I've found that my HR does not work properly if I am using any equipment that requires me to hold handles such as a stairmaster or elliptical. The HR will either go out entirely, show as too low (90 or so) or too high (190 or so). I'd say that for any 30 minute workout, 15 minutes will be one of these 3 things and the other 15 will be accurate. I contacted apple support and they had me reset and recalibrate. All that did was drastically change the calorie burn computations (50% lower now) than they were before. Did not do anything for the HR sensor. Trying to figure out if this is a defect or common. Thanks.