After many tests, Fitbit's PurePulse system is unreliable.
At rest the beat exceeds 20 / 30bpm and under stres hardly reaches the real pulsation.
Unlike other optical devices, which measure the dilatation of blood capillaries, the Fitbit Ionic performs an average calculation of 5 minutes, resulting in delays with random values, compromising the measurement of real caloric consumption.
Moreover, in the case of an accelerated beat, it does not reach the calculation alert threshold (cardio 220 - age) putting athletes at risk.
Polar Vs. Fitbit Ionc
In this TEST, I do 5 rounds in the punching bag, where Polar reports exactly the fatigue condition, and Fitbit is absent ...
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@Nettuno I hope you're doing well. Sorry for the delayed response.
Thanks for sharing your experience with the Fitbit Community. Your feedback is valuable.
If you have any ideas for features you'd like to see on Ionic; I recommend posting them at the Feature Suggestions Board. Please search if there is an existing feature asking for the same or post your original ideas.
Additionally, please check this helpful article about heart rate readings for some in depth details about this.
Keep me posted! 
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