What is the difference between Flex Gel and Flex Saline?

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Saline sensors are much faster and easier to set up and they give comparable signals to gel when sufficiently wet. The noise floor is probably just a little higher with saline- although this largely depends on preparation before applying the gel. You can use abrasives to scrape away the superficial layer of the stratum corneum (dry outer skin layer) before applying. gel, so it is possible to gel extremely low contact impedance. However, this may be problematic for children/people who won’t tolerate a lengthy set-up period.

You can record for many hours with gel sensors, whereas with saline, you will start to see signal degradation due to evaporation after about an hour, depending on conditions (very hot dry conditions and high altitudes make this worse). You can add a non-volatile component such as glycerin to the saline mixture to extend the time of use to several house, and you can also simply top up the fluid intermittently (say at 30-minute intervals) by applying more fluid through the rear of each sensor.