Measure the electrode-to-skin impedance

It is the time to actually measure the impedance of real electrodes on my actual skin. I decided to try two types of electrodes: (1) disposable ECG electrodes and (2) re-usable gold cup EEG electrodes. Here’s the story of what I found…

On the left, I’m trying ECG electrodes. On the right, I’m trying
gold cup EEG electrodes. In both cases, I’m looking pretty sharp.

Disposable ECG Electrodes: First, I decided to try some disposable ECG electrodes. These are cheap and really easy to use. They’re not very good for using in your hair, but they’re great for sticking on your forehead. For the reference and bias connections, I used an ECG electrode on the mastoid bone behind each of my ears. The picture below shows the pre-gelled, self-adhesive ECG electrodes that I used along with the clip-type ECG electrode wires (see this post for specific recommendations).

ECG Electrodes and Clip Leads That I Used on my Head

Once I stuck three of these electrodes on my head (forehead and behind each ear), I connected the lead wires to my OpenBCI V1 board using my homemade adapter.

Connecting the ECG Leads to my OpenBCI V1 Board.

After getting all connected, I activated the ADS1299’s “Lead Off” excitation signal for the channel that was connected to the ECG electrode on my forehead. As discussed in my previous post, the excitation signal is a 6 nA AC current source that the ADS1299 toggles at 31.2 Hz. The flow of current through the electrode creates a voltage that can be measured by OpenBCI just like a normal EEG signal. I configured my OpenBCI board to digitize the data and send it to the PC. On the PC, I used the OpenBCI GUI to view the data in real time and to record it for post-test analysis.

A zoomed-in plot of the recorded waveform is shown on the left of the figure below. As you can see, it has a fairly large amplitude of 508 uVrms. This corresponds to an impedance of 120 kOhm. That’s really big! Being surprised by that large value, I swapped the wires around so that I was measuring the electrode that I had been using as my reference (or “-”) electrode. As seen in the waveform above on the right, I got a very similar value. That’s not cool.

Example Waveforms Recorded While Using OpenBCI To Measure the
Electrode-to-Skin Impedance of (Left) the ECG Electrode on my
Forehead and (Right) the ECG Electrode behind my Left Ear.

Zooming out so that you can see more of my recording, the figure below shows about a minute’s worth of data. This is the full recording from which I made the excerpts shown above. The longer view below shows the story of me recording data for one electrode (up to about t = 119), of how the signal goes away as I unplug and swap the electrode connections (from t = 120 to t = 132), and then how the signal returns once I am connected to the other electrode. Again, you can see that I measured 120K from one electrode and 116K from the other electrode.

Zoomed-Out View of my Recordings Using the “Lead-Off Detection” Excitation
while Using Disposable ECG electrodes.

Gold Cup EEG Electrodes: Because I found the impedance of the ECG electrodes to be surprisingly high, I tried using some gold cup EEG electrodes. The picture below shows the electrodes and the conductive electrode paste that I used. Like with the ECG electrodes, I put one of these on my forehead, one on the bone behind my left ear for the EEG reference and one one the bone behind my right ear as the EEG bias.

Gold Cup EEG Electrodes and Ten20 Brand EEG Paste

After putting on the electrodes, I activated the “Lead Off” excitation, like before. Some examples waveforms from the data that I recorded are shown below. As expected, the waveform shape is the same as seen before, but the amplitude is different, which reflects the fact that the impedance of the electrode-to-skin interface is different.

Waveforms Recorded from the Gold Cup Electrodes During the “Lead Off” Excitation.

Notice that the three plots show decreasing amplitude, which means that I was getting better contact and less impedance in each case. What was happening?

Well, for the first waveform (the one on the left) was shows what I measured when I first attached the electrodes. It shows an RMS amplitude of 389 uV, which corresponds to an impedance of approximately 92 kOhm. This was still higher than I wanted, so I fiddled with the electrode and pushed it into my skin to try to make better contact. That’s when I got the middle graph – 230 uV and 54 kOhm. Finally, I pulled off the electrode, replaced the conductive paste, and really pressed and twisted the electrode against my skin. That’s when I got the graph on the right – 64 uV, which corresponds to 15 kOhm. That is more like the kind of value that I was hoping to see.

Below is a zoomed-out plot of the whole scenario with the gold cup electrode. Again, this is the full record from which I made the excerpts above. On the left side of the plot, you can see the 389 uV, 92 kOhm condition that I showed before. Then, you can see my multiple attempts at re-seating and re-pasting the electrode. Finally, at the end, I finally got to the 64 uV / 15 kOhm condition. So, while it does take some effort, it is possible to improve the electrical contact between the electrode and your skin.

Zoomed-Out View of my Recordings Using the “Lead-Off Detection” Excitation
while Using the Gold Cup EEG Electrodes

Why Were the ECG Electrodes So Bad? This experiment started with the ECG electrodes, which yielded a very high impedance of 120 kOhm. If I could only use the ECG electrodes, this high impedance value would have prompted me to remove the electrode, to scrub the skin (hard!) with alcohol and a rough pad, and then to attach a new electrode. Maybe this would have worked to lower the impedance, or maybe not. If it would not have helped, the problem could be that my ECG electrodes are really old. If you look really closely at my picture showing the electrodes, you’ll see that the ECG packet in the background shows a date of “June 2012”. Yikes! I have a friend who is developing a hacker-friendly EMG system (go FlexVolt!) who has mentioned to me that he has seen difficulty when using old ECG electrodes. So, I’m thinking that maybe disposable electrodes have a limited shelf life…and that 2 years is maybe too old.