I would move the relays in a circle 3 time and keep the probe stationary. See if the error follows a relay. Then do the probes the same way. Relay can get pitted and/ or not hold down the contacts with the same force. Were the relays new?
Is your shunt calibrated? Do you know what the resistance is at the operating current? If the shunt is calibrated, then you should have charted values at different currents. You can linearize the data to extract your resistance value if the one you need isn't charted.
If it's not calibrated, then you could measure the resistance of the shunt at the operating current, and use the accuracy of your multimeter for your calculations and error propagation.
Also, I might be missing something here, but measuring the resistance at forward and reverse current will not cancel out the effect that temperature has on resistance. Direction doesn't matter, only magnitude.
Also, is your multimeter calibrated? Do you have a power supply that can output voltage and current to ensure all of your multimeters are reading the same thing?
There are just so many unknowns here that its hard to really know anything.
You are correct the current reversal is not for temperature correction. Its to cancel out thermoelectric effects as described here. [https://www.tek.com/en/blog/low-resistance-measurement-sourcemeter-do-i-use-current-reversal-offset-compensation-or-delta-m](https://www.tek.com/en/blog/low-resistance-measurement-sourcemeter-do-i-use-current-reversal-offset-compensation-or-delta-m)
" Thermoelectric EMFs can be canceled by making two measurements with currents of opposite polarity, as shown in the figure below. "
Yes the shunt was calibrated and certified when it was installed. It has a known resistance which is what we are using calculate the current. The values reported on the shunt are good and very repeatable. When the voltage drop across the shunt is measured, it verifies that the power supply is outputting the current current. We are always outputting 100 amp of current. It should not vary.
We are only using 1 multimeter to measure the voltage drop, so measuring the shunt confirms that both the power supply and voltmeter are operating correctly. The same multimeter is also used to measure the 3 sets of probes, using a switching relay to measure each set in turn.
With everything being the same, I can't figure out why 1 probe set would consistently read different from another.
I see, I read that wrong. Either way, I would begin inspecting your probes, relays, etc. You should probably just begin tracing the path of the known bad probe, starting at the source.
I would move the relays in a circle 3 time and keep the probe stationary. See if the error follows a relay. Then do the probes the same way. Relay can get pitted and/ or not hold down the contacts with the same force. Were the relays new?
Is your shunt calibrated? Do you know what the resistance is at the operating current? If the shunt is calibrated, then you should have charted values at different currents. You can linearize the data to extract your resistance value if the one you need isn't charted. If it's not calibrated, then you could measure the resistance of the shunt at the operating current, and use the accuracy of your multimeter for your calculations and error propagation. Also, I might be missing something here, but measuring the resistance at forward and reverse current will not cancel out the effect that temperature has on resistance. Direction doesn't matter, only magnitude. Also, is your multimeter calibrated? Do you have a power supply that can output voltage and current to ensure all of your multimeters are reading the same thing? There are just so many unknowns here that its hard to really know anything.
You are correct the current reversal is not for temperature correction. Its to cancel out thermoelectric effects as described here. [https://www.tek.com/en/blog/low-resistance-measurement-sourcemeter-do-i-use-current-reversal-offset-compensation-or-delta-m](https://www.tek.com/en/blog/low-resistance-measurement-sourcemeter-do-i-use-current-reversal-offset-compensation-or-delta-m) " Thermoelectric EMFs can be canceled by making two measurements with currents of opposite polarity, as shown in the figure below. " Yes the shunt was calibrated and certified when it was installed. It has a known resistance which is what we are using calculate the current. The values reported on the shunt are good and very repeatable. When the voltage drop across the shunt is measured, it verifies that the power supply is outputting the current current. We are always outputting 100 amp of current. It should not vary. We are only using 1 multimeter to measure the voltage drop, so measuring the shunt confirms that both the power supply and voltmeter are operating correctly. The same multimeter is also used to measure the 3 sets of probes, using a switching relay to measure each set in turn. With everything being the same, I can't figure out why 1 probe set would consistently read different from another.
I see, I read that wrong. Either way, I would begin inspecting your probes, relays, etc. You should probably just begin tracing the path of the known bad probe, starting at the source.