I have been using a Polar H10 (chest-strap, kind of the gold standard in consumer-grade HR monitoring) for over 2 years now, and have always found the HR numbers from it to "feel" very accurate, and responsive. I got my first smart watch and Fitbit device ever--a Sense--a few days ago. Knowing that the wrist HR measurements aren't generally as accurate or reliable as the chest-strap ones, I have been wearing both during most workouts, to compare them. What I found is similar to what most others here have found, in that the HR reported by my Sense tends to be significantly lower than the one from my Polar H10. I do definitely think it has to do with your wrist movement, as, when I'm on the elliptical, the numbers tend to match very closely... but when I'm doing planks, bird-dogs, mowing the lawn, or anything else that involves moving my wrist a lot, it seems like the Sense tends to miss increases in my HR, for 30+ seconds at times. When I get back into the gym and start lifting again, I wholly expect the problem to be much worse, and suspect I will need to continue wearing both my H10 and my Sense, so I can have confidence in my HR reading. I got my H10 because I was overdoing workouts, and spending too much time in the red zone, and the H10 has helped a ton with me knowing what is "too much" now. Obviously, I can't count on the Sense for that purpose.
I definitely don't expect any watch to be as accurate as my H10, and my first reaction to the comments on this thread was just that this is to be expected when comparing a watch to a chest strap HR monitor... but I've read several comments that say other, older Fitbit devices were (and still are) more accurate than the Sense. That, to me, is the glaring problem. There is no excuse for newer, more-expensive tech by a vendor being less-accurate than older tech, and that is unacceptable, especially, as many people have said, given the price of the Sense. I do love many of its features, and have yet to find a smart watch (other than maybe Apple Watch, which won't work with my Android phone) that offers the features I want both in the device and companion software the way the Sense does. With that in mind, I'm not going to threaten to (or actually) return my Sense, like others have done here, but if they don't quickly figure out how to fix or mitigate this problem, I will definitely think twice about buying Fitbit devices in the future, and will be sure to make any friends considering one of these know about this issue.
The most frustrating part of this situation to me is that Fitbit doesn't even allow us to mitigate the shortcomings of their HR sensors by allowing us to read data from our more-accurate ones. I am now moving (from Google Fit) to the Fitbit app as my central health/wellness tracking app, so I want all of my health data to make it in there, and HR info during workouts seems like one of the more important bits of health/workout data. If Fitbit doesn't offer their own highly-accurate chest strap monitor like the H10, then they should've just supported ANT+ like Garmin does, so they could read the H10 data. With all of the insights and stats the Fitbit app provides (some of which is based on HR data), it seems like a silly oversight that they don't offer a way to get highly-accurate HR data into the app.
I hope other Fitbit customers come here and +1 this thread, and leave their own comment as well, so Fitbit sees that this is as a high priority.
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