So I used a Fitbit Versa to measure my heart rate for a science experiment and I need to record the uncertainties in my report. How can an uncertainty for measuring the heart rate with a Fitbit be determined? Is it something I can calculate with my data? Thanks. 

The measurement of a ruler, for example, is (by default) half of the smallest division you can measure. i.e. if you see something that is between 33 and 34 mm, and you estimate it to be about not quite halfway between those lines, you'd say the length was 33.4 ± 0.5 mm. (That assume you can do the interpolation, and estimate the 0.4 part) It also assumes essentiall no error is introduced on the other end of the ruler — the ruler starts at the beginning of the line.

How would you apply this concept to a fitbit measuring a heart rate?