Gas station without pumps

2012 August 15

Measuring Ag/AgCl electrodes

Filed under: Circuits course — gasstationwithoutpumps @ 13:46
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I had a better idea for measuring Ag/AgCl electrode characteristics than I mentioned in yesterday’s post Better measurement of conductivity of saline solution. Rather than buying fine silver wire and soaking it in bleach (though that may still be worth doing), I just took two of the Ag/AgCl gel electrodes I had for the EKG lab and stuck them face-to-face.  The adhesive disks do a pretty good job of sealing in the electrode gel.

I expected this electrode sandwich to have very low resistance, both because of the good properties of the Ag/AgCl electrodes and because of the short distance between the metal electrodes.  I was not sure what to expect for capacitance, as the non-polarizable nature of Ag/AgCl would lead me to expect little change in resistance as a function of frequency.

Trying to measure the resistance of the electrode sandwich with an ohmmeter is still misleading though, as it gradually creeps up as the meter charges a capacitance, going from 40Ω to 80Ω in about a minute.  Reversing the connections drops the resistance reading to 20Ω, from which it gradually increases as the capacitance is charged the other way.

Plot of resistance of Ag/AgCl sandwich made from two Vermed SilveRest EKG electrodes as a function of frequency. Note that a power-law fit seems much better than an R+(C||R) model.

I’m getting a power-law fit with a small exponent, rather than either a constant resistance or something that fits an R+(C||R) model. Looking at I vs. V plots on the oscilloscope, it looks like a pure resistance at high frequency (no phase changes), but there is some phase change at lower frequency.  The small change in resistance with frequency is not easily modeled with an RC circuit.

For the EKG frequency range of 0.01Hz to 150Hz, the resistance of the electrode pair (using the power-law fit) varies from 48Ω down to 8Ω.  Given the large resistance of the skin, this electrode resistance is so small as to be negligible. Putting small Ag/AgCl electrodes into a saline solution, however, might have quite a different effect, where the resistance of the electrodes plays a more substantial role in the electronic behavior of the system. So maybe I do still need to order some silver wire and make my own Ag/AgCl electrodes (either by bleaching or by electroplating).