ESI Resistance Standard SR1010 and Standard Resistor SR104

[BradC]

Yes, and nowadays it's quite simple to supervise humidity using integrated sensors. Since i have a lot of desiccant in that SR1010, i won't touch it any more, but observe humidity inside.
[..]

  If you don't intend to replace the "consumed" (or "spoiled") desiccant, but just observe the internal humidity, why use a non-zero amount of desiccant in the first place?

The dessicant isn't there to remove humidity, it's there to buffer it.

[dietert1]

Slowly i am coming to grip with our SR1010-1K and here is an example measurement. This is a resistance bridge with a voltage divider 10 V to 3.3 V (3x 1 KOhm) and one 1KOhm resistor serving as a 6.7 mA current source to the DUT, which is an Alpha Electronics HCZ500R00 (Vishay hermetic foil). The DUT has been in a 40 °C oven for about a week. Bridge is driven by our Geller reference.
I logged the bridge voltage difference and the temperature of the dry and boxed SR1010 with our two HP 3456A. During the night the SR1010 temperature was following the decreasing room temperature for about 5 hours. Vertical axis is 1 ppm, horizontal is 0.5 K. Red line is a calculated filter, green a line fit.
Now i can correct the SR1010 temperature dependence numerically (linear -1,84 ppm/K)

Regards, Dieter

[dietert1]

After i got about 10 uV of thermal EMF by heating a copper lug between fingers, i checked the materials used in a SR1010. I think the binding posts are gold-plated steel as well as the nuts on them (front and back). The trapezoidal plates inside appear to be copper. This construction is prone to generate thermal EMF. The other SR1010 i am operating in a box, well protected from air movement, generates about 300 nV.
Pomona 3770 copper binding posts fit, but their diameter in the back is about M4 and the holes in the plates are more like M6. And the Pomona posts come with steel nuts. So one needs to buy copper nuts and washers in addition.

Regards, Dieter

[splin]

After i got about 10 uV of thermal EMF by heating a copper lug between fingers, i checked the materials used in a SR1010. I think the binding posts are gold-plated steel as well as the nuts on them (front and back). The trapezoidal plates inside appear to be copper. This construction is prone to generate thermal EMF.

I'm having a very hard time believing that ESI would have used gold plated steel - these aren't cheap items. Have you checked with a magnet? Could someone have replaced them before they came into your possession?

[dietert1]

Of course i can't guarantee the binding posts are original, but the web is full of images of SR1010 and they look like original. And they are identical on the two SR1010 we got from two different sources.
I just checked it, the binding posts and nuts are non magnetic. They don't have the reddish shine of copper nor the golden color of brass inside, but appear white. Maybe silver? I'd prefer copper, since i am using copper lugs and copper cables.

Best regards, Dieter

[Edwin G. Pettis]

The normal material for ESI terminals is Tellurium Copper, it is a dull golden color usually, depends on the alloy's mix.

[Kosmic]

Personally I would not replace the binding post with Pomonas.

What protocol and instruments (meter / cable / connectors) are you using to measure the thermal EMF ?

[dietert1]

I am not really measuring thermal EMF, since it's a dirt effect and i think it will be negligible  once i have the correct materials. A simple test i always do is: Connect one of the resistors with four copper lugs and copper wires to the HP3456A input (input terminals are made of copper). After a minute or two i get a zero reading of -0,6 uV. This is "without" thermal EMF. Then i heat one of the lugs on the SR1010 terminal between my fingers, so there will be a 5 or 10 °C temperature increase. The HP 3456A reading slowly creeps up to about 10 uV and it slowly returns back to -0,6 uV as soon as i take my hand off. This may take two or three minutes. I can repeat that on the other terminal giving -10 uV.

The binding posts are certainly not copper, they look white inside like steel or silver. I guess the SR1010 is from a time when users were satisfied with 1 or 2 ppm results, which are easy to get if you keep the lab at 23 °C and don't touch the setup. In my current bridge experiment i have 3.3 V across the DUT and see a 0.3 ppm difference on voltage reversal, so thermals appear at a 1 uV level.

Regards, Dieter

[Kosmic]

I am not really measuring thermal EMF, since it's a dirt effect and i think it will be negligible  once i have the correct materials. A simple test i always do is: Connect one of the resistors with four copper lugs and copper wires to the HP3456A input (input terminals are made of copper). After a minute or two i get a zero reading of -0,6 uV. This is "without" thermal EMF. Then i heat one of the lugs on the SR1010 terminal between my fingers, so there will be a 5 or 10 °C temperature increase. The HP 3456A reading slowly creeps up to about 10 uV and it slowly returns back to -0,6 uV as soon as i take my hand off. This may take two or three minutes. I can repeat that on the other terminal giving -10 uV.

Interesting, I will try to repro with my SR1010.

The binding posts are certainly not copper, they look white inside like steel or silver. I guess the SR1010 is from a time when users were satisfied with 1 or 2 ppm results, which are easy to get if you keep the lab at 23 °C and don't touch the setup. In my current bridge experiment i have 3.3 V across the DUT and see a 0.3 ppm difference on voltage reversal, so thermals appear at a 1 uV level.

In the manual of the SR1010, ESI don't specifically talk about the binding post but they explain in details how the resistor were made and how they selected materials that create low EMF when coupled to copper. I would be surprise if in the end they decided to use cheap steel binding post.

Technically IET still sell binding post similar to the one used back in the days by ESI. Like Edwin mentioned, they are made of tellurium copper.

https://www.ietlabs.com/binding-posts-bp-1000.html

[dietert1]

The binding posts in our SR1010s look different. They have two different isolators (different shape and different color). The outside isolator looks dark brown and fits into the hole. The inside isolator looks like teflon and has a larger conical hole covering up the other isolators end.

By the way, the tellurium copper of the Pomona 3770 body material appears reddish, similar to the trapezoidal sheets inside the SR1010. If something looks white, probably it has little copper. Even brass doesn't look white. I know this can't replace a chemical analysis.

Meanwhile i ordered copper screws to test whether they work better and will report later.

Regards, Dieter

[MegaVolt]

I checked my SR1010 100 kOhm with a magnet. If it is not disassembled, then the magnet has nothing to be attached to.

[Kosmic]

I am not really measuring thermal EMF, since it's a dirt effect and i think it will be negligible  once i have the correct materials. A simple test i always do is: Connect one of the resistors with four copper lugs and copper wires to the HP3456A input (input terminals are made of copper). After a minute or two i get a zero reading of -0,6 uV. This is "without" thermal EMF. Then i heat one of the lugs on the SR1010 terminal between my fingers, so there will be a 5 or 10 °C temperature increase. The HP 3456A reading slowly creeps up to about 10 uV and it slowly returns back to -0,6 uV as soon as i take my hand off. This may take two or three minutes. I can repeat that on the other terminal giving -10 uV.

Interesting, I will try to repro with my SR1010.

So I tried quickly with non ideal connectors (brass banana), non ideal meter (34401a, the 7.5 meters are sleeping ). Heating for 30 sec with a hot air rework station at 100deg (the binding post was probably a lot cooler) I was able to get around 3uV offset. In 4W Ohm mode it's generating 1 to 2 ppm of noise.

Look like it's less of a problem in my case.

[dietert1]

Kosmic, this was for a binding post like the ones on the IET website you linked?

Meanwhile i found this table in a Keithley 2700 manual:

Table E-1   
Material thermoelectric coefficients   
   
Material                           Thermoelectric potential
Copper-Copper                    0.2 µV/°C
Copper-Silver                    0.3 µV/°C
Copper-Gold                    0.3 µV/°C
Copper-Cadmium/Tin            0.3 µV/°C
Copper-Lead/Tin                  1–3 µV/°C
Copper-Kovar                    40 µV/°C
Copper-Silicon                  400 µV/°C
Copper-Copper Oxide    1000 µV/°C

So it appears the thermal voltages i actual had in my setup (220 and 250 nV) are a good result even for a copper-to-copper contact.

Can anyone explain why Pomona and others use Cu-Te? Tellurium is listed with 500 uV/°C, so i'd guess even a small fraction of Te could distort everything.

Regards, Dieter

[razvan784]

So it appears the thermal voltages i actual had in my setup (220 and 250 nV) are a good result even for a copper-to-copper contact.

I get around 3 uV with Pomona 5291A cables (supposedly CuTe bananas with CuBe springs) stuck into a 34401A (Cu sockets). This is when heating the cable end that sticks into the meter with my fingers. I also see the opposite polarity when heating the other cable, and the offsets take several minutes to decay - which is to be expected.
I guess they are called "low emf" not "zero emf" for a reason
I get about the same behavior with Multi-Contact Multilam bananas (brass body, CuTe spring) and Multi-Contact LS4 (CuBe hollow bananas).
Edit: for comparison, I get around 9 uV for a Pomona 5291A stuck into a Pomona 3770 binding post (also supposedly CuTe), and on the order of 100 uV for a copper-Kovar junction (made with a pin from a hermetic transistor package -- like the LTZ1000 uses ). That is certainly not "low emf". So your binding posts are certainly not steel.

Can anyone explain why Pomona and others use Cu-Te? Tellurium is listed with 500 uV/°C, so i'd guess even a small fraction of Te could distort everything.

It's been discussed on this forum before, apparently the reason is mechanical. Pure Cu is hard to machine, CuTe is apparently much easier, while still retaining "low emf" characteristics.

[Kosmic]

Kosmic, this was for a binding post like the ones on the IET website you linked?

No, I got the old traditional ESI version.

[MiDi]

Can anyone explain why Pomona and others use Cu-Te? Tellurium is listed with 500 uV/°C, so i'd guess even a small fraction of Te could distort everything.

Someone mentioned Cu-Te is used for better machinability and I guess it is harder than pure copper.
For spring contacts e.g. in plugs there is often used Cu-Be, because it has better properties for that purpose.

I had searched for infos about T-EMF for these materials, but could not find anything.

Edit:
The T-EMF for copper to copper should depend on differences of the purity.
Theoretically if only copper from same batch is used the T-EMF would be 0.

[MegaVolt]

Theoretically if only copper from same batch is used the T-EMF would be 0.

Any material is not only copper.

[alligatorblues]

Responding to the OP, I have a SR104 that was manufactured in 1974, and I have a few reasons to believe that is authentic. The cal. Cert says so. The glue under the laminate failed on the back panel when I accidentally lost control of the cover, and let it flop open hard. The whole panel just broke loose.

No big deal to put it back with 2-sided 3M adhesive  material. And the binding posts have no banana jacks on top. They're solid. You have to loosen them up and screw them down on the leads. Which everyone should be doing anyway!

We had Keysight calibrate it, I think to 0.2ppm, if that sounds right. It was something with a '2'. When we got it back we could confirm its drift from 1974 to 2019 was 0.3ppm. I researched to check if this is reasonable, and it is. For one thing, the drift could be in different directions over all that time, thereby offsetting itself.

With no cal in between it's impossible to know. But that is my limited experience with the SR104.

[manganin]

When we got it back we could confirm its drift from 1974 to 2019 was 0.3ppm.

Don't forget the 1990 correction and the drift of the US legal ohm before that.

For one thing, the drift could be in different directions over all that time, thereby offsetting itself. With no cal in between it's impossible to know.

Stress free Evanohm drifts upwards.

[HighVoltage]

Stress free Evanohm drifts upwards.

But how do you know, you have it stress free installed?

[manganin]

But how do you know, you have it stress free installed?

Because the people at ESI knew what they were doing. They were able to release the mechanical stress so well that practically the only drift left was metallurgical.

[alligatorblues]

When we got it back we could confirm its drift from 1974 to 2019 was 0.3ppm.

Don't forget the 1990 correction and the drift of the US legal ohm before that.

For one thing, the drift could be in different directions over all that time, thereby offsetting itself. With no cal in between it's impossible to know.

Stress free Evanohm drifts upwards.

Ok, how do I find out about the correction and drfft of the Ohm?

[alm]

Ok, how do I find out about the correction and drfft of the Ohm?

See this NIST document: Guidelines for Implementing the New Representations of the Volt and Ohm Effective January, 1, 1990. It describes the correction on page 3, and pages 13-18 talk about the drift about the Ohm as it was defined before 1990.

[trobbins]

I can add present-day deviation measurements from seven SR10xx boxes made between 1979 and 1993, and also make some general comments.  Individual step deviations were measured at 21-23C using a (mostly) calibrated ESI 240 bridge and using a SR10xx step(s) of another box as the Standard for comparison.  History of the boxes is unknown, and they came from different labs.

The Factory spread data for each box is from the factory measured table fixed to each box, and afaik are calibrated absolute values, but noting that measurement temperatures ranged from 20C to 24C, and there has been a subsequent 1990 ohm correction.

The 2026 spread results could show the influence of unknown stresses as well as ageing, and the 1k box was noticeably 'worse' than the others, whereas the SR1030 10 and 10k were pretty damn good.

Given each box has 12 resistors, it may be worthwhile to select a 'better' series sequence of steps when using a box as say a 1:2, 1:5 or 1:10 or 10:11 divider.  That selection could minimise the ppm spread quite noticeably, especially if the 'end' step has low average deviation from the other series steps in the divider.  Similarly, an obvious outlier step(s) may conveniently be located at step #1, or #2, or #11 or #12, and so be excluded from typical use, although the series and series-parallel ESI accessories don't allow any flexibility in step choice.

It would be enticing to hope that an SR1030 box showing a relatively low ppm spread nowadays has a better liklihood of steps aging in a symmetric distribution, such that a series, or series-parallel, or parallel connection of steps, or a selected step, was within a few ppm of original factory measurement.

[dietert1]

When i got a 100 ohm and a 1 KOhm step box some years ago, i opened them to do a visual inspection. And those wire wound resistors looked pretty bad. Don't know whether it is plastic/glue/paint aging or whether they were used with oil inside the recipient. Those parts look "hand made" in comparision to those hermetic glas sealed resistors you find in an ancient Fluke 8502A meter. I resisted the temptation to rework the resistors, though.
In the end i installed some integrated I2C temperature sensor (SHT40 or so) inside the recipient and filled its bottom with fresh desiccant. And i made an "air tight" MDF box for the setup with 3 cm of styrofoam thermal insulation. After some weeks i got pretty impressive results. Stability of a bridge built with the 1 KOhm step unit reached 1E-8 for the measurement of a modern 500R metal foil precision resistor. This required a fairly complex compensation scheme for temperature and humidity effects. Maybe easier to put the whole setup into a constant temperature oven.

Regards, Dieter