How accurate during intense cardio spurts is heartrate reading? What is 6our highest Fitbit reading,

[Victamon]

While doing high-intensity full-effort on Precor machine, with sufficient caffeine intake, can get my heartrate reading up nearly ton200bpm (wi5h my max instantaneous >201). Yet my Sensenduring dame time showed a maxed reading of merely 160ish. 

[Guy_]

Hi @Victamon - the Sense is good for measuring heart rate at rest, it gives good resting heart rate and can do ECG.

However if you are active it may over or under read. Mine [and it's replacement] systematically reads nearly double at times, others like yourself, report under reading. The divergence for different people may be due to age, sweat skin, type of movement  or other factors.

[KB9WNS]

Get a chest strap that is compatible with your watch for better accuracy.  All of the optical wrist sensors, regardless of brand, have issues with high intensity interval training.