From: jiva_at_humboldt1.com
Date: 2001-07-11 14:54:20
Some electrode feedback from today's Biofeedback Digest:
Date: Tue, 10 Jul 2001 12:17:16 -0700 (PDT)
From: Hershel Toomim <htoomim_at_y...>
Subject: Re: Equipment -HEG
Mike,
The best electrodes are selected by their offset
potential vs skin and background noise level. The best
for both these criteria are silver/silverchloride
coated plastic buttons. They are available from all
electrode vendors. They are inexpensive and
disposable. They can be used up to 10 times. We can
supply your needs.
Gold electrodes sound nice but have a very large
offset potential. Their basic advantage is that do not
corrode and therefore have a stable potential vs skin.
The use of two such electrodes causes cancellation of
the offset if the potentials are equal. They are too
expensive to be disposable.
Some system are not sensitive to offset potential.
This feature limits the low frequency response of the
system or causes a long time constant with dead time
after large transients such as reapplying the
electrode.
HEG has no such electrodes. They are not required for
light transmission
Hershel
--------------
Date: Tue, 10 Jul 2001 23:23:35 EDT
From: Carmen5272_at_A...
Subject: Equipment -HEG
Mike,
I agree with Hershel's response regarding the nature of the
electrodes. Ag/AgCl electrodes are universally the best bet. I like
disposables because they don't age and polarize. I also prefer wet
gel to dry gel but there are good reasons for selecting one over the
other, and it might make sense to have some of each on hand. You can
still get non gelled electrodes that you gel yourself, although the
majority now are pregelled.
One thing additional. Maintaining quality connections goes beyond the
electrode makeup. The lead wires represent one of the few moving
parts in a modern system. Just like bearings and brakes on a car,
they have a limited lifespan based on number of times flexed at
connection points as well as potential for corrosion at the point
where two different metals and/or saline come in contact with each
other.
Defective leads may misbehave by generating their own signals and/or
detecting radio frequency signals, and still give adequate impedance
readings under most conditions. Since usually only one lead in an
active pair will be bad, the instrumentation amplifier in the
instrument will not recognize this sort of signal as noise. It will
think it is real information.
There is a moral here, but I am not sure what it is. You need to make
sure that your leads are always in perfect shape. However this
requires knowledge of electronics and test equipment. Even then it
isn't always easy to tell. It is likely that you will start getting
bad data some time before you realize there is a lead problem. It is
a good idea to have a complete set of substitute leads, but that is
not a cheap undertaking. I am not sure that there is a universally
satisfactory solution.
Even with Ag/AgCl electrodes (which represent the "gold standard"),
the electrode to wire interface and wire to instrument interface is
just as potentially troublesome as the electrode to skin interface.
So, stay on your toes. If a signal just does not seem to fit, suspect
the moving / flexing parts before you necessarily assume that you are
looking at real data.
Jeff Carmen
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