Self made electrode shows "RAILED"

thechargedneutronthechargedneutron India
in Cyton
Hello,
I have been experimenting with making my own set of electrodes for EEG measurement. I prepared two set of electrodes with different set of materials. The first type of electrode is working fine and giving the desired EEG signals. However, when I connect the second type of electrode (SRB pin still connected to the first type of electrode) , the signals are "RAILED". If I hold it firmly with my hands, it does go out of the RAILED state but does not give good EEG signals.

I have checked that the impedance of both the electrode is same, at around 1 ohms.

What other electrical properties or requirements am I missing?

I have also tried the following things to debug my electrodes -
1. Connected SRB electrode and N1P pin electrode and got a 0 uV output.
2. Connected the electrode with a sinusoidal signal provided through AFG (with SRB connected to the ground of the AFG). I got the desired peak in the FFT in the OpenBCI GUI.

What can I do in order to ensure a continuous and good quality signal?

Comments

  • jps2000jps2000
    Railed means the signal is out of range.
    it is the DC component of the signal
    This is caused by different electrode materials that work like a battery in acidic environment.

  • wjcroftwjcroft Mount Shasta, CA
    Thanks @jps2000.

    Commercial electrodes are made of these metal types:
    • solid tin, molded/cast or stamped
    • solid silver
    • gold plate on tin or silver
    • silver chloride coating on silver plated plastic electrodes

    That's basically it. All other common metals are problematic, due to the galvanic currents mentioned by jps.

    Regards,

    William

    PS the tin electrodes can be found quite inexpensively. The only downside is that they are noisier than the other types.

  • thechargedneutronthechargedneutron India

    Thanks for the answer.

    I now understand the reason for the signals getting railed. Is there any suggestion on how can I solve this situation?


    Should I use same kind of electrode (both SRB pin and N1P pin)? Will that help in getting non Railed Signal?

    Any other advices in this regard will be appreciated. Thanks!!

  • wjcroftwjcroft Mount Shasta, CA
    (1) what electrode metals are you using? If not one of those listed you will have poor luck.

    (2) never mix metal types, this is exactly what causes the battery / galvanic effect. A battery consists of two metal or conductor types, and an electrolyte. In the case of an electrode the gel or paste is the electrolyte.

  • wjcroftwjcroft Mount Shasta, CA
    https://electrodestore.com/collections/eeg-cups/products/cup-series

    About $1 each electrode, silver chloride gives the highest quality signal. These say they are 'disposable' but can be reused somewhat until the silver chloride coating wears off.
  • jps2000jps2000
    I have good experience also with stainless steel washers (highest grade) + electrode gel  from ecg applications
  • thechargedneutronthechargedneutron India
    edited June 2019

    @wjcroft

    1. It is basically a highly conductive cloth of Copper (Cu) and Nickel (Ni)
    2. The other electrode connected to SRB is silver chloride electrode, and it works when two of such are used to take measurements. I now understand that this may be causing me additional troubles.

  • BillhBillh
    If you are going to mix metal cloth electrodes with AgCl electrodes, it's best to use silver ones. Perhaps something like this (untested):






  • wjcroftwjcroft Mount Shasta, CA
    Bill, Jps, Neutron, thanks.

    Check out some of the figures in this document,


    Although you are likely not doing slow cortical potential work, it does contrast the stability and noise of various metal types. Tin and steel are not the best for EEG. That honor goes to silver chloride.
  • thechargedneutronthechargedneutron India
    Thanks for the comments.

    I am trying to procure Silver coated cloth electrode for my application.

    Meanwhile I tried applying silver conductive paste on top of the above mentioned  Cu and Ni cloth electrode. Although the surface is fully coated with the silver conduction gel but still I am facing the same problem.

    Can you suggest if silver conductive paste on top of copper conductive cloth should work or not?

    If it should, then I am having more issues with my setup.
  • wjcroftwjcroft Mount Shasta, CA
    Mixing metal types is never recommended. Mixed metals plus electrolyte = battery / galvanic current.


    LessEMF here in the US has various conductive cloths, some based on silver,


    You may have a similar online retailer in Asia.
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