Embodiments of the disclosed invention provide wearable devices that use a humidity sensor to measure sweat rate generated from an area of skin. A sensing chamber is continuously filled with a sweat sample, which forms a droplet and alters the humidity measured within the chamber. Once the sweat sample droplet expands to the edge of the chamber, the droplet contacts a wick and is drawn away, so the chamber can fill with a subsequent droplet. The device uses a droplet volume and the time required to reach a maximum humidity to calculate a sweat rate. A pump is used to draw old sweat sample out of the wick to allow extended device operation. Some embodiments also include capacitive sensors to perform back up measurements. Another set of embodiments includes alternatively shaped sensing chambers configured to reduce sample volumes or improve function. A method for determining sweat rate based on humidity sensor measurements is also included.
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