Differential voltage measurement using Arduino

Question

I have a electrolysis cell in which the anode is hanging using a Structure in a electrolyte. The cathode is at the bottom of the cell with electrolyte above it and anode being dipped into this electrolyte (not completely, a portion of the anode is dipped). The cell operates at 7 volts DC and 90000 amperes of current flowing through it. This is a typical electrolysis process of producing aluminium from alumina powder in a aluminium smelter. The cell is called a pot. Now the question is I need to measure a potential drop on the anode rod. We usually take a meter to measure the potential drop on the anode rod and typically the reading are in millivolts. As the readings are in millivolts, it just indicates the drop in rod voltage due to resistance of rod. Now can I measure this millivolts drop using Arduino provided the voltage at one point of anode is 5 volts with respect to ground and the voltage at the other point is 4.95 volts with respect to ground, however the difference between these voltages is 0.5 volts and this is voltage I need to measure using Arduino and some arrangements as add on to it. How can I do this? Can anyone help. If you want more details, please comment. I will also provide more details.

So far I have done is tried measuring 1.5 volts on a pencil cell using Arduino by connecting A0 analog pin on Arduino to positive directly to the positive of 1.5 volts battery and ground of Arduino to the negative of Arduino.

Can I do the same for this millivolts measurements? I understand that the ground of Arduino and the 2nd point of anode measurement will not be at same potential, so shouldn't be connected together. I am scared to burn my board, so didn't try practically and wanted to get some opinion.

Answer 1

As you do it with the multimeter: connecting ground to the lower potential (4.95 Volts in your example) and the volt probe to the higher potential (in your case 5.0 V), you can also connect the Arduino between the two potentials: Arduino GND to the lower (!) potential and A0, A1, ... or A7 to the higher (!) potential.

As long as you use the correct polarisation and the difference between the potentials is less than VCC of the ADC (5V Arduino ADCs) you should be save.

The resolution of the Arduino might be a problem. I don't know what resolution you need; 5V divided by 1024 is about 5 mV per step.

You might want to provide a reference Voltage at Vin of -let's say- 1 Volt if you need a better resolution. That would improve the resolution to about 1 mV per step. But then the meassured voltage must no exceed the 1 Volt reference voltage.

EDIT (Gerben's comment)

You could use the 1.1V internal reference of the ATMega328P (analogReference(INTERNAL))

And please let me give you a security advice: If the resistence in any part of your meassuring setup is significantly lower than the resistance between the messuring points (e.g you shorten the probe wires), then there is a chance that 90000 A running through this setup. That could burn away your hand, arm or even your life. Amperes kill; not Voltage. I've seen a wrench that was vaporized (ok, melted, ok glowing ;-) ) in a second, when it had fallen between the poles of a car battery (12 V; 300 Amps). An you operate with 90000 Amps. !

Answer 2

This is obviously a significant project, not something to economize on. I would recommend you use a separate difference amplifier to get a single-ended output referenced to the Arduino ground and scaled to maximize the 10-bit range use. The diff amp can use its own supplies (+/-7.5 or +/-15, say) with the ground tied to the Arduino ground. That way the Arduino only sees the difference, never the full 7 V no matter what.