Why do watch heart rate sensors, such as the Apple Watch, still read a heart rate when it’s not on a wrist and why is it seemingly random?

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Why do watch heart rate sensors, such as the Apple Watch, still read a heart rate when it’s not on a wrist and why is it seemingly random?

In: Technology

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The way these sensors work is that they shine a light into your wrist and measures the color of the light returning. As your blood pressure pulses for each heart beat the tiny capilary blood vessels in your skin will expand slightly making your skin more red. This is used by the watch to get a rythm which it uses to measure your heart rate. However the signal return using this technique is very noisy. So the watch have to be very liberal when trying to find your heart rythm as it can easily be drowned in other noise. So even if you remove the watch from your wrist and it will only pick up noise the algorithms might still find some sort of rythm burried in the noise. This will cause the watch to display the wrong heart rate as it have misinterpreted static noise or random signals as a heart rythm.