today I did my first pool swimming session with Fenix 6X Sapphire and I was pretty curious how optical heart rate sensor works. Actually I was surprised how well it worked.
First I swam 1km without break, then 4x50 m higher effort intervals, then 4x50m lower effort intervals with breathing on my weaker side and then 100m cool down. Watch recorded my heart rate pretty good, even intervals, clearly seeing difference between higher and lower intensity intervals.
(Blue is pace, red is heart rate)