Low ohm precision resistor standard and testing

[tszaboo]

Calibrating a 10mOhm shunt is "easy".

What is your procedure to calibrate a 10 mOhm resistor in your lab?
At how many points between a low current and full scale current do you check for linearity?


Very interesting with your 4000A of multiple current sources.
With my two Agilent 6V / 100A PSUs I just can reach 200A.

Well, we did characterization, and concluded that 2x2 point calibration is good enough for our specification.  I think we verified hundreds different scenarios on a new design. Different output voltages, currents, rise and fall times...

2x2 because sourcing and sinking current was separately calibrated. Keep in mind that we were selling machines, which had two to four ranges (50A the largest), and 80-120 channels, so calibrating these took hours. Any other measurement point would extend the calibration time by hours, which would mean increasing the lead time.

The procedure for the 50A range is the following. We had our reference 2 mOhm shunt. It was an Isabellenhütte RUGZ. 1ppm/K, 250W max power (we used 5W from this), on a big heatsink. Full scale was 100mV, this went into a 34410A. The two points were 10% and 90%. The reference shunt went for traceable calibration every year. I think it should have been more often, they drifted quite a lot.

We weren't just calibrating just the shunt of course, but the machine as a current source. Together with the AFE and the digital control values. But the principle is the same.
And I write past tense, because I'm not working there anymore. In fact, I think it was 2mOhm shunt built in, not 10... Memory.

In principle, if one can design a current source, then can calibrate any kind of shunt. The downside, you need a lot of equipment for it. For example, if we wanted to to (mis) use our stuff to calibrate a 0.2mOhm shunt, with 500A, it was just the matter of connecting them in parallel. Because the AFE is built in. But if you want to do it at home, you need 11 mulitmeters for it, simultaneously measuring all the 2mOhm shunts and the 0.2mOhm .  (Or do them sequentially, but then I'm not sure, how valid the measurements are then. )

The interesting part is, that the errors are not increasing significantly, because the voltage measurements are like a magnitude more accurate then current measurement. So if you have 10 pieces of 0.03% accurate current sources, and you use these to calibrate a shunt, then the shunt is something like 0.035% worst case.

[doktor pyta]

@Tin, HighVoltage

It seems that DCCT with higher nominal current have better parameters (noise, linearity etc related to nominal current). It may be considered as higher SNR situation- there is more magnetic flux with certain fluxgate sensitivity and earth's magnetic field.
Below photo of my first test setup. I'm building 1A, 10A, 100A precision current source using 900, 90 and 9 turns wound 900A DCCT. First tests using 90 turns are very promissing at 10A current.

[HighVoltage]

Perhaps if you really interested in high accuracy high current stuff, you'd be better off getting LEM Ultrastab DCCT.

Thank you all for the good input.

The Ultrastab IT 600-S or the IT 405-S are too expensive and I have not found a good low cost source source.

But for now I bought one of the DANFYSIK Current Transducer ULTRASTAB 867.
May be this one will work for my purpose.

Type: 867-700I
Nominal current: 700A DC
Ratio: 1:1750
Output +/- 400 mA

I am surprised how difficult it is to make some precision current measurements, even at only 1A

[Robert763]

I'm late to this thread but acouple of people have asked "why 60mV" value for shunts. This is not the only value used, 75mV seems quite common in the UK, 50, 100 and 150mV are also seen. It's related more to the impedance of the matching meter than anything else. Most panel meters for use with shunts have a 1mA movement. A coil resistance of 60 Ohm is not unusual for a 1mA movement, and a lower value will allow for a series calibration resistor.

[HighVoltage]

I'm late to this thread but acouple of people have asked "why 60mV" value for shunts. This is not the only value used, 75mV seems quite common in the UK, 50, 100 and 150mV are also seen. It's related more to the impedance of the matching meter than anything else. Most panel meters for use with shunts have a 1mA movement. A coil resistance of 60 Ohm is not unusual for a 1mA movement, and a lower value will allow for a series calibration resistor.

Thanks for this explanation.

I bought a couple more shunt resistors, one with 75mV and one with 150mV
Soon I will start measuring their linearity, when the Danfysik 867-700I will arrive.

I am expecting a really good and calibrated shunt to be a reliable source for a good current measurement, even at 100A.
But one big question is: How linear are these shunt resistors in real life?

[HighVoltage]

Perhaps if you really interested in high accuracy high current stuff, you'd be better off getting LEM Ultrastab DCCT.
They pop up on ebay from time to time to relatively cheap prices. I've got two with best linearity, Ultrastab IT-600S.
It is 1:1500 ratio CT, and with linearity spec <1ppm, offset error <20ppm and tempco <0.2ppm/K, so high chance that your measurements accuracy with these DCCTs will be limited by your burden resistor stability. There are also lower (and higher) current Ultrastab's with similar specs. Buying older Danfysik DCCT (model 867 is same as IT-600S) might save few $$.

CERN using these beasts to calibrate and maintain LHC superconductor magnet currents to ppm levels stability. You can find related research paper, covering their transfer and calibration apparatus. 

Hello TiN,

I am making progress .... to be reported.

On your site you are explaining that you are using the Burden shunt: 2.5 ? Vishay PG foil 1ppm/C 9W, CSNG
Why are you using the 74$/piece shunt resistor?
Why not directly hooking up the Ultrastab output to the DMM current input?

Thanks!

[tszaboo]

The CSNG shunt is better than the one built into the DMM. The Kethley 2002 doesnt have an excellent shunt built in, we are talking about 400 ppm accuracy for 1 year on the 200mA range. Voltage is much better of course. 3458A has 20ppm typical.
Though, I dont know how much sense the low current measurements make. I mean, he is measuring 2.5uV on a 200mV range??

But one big question is: How linear are these shunt resistors in real life?

Depends on cooling. Self heating is the biggest enemy. Look at the de-rating curves of a shunt, tells you a story. If the shunt is rated for 10W and it has a curve, which is flat to 80 degrees, and from 80 to 125 degrees it falls down to zero. That would mean, that in normal conditions, 10W will heat the shunt 45 degrees. Multiply that with the tempco...
Also, some heatsink mounted shunts will tell you the thermal resistance, but that value is usually better than this.
And a 4 terminal shunt will have a value, like 1 mOhm. Which often time means, there is 1mOhm between the sense terminal's connection, but not the entire shunt. So the total resistance is higher, leading to more self heating...

[Micke]

If anyone is interested in a high current resistor with super-performance (TC=±1ppm/°C  !!!) at great price, ELFA Distrelec have 2 m ±0.1% 250W TC=±1ppm/°C (Isabellenhütte RUG-Z-R002-0.1-TK1) at the great price 79.18 Euro (normal price 589 Euro, yikes!) you can put 350A continuously through this thing (with proper cooling of course...)
https://www.distrelec.biz/en/power-resistor-mohm-250-isabellenhuette-rug-r002-tk1/p/16057581?q=rug-z&page=1&origPos=1&origPageSize=25&simi=96.57

[HighVoltage]

Thank you Micke
I have placed an order, this will be a good comparison of linearity between a few shunt resistors and the LEM

Thank you NANDBlog for the explanation.
I will try a few current measurements with different DMMs, once I am setup for the next step.
May be I will get such a precision shunt later.

[TiN]

NANDBlog is right, as with high power systems PCR of the used resistors has big impact on accuracy. That is why I also chosen CSNG which is rated 9W to support maximum output current of the LEM's 400mA output.

That shunt is very interesting price, I wonder if this distributor can ship internationally to private customers..

[HighVoltage]

That shunt is very interesting price, I wonder if this distributor can ship internationally to private customers..

If you can not get it shipped, send me a PM and I will arrange it for you.

[2N3055]

I just ordered one RUG-Z-R002-0.1-TK1....... I needed to measure 50-100A currents many times and had to borrow shunt.. this time I will have mine.
I'll put it in case and stick it on CPU cooler...  Probably also a PT1000 for a good measure... Have to figure out mechanical package...
Anyways, thanks a lot Micke !!

[Micke]

Hi!
Always nice to help out fellow VoltNuts...    I also ordered one Isabellenhütte resistor, got it this Thursday, very nicely packed in a plastic box from factory (what else to expect...)
Heavy beast, weighs almost 300g...
It has a stamped number on the solid copper base, perhaps unique serial number?  or batch number at least...
Will put it in an aluminum box with four binding posts, high current binding posts will be 63A rated, Kelvin output will be a dual gold plated 19mm spaced version 
Getting like a 0,25W TC=1ppm/K resistor is expensive enough, but 250W... 

[HighVoltage]

Hi!
Always nice to help out fellow VoltNuts...    I also ordered one Isabellenhütte resistor, got it this Thursday, very nicely packed in a plastic box from factory (what else to expect...)
Heavy beast, weighs almost 300g...
It has a stamped number on the solid copper base, perhaps unique serial number?  or batch number at least...
Will put it in an aluminum box with four binding posts, high current binding posts will be 63A rated, Kelvin output will be a dual gold plated 19mm spaced version 
Getting like a 0,25W TC=1ppm/K resistor is expensive enough, but 250W... 

Thanks for the heads up on the 300 gramm weight, I did not see that.

I ordered two of these Isabellenhütte resistors, too good of a deal to not take advantage of.
And I also wanted to put one in a box with binding posts and separate Kelvin output to good quality binding posts.
What gold plated bindings posts will you be using?

But why are you planning to have a "dual" output?
May be something to consider?

[TiN]

HighVoltage, thanks for the offer.

I'm trying to get account first, since:

Dear customer,
registration for customers outside EU has to be done manually. Simply send us your company details to export@distrelec.com and we will send you login details to our web for further purchase online. Thank you.
Your Export Team.

.

I've sent them an email, since I'm no company.

[0.01C]

Perhaps if you really interested in high accuracy high current stuff, you'd be better off getting LEM Ultrastab DCCT.

Thank you all for the good input.

The Ultrastab IT 600-S or the IT 405-S are too expensive and I have not found a good low cost source source.

But for now I bought one of the DANFYSIK Current Transducer ULTRASTAB 867.
May be this one will work for my purpose.

Type: 867-700I
Nominal current: 700A DC
Ratio: 1:1750
Output +/- 400 mA

I am surprised how difficult it is to make some precision current measurements, even at only 1A

Ultrastab IT 600-S=Ultrastab 866 I guess ,you can find it in eBay @$89 , I bought a lot.

[Micke]

...
Will put it in an aluminum box with four binding posts, high current binding posts will be 63A rated, Kelvin output will be a dual gold plated 19mm spaced version 

Thanks for the heads up on the 300 gramm weight, I did not see that.

I ordered two of these Isabellenhütte resistors, too good of a deal to not take advantage of.
And I also wanted to put one in a box with binding posts and separate Kelvin output to good quality binding posts.
What gold plated bindings posts will you be using?

But why are you planning to have a "dual" output?
May be something to consider?

I wrote a little bit confusing I realize now... I will only have one output, a couple of years when ELFA had a big sale, I bought very nice gold plated double binding posts very cheap (think they are USA made) 

[2N3055]

Sense connectors are not a problem. I have trouble finding what 63A to 100A posts to use.. I only found 63A from Hirschmann , but they have more contact resistance than shunt (they will heat up), and brass ones from Schützinger (3 models 63A/100A/200A). They have 0,4 mOhm, much better, and 63A ar not that big...
Did anyone find other type?

[HighVoltage]

My first Isabellenhütte 2 mOhm resistor arrived already.
RUG-Z-R002-0.1-TK1

I think I will use some M6 studs, may be out of copper for a direct connection through an aluminum box,
instead of the usual binding posts.

Similar to this:

[tszaboo]

I would order a pair. Unfortunately, I know how much it costs to calibrate these. I remember the price was some 350-500 EUR each. And I'm sure everyone sees the 1K hour drift value, .2% for the .1% resistors. And they drift quite a lot, even if you don't use it near full load. I think our resistors never went above 20W load (8mOhm 50A) and the drift was still significant. But of course these were used a lot, hours every day.

That being said, they are excellent resistors. Make sure to cool it well. Bolting down to an aluminum extruded heatsink works quite well.

[HighVoltage]

I would order a pair. Unfortunately, I know how much it costs to calibrate these. I remember the price was some 350-500 EUR each. And I'm sure everyone sees the 1K hour drift value, .2% for the .1% resistors. And they drift quite a lot, even if you don't use it near full load. I think our resistors never went above 20W load (8mOhm 50A) and the drift was still significant. But of course these were used a lot, hours every day.

That being said, they are excellent resistors. Make sure to cool it well. Bolting down to an aluminum extruded heatsink works quite well.

You are bringing up some good points.

Calibration.
Do you know how they are calibrated and what labs are capable of it to full current of 350A?
May be we can calibrate them ourselfs with the LEM / DANFYSIK sensors a few of bought?

Drift
Are you talking about a long therm drift?

Heaksink
I have been looking for a large heatsink already, that will fit with the resistor together, in to a nice aluminum box.
 
Linearity
I am really looking forward, testing these for linearity up to 100A, in comparison to the Danfysik / LEM. (may be 200A, if I hook my two agilent power supplies in parallel)

[tszaboo]

I would order a pair. Unfortunately, I know how much it costs to calibrate these. I remember the price was some 350-500 EUR each. And I'm sure everyone sees the 1K hour drift value, .2% for the .1% resistors. And they drift quite a lot, even if you don't use it near full load. I think our resistors never went above 20W load (8mOhm 50A) and the drift was still significant. But of course these were used a lot, hours every day.

That being said, they are excellent resistors. Make sure to cool it well. Bolting down to an aluminum extruded heatsink works quite well.

You are bringing up some good points.

Calibration.
Do you know how they are calibrated and what labs are capable of it to full current of 350A?
May be we can calibrate them ourselfs with the LEM / DANFYSIK sensors a few of bought?

Drift
Are you talking about a long therm drift?

Heaksink
I have been looking for a large heatsink already, that will fit with the resistor together, in to a nice aluminum box.
 
Linearity
I am really looking forward, testing these for linearity up to 100A, in comparison to the Danfysik / LEM. (may be 200A, if I hook my two agilent power supplies in parallel)

We were using a local Belgian cal lab, they were calibrated up to 50A, not to their full capability. I think I saw the invoice once, I'll try to recall what was the lab. They produced a very accurate reading, I think up to 7-8 digits. We sent our 3458A also there.

We were using these as a reference resistor, "gold standard", to calibrate the shunts built into the equipment we sold. The spec of our equipment was 0.03%, max current 50A. We were using the 2mOhm TK1 part, along with a bunch of other values for less current.

Drift is the long term drift. The datasheet specifies 0.2% drift @2000h, nominal load, 85 degrees. We were using it below maximum load (in fact below 2% load and almost room temperature) and it was causing issues with our spec. Like it drifted more than 0.03%, and we had to re-calibrate stuff. While the Arrhenius Equation would have suggested less drift, so careful, when trying to apply it. Also, these were used for 5+ years, so they had time to do initial drift. Might be, that some hysteresis or other physical phenomenon...

[HighVoltage]

We were using a local Belgian cal lab, they were calibrated up to 50A, not to their full capability. I think I saw the invoice once, I'll try to recall what was the lab. They produced a very accurate reading, I think up to 7-8 digits. We sent our 3458A also there.

We were using these as a reference resistor, "gold standard", to calibrate the shunts built into the equipment we sold. The spec of our equipment was 0.03%, max current 50A. We were using the 2mOhm TK1 part, along with a bunch of other values for less current.

Drift is the long term drift. The datasheet specifies 0.2% drift @2000h, nominal load, 85 degrees. We were using it below maximum load (in fact below 2% load and almost room temperature) and it was causing issues with our spec. Like it drifted more than 0.03%, and we had to re-calibrate stuff. While the Arrhenius Equation would have suggested less drift, so careful, when trying to apply it. Also, these were used for 5+ years, so they had time to do initial drift. Might be, that some hysteresis or other physical phenomenon...

Thanks for this great info.
Can you share, which lab in Belgium you are referring to, that makes 50A calibration?

[tszaboo]

I believe it was LHM calibrating our shunts.
https://www.lhm-instrumentation.eu/kalibratie.html

It took me some time to remember.

[HighVoltage]

I made a quick test with my first Isabellenhütte 2 mOhm resistor RUG-Z-R002-0.1-TK1
At least on my Keithley 2460 it is spot on.

Next I will do the same test with a switched polarity