The heart sensor on the Mi band 2 is accurate, but it is not completely accurate. It does not track everything that goes on in the body, and that is why some people might find it inaccurate. The HR sensor is accurate for general activities, like walking and running, but it is not accurate for things like weightlifting oriqueness! The heart sensor does help with performance monitoring, but it is not perfect. It is easy to overthink how accurate a heart sensor is, and it can be difficult to use incorrectly. If you are trying to use the heart sensor to track something like weightlifting or unique activities, make sure to use the other sensor first.