Today I decided to revisit the water-conductivity experiments for the course, now that I have an easy way to do proper impedance spectroscopy (including phase information as well as magnitude), using the network analyzer function of the Analog Discovery 2 USB oscilloscope. I wanted to look at 4-electrode measurements, as well as the 2-electrode measurements we’ve done in the past.
First, I made myself a 4-electrode device, by cutting some ⅛” stainless-steel welding rod (316L rod for TIG welding) into 15cm pieces, drilling 4 ⅛” holes in a scrap of cutting-board plastic, and driving the rods through the holes with a hammer.
I then immersed the short end in tap water (using a mason jar, so that the long end stuck out the top) and used alligator clips to attach wires from the electrodes to a breadboard.
I connected the function generator through a series 1kΩ resistor to one of the end electrodes and ground to the other end electrode. Channel one of the oscilloscope measured the voltage across the 1kΩ resistor (hence the current in milliamps).
Channel two of the oscilloscope was connected to either the two end electrodes (making a 2-electrode measurement similar to what we’ve done for years in the class), or to the two middle electrodes, for a 4-electrode measurement. The idea of a 4-electrode measurement is that there is an electric field established in the bulk material by the outer electrodes, and the middle electrodes can measure that field without interference from surface effects that occur on the electrodes that are providing the current.
I used the network analyzer function to sweep from 2Hz to 10MHz. I exported the data so that I could plot it as impedance (rather than as just the dB ratio of the two measured voltages). For the 2-electrode measurement, we are measuring the impedance of the water and electrodes (voltage across the electrodes divided by the current through them), but for the middle electrodes, we’re looking at the voltage across the middle electrodes, divided by the current through the end electrodes.
The plot of the phase shows even better why 4-electrode measurement is useful:
I don’t think I’ll switch to 4-electrode measurements this year (if for no other reason than that I’d have to make a dozen new electrode sets), but I’ll keep it in mind for next year.