When conducting long-duration EEG experiments—especially those lasting over an hour—it’s common to observe a gradual decrease in EEG Quality (EQ). The primary reason for this degradation is the evaporation of the saline solution used in the felt sensors of the headset.

Why Does Signal Quality Degrade?

For example, if you’re using Flex Saline or Epoc X, over time, the saline that maintains electrical conductivity between the sensors and the scalp begins to evaporate. As the felt pads dry out, their impedance increases, which leads to a decrease in signal quality. This effect is more pronounced in environments with:

• High temperatures

• Low humidity

• High altitudes
  
  
  

In typical indoor conditions, saline evaporation can start to noticeably affect signal quality after 1–2 hours.

How to Maintain EEG Quality in Long Experiments

To ensure reliable data collection over longer periods, we recommend the following strategies:

1. Scheduled Saline Top-Ups

The EPOC X headset is designed with a convenient feature: small refill holes located behind each sensor. This allows saline solution to be replenished during the session without removing or disturbing the headset. As a best practice, consider scheduling saline top-ups every 30 minutes as part of your experimental protocol.

2. Add Glycerol to the Saline Solution

Another effective method is to mix 5–15% glycerol into the saline solution. Glycerol is a hygroscopic (moisture-retaining) compound that significantly slows down evaporation, thereby extending the effective life of the saline.

Additional benefits:

• Helps maintain lower and more stable impedance

• Glycerol is hypoallergenic and commonly used in skincare products

However, be aware that glycerol has a slightly greasy texture, which some participants may find uncomfortable.