A bit over a year ago I took the time for little experiment and made practically simultaneous heart rate measurements using a fairly recent FitBit model, anothe

A bit over a year ago I took the time for little experiment and made practically simultaneous heart rate measurements using a fairly recent FitBit model, another smart watch (forgot which one), the heart rate reading from a medical arm cuff blood pressure monitor and manual counting over a minute. In a rested state, repeated five times. The arm cuff and manual measurements showed very limited variablity over different measurements and were in very good agreement with each other (like +/- 4 beats per minute). The smart watches were all over the place, showing deviations of up to twenty beats per minute from the former two methods / each other, varying by over 20 beats per minute over consecutive measurements, and not even showing signs of convergence over repeated measurements. My conclusion (at least for my skin type, which does affect accuracy for watch-based measurements): smartwatch-based heart rate measurements are probably best used as slightly biased random number generators. If tracking blood pressure is much harder than the heart rate, as they say in the article, I don't even want to know.