Calibrating a 100A DC shunt

[enut11]

I want to characterise a second hand 100A shunt that I purchased on eBay. Turned out to be a bit more expensive than a new Chinese one but hopefully better quality.
https://www.ebay.com.au/itm/255796427831?var=555732568878
I have on loan a calibrated 100A shunt. I opted to test at around 20A using a HP Server 12v/600w supply and a stable 300w fan cooled programmable resistor load set to 0.45ohm.
According to the calibration sheet that came with the reference shunt, testing was done at the 4 minute mark at 20A, 30A and 50A DC current.
The biggest challenge with high current testing is the impact of heat on all components – power supply, cables, connectors, shunt and load.

[enut11]

The setup that I started with was to connect everything in series:
PS  >  DUTshunt  >  REFshunt  >  LOAD
and simultaneously measure the voltage across the shunts using two mains operated bench meters. The power supply negative terminal was not earth grounded.

Attached chart shows the results of my first test, DUT on the left and Reference shunt on the right. The test lasted 5 minutes. I have scaled the Y-axis steps to be the same on both graphs.

[enut11]

My initial analysis is that the DUT settles down after about 1.5 min and has a lower slope thereafter compared to the Ref which seems to continue upwards.
At the 4min mark, the DUT drops 14.056mV compared to Ref at 23.29mV.

Calibrated resistance of the Ref at 4 minutes is 0.999025mOhm. This computes to 23.31A test current.
At the test current, DUT resistance computes to 0.603 mOhm, ie 0.5% high.

A picture of the monster 300w load below. 3 fans, one for each tube (not shown) and one above the hotter part of the 2 stage resistive element. The DUT is the white shunt.

Comments on test setup and results welcome.
enut11

[enut11]

I tried to measure the change in the 300w resistor during the test using an isolated handheld DMM.
EDIT: calculated change over 4 minutes is 0.125%

[beanflying]

Maybe time for a glycerine or oil bath to keep it more stable?

Either that or buy one of these bad boys   eBay auction: #325785913564

[PartialDischarge]

My impression is that the use of that kind of load resistor totally invalidates your results. Simply put its resistance is too dependant on temperature (or power consumed too high) for you to have a precise constant current setup

[enut11]

Thanks @beanflying but the Valhalla is a bit above my $50 budget!

@PartialDischarge
Yes, the small change in voltage (~0.13%) across the 300w resistor will affect the test current but, as both shunts are in series, would not the relationship between DUT and Ref still be valid?
enut11

[bdunham7]

would not the relationship between DUT and Ref still be valid?

It would, and you should be plotting that ratio rather than the expanded plots of their actual values.  The overall curve that they both exhibit is probably mostly the current actually changing.

[PartialDischarge]

Ok, I see your idea now. Although the reference shunt voltage does not seem to be stable, contrary to your DUT.

[trobbins]

The reference shunt likely has a subtle varying tempco difference than the DUT, due to construction of the sensing resistance element and the resultant thermal conduction to the ends, versus to ambient.

Normally, shunts like the reference are turned on their side so that the resistance element has a lower Rth to ambient.  You could also include some airflow to further suppress any tempco contribution.

For most power applications, commercial shunts I've come across like those two would typically have a 1% tolerance from the manufacturer. 

I recently tested a batch of ten 5A 100mV marked "class 0.2" shunts from A.J.Williams (Australian) and was able to 'confirm' that shunt resistance varied between all units by less than 0.17% max.  Those shunts were circa 40 years old, and of unknown use history, but given the class rating and serial numbering and no outlier measurement results, it does appear that they were originally made with a 0.2% tolerance.  I could also conjecture that the three shunts closest to the median value are at worst <0.1% tolerance from the original manufacturer reference, and likely to have a lower tolerance, but haven't as yet been able to further that view.

PS, thanks for copying the shunt calibration report.

[enut11]

Ok, I see your idea now. Although the reference shunt voltage does not seem to be stable, contrary to your DUT.

I guess this is why the cal lab specified a time (4min) for the test to be taken.

[enut11]

would not the relationship between DUT and Ref still be valid?

It would, and you should be plotting that ratio rather than the expanded plots of their actual values.  The overall curve that they both exhibit is probably mostly the current actually changing.

Thanks @bdunham7, I will have a look at that...

[mendip_discovery]

IIRC the Fluke Bible has a method for measuring shunts. I haven't tried it yet.

I have mostly relied on the calibrator and its specifications for keeping a steady current but that is for 5min on and 5min off which I do for 5 forward reading and then 5 reverse. Taking the average of all the readings to get a resistance value.

The method my ex lab manager had was use a known shunt to confirm the current going through the unknown shunt and use that to correct the current and then do the maths. But he had two multimeters, broke one before he left.

Talk of shunt measurments before have suggested and idea to measure the temperature, and possibly using a small diy oven to change the temperature to simulate the effects of heating on the shunt. Using the temperature of the shunt during measurments when in use to correct the resistance measurments.

[Berni]

I did testing on 100A 300A even 1000A shunts before.

The thing you want to mostly test for is temp co. Some of the cheep shunts have huge variations due to temperature. The fact that these things are run hot also causes issues with thermocouple voltages at the sense terminals.

The way i did testing was to use a 500A lab PSU to test the current capability. This was used to heat up the shunt, if it didn't get hot enough it was wrapped in a cloth to help it get there. Then once that is done, the setup is switched over to a programable lab PSU that sources 9A trough it, a DMM with a 10A range checking the current and then another DMM reading the sense voltage. Another DMM is used to monitor temperature via a thermocouple. A script was used to toggle the PSU between 0A and 9A, continuously collecting measurements once every few seconds.

At the end the whole thing is put into a graph to show the results.

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

[enut11]

The reference shunt likely has a subtle varying tempco difference than the DUT, due to construction of the sensing resistance element and the resultant thermal conduction to the ends, versus to ambient.

Normally, shunts like the reference are turned on their side so that the resistance element has a lower Rth to ambient.  You could also include some airflow to further suppress any tempco contribution.

For most power applications, commercial shunts I've come across like those two would typically have a 1% tolerance from the manufacturer. 

I recently tested a batch of ten 5A 100mV marked "class 0.2" shunts from A.J.Williams (Australian) and was able to 'confirm' that shunt resistance varied between all units by less than 0.17% max.  Those shunts were circa 40 years old, and of unknown use history, but given the class rating and serial numbering and no outlier measurement results, it does appear that they were originally made with a 0.2% tolerance.  I could also conjecture that the three shunts closest to the median value are at worst <0.1% tolerance from the original manufacturer reference, and likely to have a lower tolerance, but haven't as yet been able to further that view.

PS, thanks for copying the shunt calibration report.

@trobbins
Here are some pix of the Aussie cal kit. The second shunt is 25A

[enut11]

Cal certificate for the 25A shunt

[enut11]

would not the relationship between DUT and Ref still be valid?

It would, and you should be plotting that ratio rather than the expanded plots of their actual values.  The overall curve that they both exhibit is probably mostly the current actually changing.

This is the ratio of REF/DUT shunt voltage at 23.3A. The X scale spans 5 minutes. Apart from an initial dip at the beginning, the DUT still has a lower temperature coefficient.

[Smokey]

from the ebay listing description:

"Rated current: 100A or 150A
Voltage drop: 60mV"

If it's 60mV drop at 100A, that means about 0.6mOhms.  You measured 0.603 mOhm.
Seems pretty close to me.

[trobbins]

Perhaps the ratio plot in post #16 is highlighting the rise of temp of the element in the REF shunt, and it having a changing tempco just above ambient temp, as it moves from ambient temp to stable temp.  You may get different characteristic curves if, for example, the REF shunt only was kept cooled with a fan, or in the alternative the DUT shunt was kept forced-air cooled.  You may also be able to measure the temp rise on each element surface using a small thermocouple, or IR spot meter.

Without stress during life, I guess one could expect quite low aging impact on the 2003 cal of the REF shunts.

[beanflying]

If you are interested @enut11 drop me a PM and I will send you my little set of Shunts to play with. I have nothing planned for them for the next few months and have other options if I need to measure something anyway.

10, 100 and 500A 0.25% Murata's with Manganin elements so the tempco is low.



Interesting read for continuous operation they suggest derating them by a chunk https://www.farnell.com/datasheets/2237929.pdf

[bdunham7]

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.

[trobbins]

I also have a penchant for shunts ranging up to 500A 50mV, and am working towards some absolute accuracy tests, but that may take a few years and a step up in other calibrated equipment.  The big vintage Tinsley has a 1984 measurement to 0.01 micro-ohm (and uV) resolution at 1A but I don't know the origin of that, and I know of an even larger Tinsley that has a similar vintage measurement, so the hope is to one day see what age-related variation may have occurred.

Ciao, Tim from Melbourne

[beanflying]

Williams made some high grade stuff. Not really a shunt but it is 10A rated  I added this one to my collection a year or so back.

[mzzj]

Your setup looks quite horrible thermal emf-wise. Ditch those alligator clamps and wire the sense cables directly to current shunts without any connectors.
10uV thermoelectric voltage will ruin your whole measurement in this case!

Would be also good idea to measure the thermal EMF before and right  after the test to get idea how good the setup is.
Continue the logging while you disconnect the measurement current to see how many uV's of thermoelectric error you have after the measurement.

Also use enough heavy gauge wiring for the current connections and crimp proper terminals on the wires.

Having said that your results look about as good as you can expect with this level of shunts. Calibration certificates for the reference shunts leave some open questions.
After improving your wiring you could add forced air cooling alternately to DUT or REF shunt and plot the results.

Your measurement drifts about 0.1% and  20 degree temperature rise with 50ppm difference in resistor tempco's would be already enough for this.

Also read this if not already familiar: http://ohm-lab.com/pdfs/Shunt%20Calibration.pdf

[enut11]

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