Calibrating a 100A DC shunt

[enut11]

Apart from an initial dip at the beginning, the DUT still has a lower temperature coefficient.

The maximum difference in tempco is on the order of 0.05%, whereas the absolute difference between the two appears to be about 0.5%.  I don't see how you can conclude that one or the other has the lower tempco, just that they both seem close.  That seems a pretty good result unless you are hoping to characterize the DUT to an accuracy much better than its 0.5% spec.

Thanks @bdunham7. If the two shunts had similar tempco, I would expect the ratio curve to be flat?
enut11

[bdunham7]

Thanks @bdunham7. If the two shunts had similar tempco, I would expect the ratio curve to be flat?

The closer the flatter, although the scaling you choose for the graph affects how it looks--that would look pretty flat on a graph that went from 0 to 2, for example.

Keep in mind that we're using the term "tempco" as a rough approximation for how the shunts behave as they warm up from a fixed current.  In fact, it is possible that one is quite a bit warmer than the other and that temperature gradients and thermal voltages are part of the equation.  It might be interesting to compare them if your DUT was aggressively cooled, for example, while the one with the certificate was treated just as stated and everything was measured after 4 minutes as stated on the certificate.

[PartialDischarge]

The other method i also tried is using a Danfysik Ultrastab flux gate current transducer (very accurate and linear) to directly measure a 300A current pushed trough the shunt. Also works well

Thanks for the idea, just bought a couple of ULTRASTAB 867 at a very low price: https://www.ebay.com/itm/235153418291

Also didn't know but there are Murata 0.25% shunts on digikey until supplies last:

[beanflying]

Likely similar pricing but I got mine via E14/Farnells https://au.element14.com/c/automation-process-control/panel-displays-instrumentation/current-shunts?brand=murata-power-solutions which might suit others.

[Chance92]

Thanks @mzzj, that 'ohm' article is great and quite an eye opener. Currently away from home but I will certainly have a good look at my setup.
@beanflying is sending me his super Murata shunts to play with and I certainly need to up my game to do them justice.

What a minefield!
enut11

I'm going to calibrate the same 100A shunt with a calibrator and a current amplifier. Maybe we can compare results.

[Berni]

The absolute precision of shunt resistors does not really matter that much.

You can calibrate your shunt as long as you have an accurate way of measuring voltage and current. These are both things that bench DMMs can be very good at. Metal resistors like this don't suffer from any weird non linear effects, so you only need one calibration point to be confident in the whole range. So no problem in calibrating a shunt at 10A then extrapolating your calibration out to measure a unknown 100A current. But they do have a temp co! So the calibration is only valid at that temperature! Waiting for it to heat up is no excuse as it won't heat up the same every time.

The actual problem is that shunts get less repeatable once scaled up to 100s of amps. These shunts have very very low resistances, so you get very tiny voltages. This means thermocouple voltages matter a lot and can wreck your accuracy. At these currents things get hot, making the problem worse. The big block of brass that feeds the resistive element with current also has resistance and tempco of its own, so it can add some phantom voltages. To make things even worse the way you connect to the brass block even makes a difference, since turning a cable lug 90 degrees makes the current enter at a slightly different spot, taking a different path trough the brass block as a result.

I had shunts that show considerable differences in performance from just changing how the sense wire is connected to the screw.

So unless you have a really dialed in setup don't expect your measurements to be reliably within 1% by just buying a fancy precision resistive shunt. If you are after precision the LEM ultrastab transducers are way superior, and aren't even all that expensive on the used market.

[alm]

Funny how this is the third thread about the subject of high current in a short time. They are distinct topics, like one was about AC, but still interesting that this is apparently the high current season:

• Masuring 50A DC
• Calibration of Rogowski Coil at ~400 A AC RMS 50 Hz

So unless you have a really dialed in setup don't expect your measurements to be reliably within 1% by just buying a fancy precision resistive shunt. If you are after precision the LEM ultrastab transducers are way superior, and aren't even all that expensive on the used market.

I agree with everything you say. The document mzzj linked to earlier also describes testing multiple ways of attention sense wires. I also like DCCTs like LEM Ultrastab.

Although to some degree making shunts bigger or adding active cooling can increase the voltage drop. You often see that with lab-grade shunts that the shunts get bigger the higher the current so they can dissipate more. See for example this 15A shunt designed for dissipating up to 22.5W. Or the Fluke A40B series of shunts. Compare the size of the 100A shunt to the 1A shunt.

Another option is integrating an amplifier with the shunt so any uncertainty due to connection to the shunt and thermal EMF is handled within the box and can be measured as part of the calibration. This is how the Valhalla 2575A active current shunt works: both active cooling and built-in amplification.

[mendip_discovery]

That's part of the fun of the metrology side of things. TEAS is the beginning then you get into metrology and then they fun begins.

The main thing is to find a way to get consistent and repeatable results. Document what you do and have fun.

[enut11]

The other method i also tried is using a Danfysik Ultrastab flux gate current transducer (very accurate and linear) to directly measure a 300A current pushed trough the shunt. Also works well

Thanks for the idea, just bought a couple of ULTRASTAB 867 at a very low price: https://www.ebay.com/itm/235153418291

Also didn't know but there are Murata 0.25% shunts on digikey until supplies last:

@Berni, @PartialDischarge
Can you tell me a bit more about the Ultrastab devices? How do they work?  Are they useful under 50A?
enut11

[Berni]

Yep they are incredibly linear and stable once powered on.

I forgot the exact model i have, but i think it is designed for 500A, yet it can still easily measure a 10mA current trough it accurately.

The big difference that makes them so good is the flux nulling method. The magnetic sensor inside is only looking for zero, adjusting the current in the coil to get it there. This current in the coil is what is sent out as the output, so your output is the very same current made from the very same electrons that flown trough the coil to null out the measured current. So the conversion factor is purely dependent on the number of turns around the core that forms a transformer. Tho yes the zero stability of the magnetic sensor still matters, but they are specially designed sensors for that exact task of looking for 0 flux, designed to be as stable as possible at that point.

The 1 problem is the stay magnetization in the core and the earths magnetic field. So they do need the 0A to be calibrated on first power on. But 0A calibration standards are really cheap, it is simply leaving the center of the transformer empty.

[guenthert]

[..]
I forgot the exact model i have, but i think it is designed for 500A, yet it can still easily measure a 10mA current trough it accurately.
[..]

Interesting.  Funny that you picked 10mA as example.  I couldn't help but to think of Cern's 10mA reference [1].

Would it then also be possible to measure (i.e. compare) very small currents?  That is, if the induced (very small) current is compared to the unknown current, while a known (e.g. aforementioned 10mA or much less) is fed through the main port?

Tho yes the zero stability of the magnetic sensor still matters, but they are specially designed sensors for that exact task of looking for 0 flux, designed to be as stable as possible at that point.

So the question is, how precisely can those transducers determine 0 flux, but perhaps this belongs in another thread  .


[1] http://cds.cern.ch/record/643294/files/cer-002399331.pdf

[Berni]

I can give it a try at exactly how good the Ultrastab flux gate is at low currents.

I never actually used it for measuring small currents because the main reason why i bought one is to give me the ability to measure in the range of 10A to 300A with a reasonable amount of confidence. I just tried putting a few miliamps trough it once to see how usable it is down low (as this is what currents clamps are bad at) and i was very impressed with its performance.

The whole unit does heat up a fair bit as it runs (since the current being fed into the coils is provided inside by a linear AB class amplifier) and i never had to re-zero it once it warms up, so id assume the temp co of the flux sensor is also impressively low. The datasheet for a LEM IT 200-S ULTRASTAB claims a initial offset of 80ppm with 2 ppm/°C temp co and 1 ppm per month stability(for a 200mA test current). Those are really impressive figures. You will have a hard time finding a resistor this stable.

[mzzj]

I can give it a try at exactly how good the Ultrastab flux gate is at low currents.

I never actually used it for measuring small currents because the main reason why i bought one is to give me the ability to measure in the range of 10A to 300A with a reasonable amount of confidence. I just tried putting a few miliamps trough it once to see how usable it is down low (as this is what currents clamps are bad at) and i was very impressed with its performance.

The whole unit does heat up a fair bit as it runs (since the current being fed into the coils is provided inside by a linear AB class amplifier) and i never had to re-zero it once it warms up, so id assume the temp co of the flux sensor is also impressively low. The datasheet for a LEM IT 200-S ULTRASTAB claims a initial offset of 80ppm with 2 ppm/°C temp co and 1 ppm per month stability(for a 200mA test current). Those are really impressive figures. You will have a hard time finding a resistor this stable.

If you use the Ultrastab fluxgate sensor in fixed position and zero out the initial offset you can get better than 0.5ppm of full scale output in the low end.
For example typical 700A Ultrastab sensor has enough good short-term stability and linearity to measure with 0.35mA accuracy at lower end of the range.   
(IIRC the 600A unit I tested was as good as 0.15mA offset stability/repeatability.)
In practice this means that you can measure 350mA with 0.1% accuracy. Or 3.5A with 0.01% accuracy.
Loop the primary wire 10 turns trough the hole in the fluxgate and you get 0.1% accuracy down to 35mA with a maximum range of 70A.
100 turns primary would be still feasible if you really want and then you can get measure down to microamps with 7A maximum range.  1uA resolution with 7A maximum would be still feasible with these.
 

[enut11]

It occurred to me that the physical mount of a conventional current shunt may affect the stability of the output signal.
If both end blocks are rigidly mounted, would heat cause stress at the junction of the resistive element and the blocks.
If so, would this affect the output signal?

[mzzj]

It occurred to me that the physical mount of a conventional current shunt may affect the stability of the output signal.
If both end blocks are rigidly mounted, would heat cause stress at the junction of the resistive element and the blocks.
If so, would this affect the output signal?

For sure it is going to have some effect. Just look at the trouble and expense standard resistor manufacturers have gone trough to avoid stressing the resistive element.
BUT the effect is probably ballpark of tens to hundred of ppm, ie less than 0.01%

edit:
Best I could do right now in home lab was two HP 6632B's supplying 10A total to a 60mV 150A shunt and 34401A measuring the shunt voltage.
Pressing with plastic rod as hard as I can(about 200N give or take) on the middle of the shunts resistive element I see no change(less than the 0.1uV resolution of the 34401A) so that would be less than 0.0025% change from physical stress.

6632B Power supplies and 34401A are remarkably stable for short perioid of time, I have no problem staying within 1 digit (0.1uV) over a short time span like 2 minutes.

[tszaboo]

Pro tip, once you calibrated your shunt, flip it around and see if it measures the same value the other way.

[Irv1n]

 This photo of our cal bench: dci source, 2182, standard shunt 100A, 500A, 1kA, 2kA

[Berni]

That is a rather fancy looking setup. Thanks for sharing.

[PartialDischarge]

is that a pneumatic contactor to disconnect the shunt?

[Irv1n]

is that a pneumatic contactor to disconnect the shunt?

Yes

[enut11]

After a couple of weeks off with health issues and other distractions I am now looking to push on with this project.
The Murata 0.25% shunts from @beanflying have arrived, thanks.

Learnings so far:
1) The 4W setups were poorly implemented. Docs supplied by Forum members will help here.
2) Too much heat in the load resistors. The test voltage will be halved to around 6v.
3) Orientation of shunts is important. Stability is improved if the shunts are mounted to facilitate natural air flow.
4) Tests will be carried out over 1 min instead of 4 min. (EDIT: 2min to allow for the PS  and load to stabilize)
5) Shunt element temperature measurements and forced cooling will be considered if necessary.

The contenders below, from L to R:
- The shunt that started all this, 100A 0.5% form Bulgaria (white base)
- My home made 50A shunt (wood base)
- Murata 0.25% 10A, 100A and 500A (nice)
- 25A and 100A with 2002 calibration certificates
enut11

[joeqsmith]

Top left Weston 500A, Wade Allen 200A, Weston 10A.  Top right, Weston 150, 100, 50A.  Front center,  GE 100A 0.2%. 

No idea about any of these as I really have no way to measure them.  They would need to be sent in for calibration.   After I was given the old GE shunt, I tried a few tests with it.

[enut11]

@joeqsmith, nice collection of old shunts. They were not only practical but works of art. Love those old knurled and tabbed knobs.
With your GE 100A 0.2% shunt and a decent multimeter you can calibrate your own shunts. BTW, what is your power source?
enut11

[joeqsmith]

@joeqsmith, nice collection of old shunts. They were not only practical but works of art. Love those old knurled and tabbed knobs.
With your GE 100A 0.2% shunt and a decent multimeter you can calibrate your own shunts. BTW, what is your power source?
enut11

There is nothing I can do to calibrate anything.  Nothing I own is in current calibration.  Most of my experiments are making relative measurements and I can tolerate a fair bit of slop, so I run some cross checks and call it a day.  If I required absolute accuracy, the only thing I could do would be to send them in and request a report.  I've never looked into it.

The power supply shown in that last video has changed over the years.  Watch the first few minutes for details on the current (no pun intended) setup.

[enut11]

@joeqsmith. Wow, impressive setup! I see you test at very low voltages. Are you testing with DC or AC?