how to measure electrode impedance?
(apologies in advance if this should go in Hardware or if another thread already -- please merge if so)
Is there a way to measure the impedance of the electrode to skin contact, via the OpenBCI board and/or software? Or is the only method to disconnect the electrode and reference wires from the board and use an ohmmeter to manually test it?
What do you guys do to make sure you're getting low impedance between electrode and scalp? I have the dry, spiky electrodes from the kit ... and wonder if they're actually making good electrical contact.
Is there a way to measure the impedance of the electrode to skin contact, via the OpenBCI board and/or software? Or is the only method to disconnect the electrode and reference wires from the board and use an ohmmeter to manually test it?
What do you guys do to make sure you're getting low impedance between electrode and scalp? I have the dry, spiky electrodes from the kit ... and wonder if they're actually making good electrical contact.
Comments
http://openbci.com/forum/index.php?p=/discussion/601/measuring-electrode-impedance-w-gui
There are a number of other posts that discuss details of the impedance algorithm,
http://openbci.com/forum/index.php?p=/search&Search=impedance
http://openbci.com/forum/index.php?p=/discussion/84/ads1299-electrode-impedance-measurement-algorithm
http://openbci.com/forum/index.php?p=/discussion/781/how-to-get-heart-srp-electrode-impedance-alpha-waves-from-the-brain-at-the-same-time
http://openbci.com/forum/index.php?p=/discussion/668/recording-impedance-values-and-voltage-values-at-the-same-time
It says "the required procedure was to abrade the skin to achieve a scalp-electrode impedance of less than 5 kΩ. To achieve such impedance levels, skin abrasion is required. Abrasion removes the surface epidermal layer, which has higher impedance than the underlying tissue."
If that's true, then how am I getting such low impedances with just the dry, spiky electrodes used in the headset?? I'm not even using gel.
And then perhaps a follow up comment: while I'm no expect, I am not so sure that I agree that just because you have low impedance, means you're getting accurate EEG measurements. Perhaps that statement is wrong to the point of being silly. But to my way of thinking, low impedance just means the electrode conductors of the channel and the reference have good electrical contact with the scalp/ear skin, and thus are well able to pass current along the surface of the skin.
Why am I even saying this? Because I wonder if the geometry of the spiky electrode is in fact "limiting" in its ability to detect neuronal (EEG) activity. Yes, great impedance (gets through hair, without mess of gel), but does that in fact mean good EEG detection??
Pretty much just wondering aloud. But I do think the geometry matters. There is a guy at U of Rhode Island who has great results using "Tri-Polar Concentric Ring" electrodes. Anyway, rambling now ...
That doesn't seem right, to me. I doubt that the neurons are directly supplying charges to the conductors, through the skull. If that were the case, then what you said would make sense. Rather, I think the EEG sensors work more like how TV antennas work. And in that sense, the shape of the antenna does affect reception.
Conductivity between the sensor conductor and the scalp is meaningful for impedance, obviously.
Or like I mentioned before, the guy at URI uses concentric rings to obtain even higher performance.
http://www.cognionics.com/
Yes, at the scalp you are measuring ensemble electric (or magnetic in the case of MEG) combined field effects which result in an electric field (micro voltage) potential difference between the electrode being measured and the reference electrode.
But this is not like the static (high voltage) electric fields measured in physics lab experiments. Where the electrode shape concentrates charge. Nor is it like RF radio frequency electromagnetics, where again the antenna shape effects the received signal strength.
By the time the summed charges reach the scalp, it's a simple micro voltage potential difference. So flat or comb or gel type shapes don't show any appreciable differences.
http://egr.uri.edu/wp-uploads/neurorehabilitationlab/IEEE-TBME-vol53-no5-pp926-933-2006Besio-et-al.pdf
And the Rhode Island PhD thesis using them,
http://digitalcommons.uri.edu/oa_diss/69/
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I'm still unclear how the three rings in the electrodes (PC board apparently) are connected to amplifier channels. I think one conductor may act as a reference and the other two conductors are two separate channels that are then mathematically combined as in the paper.
I've only been to one talk by Besio, so that's how I knew of them. I guess he was at LA Tech before. Anyhow ...
"By the time the summed charges reach the scalp, it's a simple micro voltage potential difference. So flat or comb or gel type shapes don't show any appreciable differences."
I don't see why the relatively low strength of the summed electric fields from the action potential currents means that you can completely ignore electrode shape, and just throw any old piece of conductor on your scalp. And even if that were the case, then what's the point of insuring low impedance between the electrode conductor and the scalp? I guess the larger the air gap, the larger the distance between the source of the field and the electrode, and thus the smaller the signal strength. But that also means because of the spikes on the dry comb, the bulk of the electrode conductor is sitting off the scalp surface ...
Appreciate any further comments.
http://www.cognionics.com/index.php/technology/download-now
Some of them show comparison graphs and such with dry combs vs. gel / paste systems. They conclude no significant differences.
Spikes on the combs are directly digging into the skin layers. So distance of the comb framework / backing makes no difference. There are a range of papers published on the electrical and mechanical characteristics of the skin / electrode interface.
considerably MORE pressure on the skin than with cups and paste. On the
other hand, impedance with the dry combs or cones can vary more for
this same reason. Impedance of gel or paste electrodes is generally more
constant during a recording session.