ESI SR1 Standard Resistor Current Rating

[deepskyridge]

I just got a ESI SR1 100 ohm resistor for my lab. It's current rating is 71 ma.

Some others have a 100 ma. rating.

Any idea why there is a difference ?

Thanks
Gary

[Dr. Frank]

71mA are 500mW, 100mA are 1W.
Both values are way too high for a standard resistor, 10mW should be the limit to avoid drift.
These currents might be the absolute maximum currents, I assume.

If these resistors are somehow cooled, e.g. by oil, that might be another story.

So the difference of 0.5 vs. 1W will be directly related to the drift specification of the different resistors.

I also think, that the nominal current in both cases will be 10mA, for minimum drift.

Frank

[deepskyridge]

Thanks for the info, the resistor should be here today.

I will post teardown pictures in the next couple of days.

Thanks
Gary

[Dr. Frank]

Uhh, if that's a good standard resistor, I wouldn't do a teardown.. too risky to destroy something, or to bust the calibration, if existent.

Frank

[TiN]

Then it will turn into repair thread! 

I'd join in recommendation of using standard resistor at low currents only (<few mA levels) or only for comparison with working resistors.

[Edwin G. Pettis]

The SR-1 series has a single mica carded resistor, depending on the value, different sizes of wire; other than that, they all look pretty much the same inside.  As long as you are careful , there is nothing inside that you will damage by opening them up.  These are essentially bottom level transfer standards, not quite as good as SR-1010s but essentially the same resistors inside.

The resistor's power ratings are valid as long as you observe that operating them at higher power levels are going to cause some shift in value, running them at 0.5W or 1.0W is allowed as long as you understand the consequences of doing that.  If you want to use them as a genuine transfer standard then you should keep the power below 100mW at most, 10mW is very conservative.

[Vgkid]

The ESI SR1 resistors are held together by the indented cups(not sure what they are called) around the bottom bannana jacks. I have pulled apart my 1k SR1.
E.G. Pettis is correct on their construction.

[zlymex]

I had several SR1 resistors and 100 Ohm is one of them. I opened them the moment they arrived and here are some photos of my SR1-100.
It rated 100mA from the cover and from the datasheet(of Tegam)

[Dr. Frank]

What, no Kelvin connection?
And I was right, 1W is absolute max. rating, giving 100ppm drift by heating, 10mW will give quite reasonable 1ppm. 100mW already equals the initial calibration.
Anyhow, this is no real "standard".
Frank

[manganin]

Anyhow, this is no real "standard".

According to my experience 1-2 ppm per Celcius and per year can be expected for 100 ohm and above after the first 10 years. And the 50 ppm "lifetime" drift mentioned in the data sheet is also realistic. Quite good if you think about the very simple construction and the low price.

There *IS* one exception: the SR1 10K resistor is absolutely stable with time, temperature, and humidity.  I suspect it is made with the same Evanohm wire as are the SR104's

manganin is used for 100R and below.  10K and above are most likely made with Evanohm wire.

They are all manganin (family). Soldered connection wouldn't be possible with Evanohm wire.

What, no Kelvin connection?

Threaded banana plugs (supplied with the resistor) on bottom and binding posts on top. An economy solution but works fine.

[zlymex]

They are all manganin (family). Soldered connection wouldn't be possible with Evanohm wire.

I have a box of Evanohm wires, I can solder them easily with the help of a special flux powder.
The powder is very similar to rosin(none-toxic, none corrosive) which I bought 9 years ago but never seen it again.
I use this powder throughout the years to solder hard-to-solder metals such as steel, stainless steel, electric furnace wire, 18650 cells.
(the last two pics were from the seller)

[manganin]

I have a box of Evanohm wires, I can solder them easily with the help of a special flux powder.

The question is if it makes a connection that is stable and reliable in the long term.

Edwin Pettis would probably be the best person to answer.

[zlymex]

The question is if it makes a connection that is stable and reliable in the long term.

Edwin Pettis would probably be the best person to answer.

Without the flux, solder and steel has phobic response, solder hates steal.
With the flux, solder and steel has philicity response, solder likes steal. That is, the solder will soak the steal and completely wrap around the steal just like solder to copper.

As far as I know, Edwin Pettis's soldering is not tin solder for Evanohm.

[Edwin G. Pettis]

According to metallurgists and industry experience (mine included), you cannot solder to Evanohm alloys and get a bond like with copper or silver, what you get is a mechanical joint in effect, better than a crimped joint perhaps but still only a mechanical joint.  The long term stability of this joint is guaranteed to be less stable than a welded joint.  As evidenced by long term measurements of resistors with non-welded joints, they are significantly less stable with time.  They are unstable even sitting on a shelf and will exhibit high drift with time.

If you can tolerate long term drift of 100 PPM or more over time, then your mechanical joint will suffice, otherwise you are wasting your time soldering.  While there are fluxes which will adhere to steel and stainless, they cannot be used with Evanohm and there is no flux that works with Evanohm alloys.  Welding is the only option for stability and it must be done with compatible materials, you cannot weld Evanohm to copper although you can weld it to alloy 180 with fair success.  All of my resistors are welded with 100% compatible materials.

[zlymex]

I would imagine that your powder flux is some kind of acid-- perhaps boric acid?  If so, you would want to neutralize the solder joint with something to prevent future corrosion.  Have you had the powder analyzed?

I didn't have the powder analyzed but it looks to me not acid, and I checked my old solder joints with the flux without any sign of corrosion.
There are many people that brought the flux powder and it seems all give positive feedback.  Here is the original seller's thread and some other thread of comments. There are argument as whether the powder is boric acid or not but no conclusive result. More people believe that it is a mixture of many substances.
http://bbs.leowood.net:88/forum_read.asp?id=1621830
http://www.crystalradio.cn/thread-188820-1-1.html

[HighVoltage]

I just bought an ESI SR1-10k in "new" condition with original paperwork.

On my 3458A it shows a value of 10.000,03 k
The calibrated value printed on the SR1 is 10.000,0 k from 9-29-86
This was probably a very special resistor in 1986.

And the first thing I did after measuring it, was to open it up.
Here are some pictures.

I was surprised that it had only 2 terminals and not a true 4 terminal Kelvin connection.
But after opening it up, it was clear, that there was no space for two more terminals.

[HighVoltage]

Interestingly, this SR1 comes with 3 screw-in gold plated banana plugs for the back.
This is obviously to plug this resistor directly in to an instruments binding posts without wires.

And it fits perfectly for the 3458A
However, the ground / guard plugs in to the current input of the 3458A, because the guard terminal is on the bottom left of the 3458A

So, my question is, for what instruments was this SR-1 made to plug in directly, including the ground / guard plug?

[Echo88]

For example the PREMA 6047/8 should fit:

https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/?action=dlattach;attach=80175;image

[alm]

Would there even be a point in connecting the guard like this? As far as I know DMMs do not use a driven guard, so unless you connect the guard to low, you basically connect one floating shield to another floating shield. I would expect it to be more effective to leave the guard connected to the force low terminal and either connect the shield to the same low, or to earth ground.

[Conrad Hoffman]

Be careful with mystery chemicals. Decent fluxes can contain fluorides and other nasty stuff. I've fooled with both soft soldering and silver brazing of nickel based resistance alloys and it may be possible to get a reasonably stable joint with the right materials and techniques, but welding is the way to go, especially for small wire.

[ManateeMafia]

Fluke 8502A/05A/06A have the guard below the LO.

[HighVoltage]

Thanks, I had never seen a Fluke 8502A before, so I had to look it up, picture below.
Too bad this did not become a standard, with all of the three most important connectors in one line.

[Vgkid]

Solartron 7065/55 has the guard below lo.

[saturnin]

Interestingly, this SR1 comes with 3 screw-in gold plated banana plugs for the back.
This is obviously to plug this resistor directly in to an instruments binding posts without wires.

And it fits perfectly for the 3458A
However, the ground / guard plugs in to the current input of the 3458A, because the guard terminal is on the bottom left of the 3458A

So, my question is, for what instruments was this SR-1 made to plug in directly, including the ground / guard plug?

You can plug in your SR-1 to 3458A's Hi/Lo Sense binding posts (in this configuration, the guard terminal is connected too). Then connect 3458A's Hi/Lo Inputs to the top of SR-1 and enable 4W mode...

[Edwin G. Pettis]

The SR-1 series are what are termed 'working' standards, they have the least tightest of specs and of course cost less.  They were intended to calibrate medium accuracy instruments that were two terminal, such as Wheatstone but were also given the additional contacts on the back so they could be stacked in series/parallel and a quasi-four terminal hookup.  You can of course take an more accurate reading of them and use that right away but their short/long term drift is not that great compared to transfer standards.  They were not processed for higher stability like the SR-1010s, operating them at or near their maximum current ratings briefly will have little effect on their stability, running them at or near maximum current for sustained periods will cause them to drift a bit but they will not get hot enough to change their characteristics permanently.

When these were originally designed, instruments like the 3456A or 3458A did not exist nor are they designed to 'plug in' to them, they have the standard 0.75" C-C spacing which has been around for decades but that doesn't mean they were meant for something that didn't exist yet.  They are not really good enough to cal a 3456A even with the pseudo four terminal hookup, they were meant for instruments no better than 0.05% at best, something on the order of 4.5 or possibly 5.5 digit resolution with 2-wire resistance measurement.  The third terminal is not so much a guard terminal as a ground terminal for the case to minimize noise pickup.  Remember the minimum accuracy rule, the reference should be at least 5 times more accurate than the instrument's accuracy, 10 is even better.

Never mind the market hype, these are not one off copies of SR-1010 resistors even if they look just like them, they are not processed the same.  I have a copy of the BOM price list for the SR-1 series, the resistor is one of the cheaper parts in it.

 

[Micke]

I also bought a NOS SR-1 10k (was 25% discount at the moment) from the same seller as HighVoltage, mine was slightly older from 1978, but had drifted same amount upwards 
Written value is 9.999,9k and I measured 10.000,02k with 3458A at work.