I had been using an Alta HR since December 2018. Last week I switched to a Versa 2 so that I can take advantage of its swim tracking capabilities.
I am not surprised the the devices are not calibrated exactly the same, but I am surprised at how large the differences are in the "total calories burned" model are, given nothing else has changed in terms of exercise.
With the Alta HR, a linear regression between 80 "total calories burned" and "active minutes" pairs produced a pretty good fit which indicated that for my typical exercise regime (2-3 hours walking/day) I burnt 6.6 cal/min during exercise (not including passive calories) and 1.37 cal/min (1972 cal/day) for everything else (including BMR).
Since I have been wearing the Versa 2 for the last week, the regression through that (admittedly much smaller set of data) yields 3.6 cal/min (not including passive calories) for exercise and 1.6 cal/min (2317 cal/day) for everything else (including BMR). These are quite large differences.
So my questions are:
- how does FitBit estimate the passive calorie burn rate, and why do the two devices use such different rates?
- what explains such large calibration differences between devices?
- are these calibration differences likely to reduce over time?
- is the expectation that two different models of FitBit device would have such a large calibration difference?
- would I expect to see similar calibration differences if I switched between two FitBit devices of the same model?
