What sensors do smartwatches use, and how do they process the data? For example, what physical principles underlie mechanisms that measure heart rate or movement? How accurate is real-time data processing—does all the data stay on the device, or is some processed elsewhere?
How do the sensors in smartwatches work?
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Most of the sensors in smartwatches are actually based on the same principles used in smartphones and other portable devices. For heart rate measurement, a photoplethysmography (PPG) sensor is commonly used—it detects changes in blood flow under the skin using infrared light. The "Doppler effect" from basic physics also plays a role here: the movement of blood cells reflects light, causing small shifts in wavelength, which helps calculate the pulse rate. Motion sensors (accelerometers and gyroscopes) rely on Newton’s second law—they collect acceleration and angular velocity data based on physical inertia.
The real-time data processing part is the most interesting detail. In high-end models (like the Apple Watch Series 8 or Garmin Venu 3), sensors process data directly on the device to extend battery life—especially for heart rhythm analysis. Raw PPG waveforms are first filtered through hardware, then machine learning models filter out noise and improve accuracy. However, in some budget watches, data is often sent straight to company servers before reaching the main processor—a factor that reduces accuracy, as network latency or data compression can affect measurements. So, to answer the question "Does everything stay on the device?"—yes, in premium models, rarely in budget-friendly ones.