Hello, new Fitbit user here!
I've found that the Fitbit Sense's HR is not accurate. I tested with a Fitbit Charge 3 and Samsung Health heart rate sensor on my phone.
When I only move (go back and forth a few times) with my left arm (that's where my Fitbit Sense is) my heart rate suddenly sprints from 63 to 72, 75, 80, 85, 90. In some cases even 100 bpm, while the Fitbit Charge 3 and Samsung Health consistently indicate that this is 63 / 65 / at most 68. I read everywhere that this is a known problem. Can this be solved with a firmware update or is it the hardware? I come from a Samsung Gear S3 where the heart rhythm sensor no longer works after 5 (!!!) years, so I want to improve on that and not have that problem again with a fitness watch. Then I might as well bring it back, but I don't want that at all! It's perfect for me in every way except the most important: the heart rate sensor. I was introduced by someone else with the Fitbit Charge 3 and I liked it, hence the upgrade to the Fitbit Sense.
