdev_473371.pdf****This is the quick start guide that I normally recommend as a starting point.https://engineering.purdue.edu/~aae520/Labview_manuals/QuckStrt.pdf

author: Electro Fan
author_rank: Super Contributor
timestamp: September 06, 2020, 10:41:58 pm
content: Quote from: joeqsmith on September 06, 2020, 09:27:41 pmJoe, Thank you very much! EF

author: klausES
author_rank: Regular Contributor
timestamp: September 07, 2020, 12:59:03 am
content: The use of four individual clamps can sometimes be advantageous over the usual Kelvin clamps.Not only because of the possible local separation of the respective contact points for electricity and measuring points.After using several sets of different types of Kelvin test cables (from cheap to a little more expensive),I always return to the "four individual" terminal solution in cases of doubt, although the Kelvin are of course more convenient to use.As a reference for checking ohmmeters around such instruments today and e.g. to compare in one or more years later(only comparisons, no absolute values) I like to use stainless steel rods with calibrated diameter and length as well as precisely defined points to clamp on.Their conductance is (at short distances) more practical than, for example, if you don't want to have a lot of zeros to the decimal point.Then there is another point that always amazes me.Often measure low resistance e.g. often emitter resistors of various types.With absolutely the same construction types (e.g. a cement around a helix type of a series from one manufacturer)one expects the same temperature coefficient, so I mean at least one in the same direction.But there are always individual specimens with a reversed temperature coefficient.At first I only thought of measurement errors on my part, but it doesn't seem to be because of that.That shouldn't be the case, of course. Especially not in areas of application in which the direction of the temperature drift is important.Has any of you seen this?

author: coppice
author_rank: Super Contributor
timestamp: September 07, 2020, 01:28:58 am
content: Quote from: klausES on September 07, 2020, 12:59:03 amKelvin clamps aren't the greatest things when the resistance value is really low. You really need to apply the test current slightly upstream and downstream of the measurement connections. Kelvin clamps apply the current and measure at around the same point with rather vague results.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 07, 2020, 01:46:04 am
content: RAP show on measuring low resistance. There's a few nuggets in there:I have a fairly large supply that I run from Labview that I use to look at smaller resistances (like DMM shunts, DMM test leads). The leads are two gold plated brass clamps. The supply can run constant current up to a bit over 200A.Here I was using this setup to attempt to design and make a shunt for a cheap handheld DMM. You can see some of the tests.https://youtu.be/ns02v6ebCGU?t=453

author: Electro Fan
author_rank: Super Contributor
timestamp: September 07, 2020, 06:26:18 pm
content: Quote from: coppice on September 07, 2020, 01:28:58 am"You really need to apply the test current slightly upstream and downstream of the measurement connections."When using the four individual terminal solution how do you determine how far upstream and downstream to apply the test current? Thx

author: bob91343
author_rank: Super Contributor
timestamp: September 07, 2020, 08:30:57 pm
content: It's a mechanical problem. When you connect to a wire, you don't usually connect to the entire cross section, but to a few places on the periphery. Thus, the current distribution isn't uniform in that area. So in order to decide 'how far upstream' you must model the finite size of the parts and look at the distribution of current. When the current distribution is uniform enough that it doesn't affect the readings significantly, you have connected it satisfactorily.It's not that easy. Perhaps someone can devise an algorithm to help decide how far from the connection point it's safe to measure. If you insist on a 'rule of thumb' and are willing to accept the pifalls thereof, I would guess you'd move at least several wire diameters from the sense point, assuming the distance between sense points isn't very small.A good way would be to experiment with various distances and draw a scatter diagram in an attempt to look for an exponential trend in the errors.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 07, 2020, 08:54:03 pm
content: Quote from: bob91343 on September 07, 2020, 08:30:57 pmThanks and fair enough.... seems roughly consistent with what I'm finding.