I can't measure my heartrate which is main reason I wanted a fitbit, but my husband bought me the zen watch. The screen that I'm supposed to be able to measure the heartrate on has literally been visible 1 time and when i touched the watch as instructed the screan disappeared and i haven't been able to pull it up again. The only consistant wellness screen i can see only counts steps and measures sleep. How do i measure heartrate and is that a primary function of this watch?