0,1mm is easy to achieve with steady hand, with aditional gentle filing it should be possible to improve precision by another order of magnitue. As you said strain would be a major concern. The initial strain from the wining process could be eliminated by annealing the assembly after winding.
On the other hand since glass has much lower tce than the metal wire, we could expect the wire to slacken when temperature is hot and to tighten (and get strained) when cold. If the wire is wound in figure of eight fashion there would be enough slack to prevent this from happening in most cases.
To prevent corrosion either immersion in oil or enclosing the resistors in inert gas should do the trick. It seems like a big deal but after the trouble undergone to achieve 1 ppm it should seem trivial.
You cannot really anneal it. The wires are coated in varnish, so the heat would destroy the varnish. When making the Thomson Standard one Ohm resistors, they anneal the wire at something like 500 degC from memory. Not sure if it is possible to do that with a large value resistor on a former.
The other was that soldering was very risky business and I always needed to have a few extra matched resistors to fine tune a decade after it was all assembled. This was true for the wire wounds as well, and I did use heat sinks.
A 1K pot is way to big. You will regret it. If you have resistors that match between decades to at least 0.05%, you don't want the pot doing much more then that. I would have the 24.3K (25ppm) and a 820 ohm (1% 100ppm) in series and put a 10K pot across the 820 ohm resistor. I prefer this circuit as only a fraction of the current is going through the pot wiper.
There are some really good "800 series" wires from Kanthal (formerly H.P. Reid in Palm Coast, Florida, USA) that have both low TC and low thermal emf to copper. So who's ready to machine up some tellurium copper binding posts?
There are some really good "800 series" wires from Kanthal (formerly H.P. Reid in Palm Coast, Florida, USA) that have both low TC and low thermal emf to copper. So who's ready to machine up some tellurium copper binding posts?
I can't seem to find any mention of 800 series resistance wire. The best wire from Kanthal current products seems to be +/- 15ppm/C coefficient wire.
http://www.kanthal.com/products/materials-in-wire-and-strip-form/wire/resistance-heating-wire-and-resistance-wire/list-of-alloys/
The tellurium is for machinabilty not electrical properties. Pure copper is nasty to machine.
A thing I have never understood.
Why is gold plated tellurium copper alloy better for EMF then gold plated copper or gold plated brass terminals? I cannot see how the metal under the plating makes any difference.
Now if you are talking about unplated terminals , then it does make sense to me.
Richard
As far as I can see, if the connector is gold plated, and you are not touching the underlying base metals at all, then any gold to base metal EMF voltages are irrelevant. All that matters is the test lead to gold EMF voltage.
Richard
As far as I can see, if the connector is gold plated, and you are not touching the underlying base metals at all, then any gold to base metal EMF voltages are irrelevant. All that matters is the test lead to gold EMF voltage.
Richard
I am not an authority, I am just thinking out loud. IMO you cant pick and choose what different metal to metal joints you are going to account for. Every single connection in a circuit that is a dissimilar metal counts as a themocouple. and the connection of a plating to the base metal counts just as much as a copper wire crimped to the gold plating.
As far as I kinow the EMF of a thermocouple is not generated at the junction of the two metals. It is generated in the thermal gradient of each wire. So how does this apply when you have few microns of plating?
Hold on, does oxygen free copper 101 turns into copper oxide? The HP34401a input connectors are all made of copper.
What about the precision multi pulse capacitor discharge spot welders for making the connections to the resistance wire. Would the extremely quick but high temperature be worse for long term stability.
Just thought that it would be interesting to show a few pictures of an old ESI dekapot DP1311 (1OK) Kelvin Varley divider for reference on how a commercial product was built.
note; the 34401 has been set for an input R of 10g ohm.
(Since this is not a real review, and not many would be interested anyway I thought this would be the best place to post, hope no one minds).
cheers
John