Calibrating a 100A DC shunt

[enut11]

A lot of discussion above is about the tempco effects of different shunts, being related to either size or construction material.
In my tests, even at 20A, the shunt elements rarely changed by more than a couple of degrees C. Dissipation power= 20x20x0.001=0.4W
Seems to me there are other effects at present unknown.
enut11

[PartialDischarge]

I have a gwinstek mili ohm meter and in my experience the real problem with low valued shunts is where you take the measurement, errors become bigger wrt to the nominal value of the shunt.

[Berni]

That is because you are using a very low resistance shunt at low currents. So you also get a tiny signal out of the shunt that is more difficult to accurately measure (especially due to thermocpuple voltages)

Once you go >300A the pesky square in P= R * I^2 starts to become a problem since even a very low resistance shunt will be dissipating 10 to 100W of power, so things will certainly get rather hot even if it is a huge shunt.

Well made shunts can have impressively low temp co. but i have also seen some cheep chinese shunts that had horrifically bad temp co. like 1000ppm/°C or something. The alloy mix for the resistive material has to be carefully tuned just right to get that low temp co. This is why good shunts can be quite expensive.

[tszaboo]

A lot of discussion above is about the tempco effects of different shunts, being related to either size or construction material.
In my tests, even at 20A, the shunt elements rarely changed by more than a couple of degrees C. Dissipation power= 20x20x0.001=0.4W
Seems to me there are other effects at present unknown.
enut11

A few things. Dissipation goes up with square of the current. So 100A is 25 times as much dissipation. And nonlinearity comes with this.
Often times for four wire shunts, the actual value of the high current path is a lot higher than the nominal value. You have the nominal value between the 4 leads and some extra for the leads leading there.
When measuring 20A*0.001mOhm = 20mV, thermocouple effects will effect matter. I had a design where the leads were connected to different sized PCB planes, and the shunt was measuring current differently in either direction.
The better ones also have a voltage coefficient defined, that I'm not even sure what is the mechanism for that.

[Kleinstein]

Ther thermal EMF of the shunts can be quite an issue, if both sides of the shunt see different heat loads. A fuse in the path can be an issue here as it may produce significant heat on one side.
Getting low thermal EMF is a quite important parameter for the shunt at high precision, high power.

Many shunts are made from manganin and this material usually has a relatively large 2nd order TC that is not easy to trim / adjust by heat treatment. So even of low TC at room temperature the TC can get higher when going really hot. So the problem can be getting worse even faster than I²

[mzzj]

Some further reading: https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=32286

[Smokey]

Another thread here I just stumbled upon talking about calibrating a 100A shunt.
https://www.eevblog.com/forum/metrology/low-ohm-precision-resistor-standard-and-testing/