HYDR8 is a wrist-worn hydration stress monitor built around a XIAO ESP32-C3, created by Ayushmaan after repeated flow-state work sessions left him with headaches and a full water bottle. It reads heart rate, blood oxygenation, skin temperature, and ambient temperature and humidity, then computes a single heat and hydration stress score from 0 to 100.
The device does not measure hydration directly. It cannot. Instead it learns your personal resting baselines and uses deviation from those numbers to generate actionable output: not a generic alert, but a specific instruction like 'drink water' or 'find shade'. The wearable face stays blank except for the time, surfacing a message only when the score demands it. The ESP32 hosts a full dashboard accessible from a phone.
This is a prototype, not a medical device, and Ayushmaan is clear about that distinction. What makes the full Instructables writeup worth reading is the sensor fusion logic, specifically how five input streams collapse into one scored output and how the personal baseline calibration works. The architecture is replicable with off-the-shelf parts and has obvious extensions into heat-stress monitoring for outdoor workers or athletes.
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