Faulty Sensors - Geodesic Sensor Net Technical Manual

11: Troubleshooting

Faulty Sensors

The following guidelines are useful to determine whether an individual sensor is
faulty.
Unacceptably High Impedance
• Check for contaminating substances on the subject's scalp or hair. Natural oils,
dirt, and hair-styling products can all affect contact quality.
• Immerse the sensor end of a Net in a bucket of electrolyte, and use your
acquisition software to measure sensor impedances. If a sensor's impedance is
above 10 kΩ while immersed in the bucket of electrolyte, it is likely to be faulty.
(All sensors are certified below 3 kΩ in new Nets, but may increase slightly in
impedance over time.)
• See "EEG Contains Excessive Noise" on page 108.
Sensor Consistently Faulty in Some Way
It is always a good idea to wait for a sensor to fail for more than one recording session
before replacing it.
Note: It is also a good idea to keep a logbook in the data-acquisition area of your facility,
for keeping track of which sensors on which Nets appear to be out of order.
Faulty Sensor Tied to a Specific Net
Check whether a different Net apparently has the same faulty sensors. If it does, then
the problem is actually not related to the GSN sensor array but resides in the GSNIC,
the amplifier, or elsewhere.
If a different Net does not exhibit the same faulty sensors, then the problem likely is
local to the Net with the faulty sensors.
112
Geodesic Sensor Net Technical Manual
S-MAN-200-GSNR-001 • January 31, 2007