L@@KING for gold-plated tellurium-copper banana plugs

[alm]

A few empirical points to support my argument that plating matters: High-end references and meters will often use a copper alloy instead of nickel or gold plated binding posts. I also remember a post on one of the test equipment related Yahoo! groups (hp_agilent?) about someone who asked the UK equivalent of NIST about what they used as cables for connecting to voltage standards. They used standard connectors, but send them off to chemically strip the nickel plating before use. This also argues that plating is a big deal for precision measurement. Cleaning copper connectors before use (or air-tight crimps) is important because copper oxide produces a substantial offset voltage. See also the Keithley low level measurements handbook (available for free from their website). You can also find some volt-nuts posts of people complaining about unstable readings with plated connectors.

Is it that plating is thick enough so a thermal gradient is developed that is large enough to produce a significant potential (note that metrology is dealing with µV and nV), or is the Kasap paper oversimplifying the underlying physics? I haven't been able to find anything to support the latter (yet). This book (section 8.08) appears to do a decent job explaining the Seebeck effect, but with that model I don't see a reason either why copper-other-metal-copper would create any offset voltage if there is no temperature gradient across the other metal. So my conclusion is that there often is a temperature gradient across the plating .

Does anyone feel like estimating the temperature gradient that you can expect to occur across a thin layer of coating given the fairly small (a few K) difference in temperature across the total connector? 1 mK difference across nickel plating would result in 10 nV potential across the plating. Which is consistent with this post. Of course a solid gold connector, or a connector with much thicker plating, may produce more voltage.

[quarks]

They look nice but It does not say what the material under the gold plating is.  And that goes back to my question of how much thermal EMF effect can the plating have at such thin layers and no one seems interested in discussing that topic

When looking at the offered price, I guess it is probably not a special "low emf" material.

About plating I am still very interested and made some more experiments with pure copper, copper-silver, copper-tin and copper-gold.

I was quite surpised, that tin plating does not seem to be bad at all, therefore I odered some tinned spade lugs and cables. But I have not found any Seebeck numbers on copper-tin, so if anyone finds some, please share.

Just to see how nickel plating will be, I also ordered special high temp cables.
As soon as I get these I can give an update if there is interest.

I also made an experiment with making my own crimp spade lugs out of pure copper (but failed so far).

[AlfBaz]

1 mK difference across nickel plating would result in 10 nV potential across the plating.

That 10nV potential you speak of "across the plating" is that the potential in the plating only? If so then whilst 10nV sounds large what would be the net voltage generated given that the Seebeck coefficient of the material its attached to is not 0

[alm]

10 nV would be the voltage between the two pieces of copper at both sides of the nickel. I got the number from a table with Seebeck coefficients relative to copper from the NI website (10 uV/K). Seebeck coefficients are usually quoted relative to platina, which is not that useful since platina wires are fairly rare in electronics.

[alm]

Why do you buy low-EMF binding posts but insist on gold plating? Either you subscribe to the theory that plating matters for thermal EMF, in which case gold (over nickel?) plating is bad news. Or you assume that only a significant length of material will have a substantial impact on thermal EMF, in which case something cheap like nickel plated copper will be just fine.

If you go this route and worry about thermal EMF, I would recommend putting the binding posts in good thermal contact to keep the binding posts and plugs all at the same temperature. I'm quite sure this is discussed in the Keithley low level measurements handbook.

[Rufus]

10 nV would be the voltage between the two pieces of copper at both sides of the nickel. I got the number from a table with Seebeck coefficients relative to copper from the NI website (10 uV/K). Seebeck coefficients are usually quoted relative to platina, which is not that useful since platina wires are fairly rare in electronics.

1 cm^2 of 25um Nickel plating with 1mK across it will be transferring about 1/3rd of a watt. 1/3rd of a watt will warm 10g of copper by 1mK in about 10ms.

Hard to see the plating being significant. The leads would have to be acting as heat sinks for the equipment and you are only going to see effects of differential heat sinking between leads.

[alm]

1 cm^2 of 25um Nickel plating with 1mK across it will be transferring about 1/3rd of a watt. 1/3rd of a watt will warm 10g of copper by 1mK in about 10ms.

I agree that this sounds very unlikely, although I know some instruments where leads were substantially warmer than ambient. Those are not usually precision voltage references and multimeters, however. Still, people observe effects with plated wire and connectors. Is there something else going on that produces a thermal EMF due to the temperature differential between two jacks, even though both are mostly copper with a thin layer of plating?

This is why every high-end ultra precision instrument (that I know of) uses gold-plated CuTe binding posts.

But do these high-end instruments (eg. the Pomona low thermal EMF binding posts) use regular copper-nickel-gold plating, or an alternative that might produce less EMF? Gold over copper is quite good, for example, but the gold gets contaminated by the copper and eventually tarnishes. I'm sure there are other options.

[quarks]

All low emf posts/terminals and spade lugs I know of, I belive are direct gold plated or unplated. I do not think/hope there is nickel under the Gold plating.

[alm]

As long as you can find direct gold over CuTe, like the Pomona binding posts presumably are, then I agree that would be a superior solution. But you can pretty much assume that any commercially available gold plated copper that is not expressly marketed as low EMF will have nickel plating in between. And that would mostly defeat the point of buying low thermal EMF binding posts.

If you are not able to find banana plugs without nickel plating, then another option might be chemically stripping nickel plated banana plugs. t involves some strong acids, so I wouldn't want to try it at home, but some people on the volt-nuts experimented with it. You may also find a company to do this for you. This is what the former NWML reportedly did.

[robrenz]

Keep in mind in your calculations that the "µ" in plating thickness is micro inch not micro meter or micron.

[alm]

Unless its copper plating over FR-4, otherwise known as a PCB . Then it's µm for consistency sake.

[robrenz]

Ok but all the connectors I have checked out the plating thickness is in micro inches.

[robrenz]

This is why every high-end ultra precision instrument (that I know of) uses gold-plated CuTe binding posts.

But do these high-end instruments (eg. the Pomona low thermal EMF binding posts) use regular copper-nickel-gold plating, or an alternative that might produce less EMF? Gold over copper is quite good, for example, but the gold gets contaminated by the copper and eventually tarnishes. I'm sure there are other options.

The 34420A nano volt/micro ohm meter uses bare copper connections and how they keep it clean

[robrenz]

I was not trying  to be a nitpicking jerk with that, sorry it came across that way   It was an interesting example of a reputable manufacturer choosing to not use a plating at all when nV are involved even though it has the disadvantages of possible/probable oxidation issues.  It is also my defense that Deoxit is not snake oil.

[quarks]

Good candidate CuTe binding posts are from Pomona (3770), Multi-Contact (PK4-TS), IET (BP1000) and jswilley (2758).

I double checked these parts and found statements of direct Gold plating only at
http://jswilley.com/Accessories.html

For all others I still believe there is no nickel underneath the Gold. But it is strange that it is not documented.

[grenert]

I was not trying  to be a nitpicking jerk with that, sorry it came across that way   It was an interesting example of a reputable manufacturer choosing to not use a plating at all when nV are involved even though it has the disadvantages of possible/probable oxidation issues.  It is also my defense that Deoxit is not snake oil.

Anyone know if adding the Deoxit Preservit/Shield to keep the contacts from oxidizing after cleaning would cause a problem (EMF-wise)?

[robrenz]

Caig technical page read the pdfs linked on the left side of the page.   scroll down to the last three charts at the bottom and take a look at them.

[Alexei.Polkhanov]

To keep the thermal EMF's down, I would like to find some banana plugs made of gold-plated ...

If not a secret what exactly are you measuring and what is your setup that make it so important. I believe for certain measurements there are ways you can compensate for EMF if it is important. Bridges, 4, 5 terminal sensing, AC sensing instead of DC etc. How did you concluded that effect of EMF is greater than your measurement uncertainty for whole setup? What are your calculations?

[Alexei.Polkhanov]

Thermal EMF's are a known problem that comes into play in a major way below 100uV (or so).  A 6.5 digit DMM is a 1ppm device that can get down to 100nV (or even lower on some models). 
...
The *ideal* binding post would be something that ...

As for doing all of the calculations (etc.)-- there is simply no need for this.  It is a well known problem that you can read about in various peer-reviewed publications.  The solution is also well known, but the components for what I want to do are scarce-- hence, this thread...

Well yes, you jumped right to the solution. Only piece of information I got is that you are measuring VOLTAGE, but what for? Is it a sensor with voltage output? Thermocouple? Pressure sensor? Is there a signal conditioning involved? What is the accuracy of the sensor itself? - this is what I meant by "whole system".

I looked in a user manual of my DMM - it looks like gold is not what I need - see attached image. Table is a bit confusing but I assume it is C above 0 otherwise it would be in Kelvin.
Cadmium-Tin solder must be Cd70Sn30 (70% cadmium, 30% tin, that is by weight I assume).

From all reasonable combinations looks like you should expect 0.3 * 25C = 7.5uV EMF error. That is why I am wondering what kind of measurements are you making where 7.5uV matters?

[quarks]

I have made some experiments to build my own high quality low EMF adapters.

Here are two examples (see att. pictures).
The first/easiest is to use POMONA 3770 + 3772 + 4892.
EMF wise, the 4892 is the weak point (Gold plated brass and Gold plated Beryllium Copper spring).
Nevertheless I find the result quite good.

The second one needs some more work and I need to order more parts to finish it.
Baseplate is Teflon for max. isolation resistance, binding posts are Multi-Contact PK4-TS. Weak point is also the plug (Gold plated brass and Gold plated Beryllium Copper spring, but I will probably order goldplated CuTe replacement as soon as I have an idea how I can put them together without soldering (want to crimp or screw it)).

[mos6502]

Doesn't your multimeter offer something like a short calibration or offset removal? That should be enough to eliminate any parasitic thermocouple effects, as long as the temperature in your lab is constant.

Of course, if you're measuring resistance instead of voltage, it's better to use an LCR meter with 4 wire connections. The AC will eliminate any thermocouple offsets.

BTW, do you have a pic of those split banana plugs? Are they available separately?

[robrenz]

Nicely done quarks! Next step is a lathe and some tellurium copper bar stock

[quarks]

@mos6502,
I do not know if you asked me or DiligentMinds.com, but we both have the same TEK DMM4050 and want to be able to plug in copper wire or spade lugs for lowest possible EMF meassurement, that is why an adapter like this is needed.
About the split banana plugs, there are Fluke TL2x4W-PT II I (2x4-Wire Ohms Tweezers) Test Leads, which I do not have.

@robrenz,
 I need to find a good way to put the binding posts and the bananas together. Without having your skills I need to crimp or maybe screw them together.

[robrenz]

By any chance does the set screw thread in the banana plug match the stud thread of the banana jack? If so you could remove most of the banana plug on the right leaving just enough threads to use it as the nut to hold the jack in the Teflon block. That would also require making the stud length of the jack just the right length so the Teflon gets grabbed just before the jack threads bottom out in the plug threads. Since the bearing area of the plug is very narrow it would easily extrude the Teflon so you would want to use a washer to get a larger bearing area on the Teflon.

[quarks]

The MC PK4-TS have 4mm thread and the banana LS410-TS CuTe 4mm plugs (see att. pict.) I want to order have no thread (but solder Connection, which I do not like). If I cut it as you suggest, drill a hole and make a 4mm thread inside, there is 0.5mm wall thickness left, which should be stable enough, but the Gold plating is gone.

If I switch to Pomona 3770 with the imperial thread, it is just a little bit larger than 4mm, maybe a pressfit is then just stable enough, what do you think?