Should residual sodium be considered in measuring sodium content of sweat?

I read an article, "[Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis](http://doi.org/10.1038/nature16521)", which describes the design of a wearable band that detects various components in sweat in order to monitor the wearer's health. One of the components detected is sodium.


My thought is that as the wearer sweats and the sweat evaporates, there will be a small amount of sodium left on the skin. When he sweats again, there should be an increase in the concentration of sodium due to the addition of more sodium from the new sweat to the residual sodium left on the skin.


I didn't see this addressed in the paper and am unsure if my thinking is correct. If I'm correct, wouldn't the sensor then need to have a component that does a calculation of how much sodium is due to the new sweat and how much is residual? Would it need to "know" how much sweat was produced to achieve this? What if the sweat under the band doesn't evaporate at the same rate as the surrounding skin due to being trapped under the band?


