Got my Series 2 on launch and seem to be having an issue where it struggles to detect heart rate during exercise. I'll need to wipe the sensor and my arm before exercise for it to work properly at the start. Sometimes i'll need to take it off during and give it a wipe as well. My heart rate will go up to around 150bpm and then the next minute the watch will tell me its 70bpm and wont correct itself until I wipe the sensor which will affect the amount of calories it thinks i've burnt during the workout. Is this a fault or do other peoples do this too? Obviously it's the sweat that is affecting its ability to work but should this happen seems like it defies the point a little bit?