Salt solution problem

[Sajk]

We inadvertantly did the bucket test for testing electrodes with iodized salt. Once we realised what we did, we immediately rinsed the electrodes with water and used regular salt but we can't seem to get the impedences down. Has anyone else encountered this, and is there any way to salvage the electrodes?

thanks

[Coen]

We have always used iodized salt for all our tests since the introduction of our active electrodes 12 years ago, and we never saw any adverse effects. Your problems must be caused by something else.

Is the CM in range ? How high are your offsets?

Electrodes with high and/or unstable offsets can often be improved by cleaning the tip with sandpaper, followed by soaking in salt water for a couple of hours. If no improvement is detected after 10 hours or so, then further soaking is usually not worthwhile anymore.

Best regards, Coen (BioSemi)

[Sajk]

Thanks Coen that's very reassuring, perhaps we haven't ruined our
electrodes!

We have just started to do the bucket tests as part of regular maintenance
on a number of electrode sets, both individual flat type (EXG) and sets of
32 pin type (with/without CMS/DRL).

We noticed that initially the offsets and recordings were very good for a
relatively new set (~10) and not so bad for an older set (~20). After doing
some recording of these sets we then found that repeating the test on the
same electrode sets the offsets were at maximum levels (as you would see
with no electrodes plugged in).

After some experimenting and swapping individual electrodes out of the
bucket we found that after removing Fp1 from the bucket with the new set we
could again record a reasonable offset but with irregular/random 'popping'
of all the remaining electrodes. We then terminated the testing as we
discovered we had iodized salt and incorrectly thought that could be the
issue.

Also we noticed when emptying the 'bucket' (1 litre/1 teaspoon salt) that
there was a small but noticeable amount of a black residue in the bottom of
the bucket.

We will attempt these tests again using the information you provide but
would appreciate any advice you might have regarding our observations above.

[Coen]

Always start with checking whether the CM is in range (blue lamp continuously on). With CM out of range (blue lamp blinking), no valid data con be recorded since the the power supply to the active electrodes is cut off. CM out of range may be caused by one or more defect electrodes. In your case, a defect Fp1 most probably causes the CM to go out of range. Please sent the set to your local representative for replacement of the Fp1 electrode.

With the CM in range, offsets should be stable and approximately within the +50 mV to - 50 mV range when testing in salt water, and approximately within the +100 mV an -100 mV on the skin. A stable offset with not too much differences across the electrodes is more important than a value close to zero.

The black residue is probably a AgCl. Silver from the electrodes go into solution, and the Ag+ ions combine with the Cl- ion from the salt. NaCl is not very soluble in water, and precipitates as a black residue. Nothing to worry about.

Best regards, Coen (BioSemi)

[Sajk]

Thanks for the prompt reply Coen,

Wanted to let you know that we re-ran the tests on 6 electrode sets (3 pairs) and found that the offsets were stable and variation within an electrode set was less than 10mV (no single electrode offset was greater than 10mV).

We found that the "popping" during the bucket tests we performed previously was caused by vibrations/air flow in the room and unrelated to electrode function. We also found that 2 of the "A" electrode sets contained a faulty Fp1 electrode and 2 of the "B" sets contained a faulty Fpz electrode.

It was however, intriguing to note that each of the electrodes which we found were faulty were all on pin 1 of the cable connector (the side with the red stripe). We wondered whether this was merely a coincidence and if there is anything we could do here in house rather than sending it off to our local representative as it is rather expensive to do so.

Regards
Sajk

[Coen]

Failing of the first or last electrodes of a set is usually caused by broken wire conductor near the connector. The break is caused by (strong) sideway pulls on the flatcable with the connector still plugged into the AD-box. It is a case of abuse, seen with only a few customers. The set can be repaired by mounting a new connector (with the cable shortened approx. an inch). Please sent the sets to your local representative to replace the connectors.

Best regards, Coen (BioSemi)