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Have you cleaned the back of the watch properly? Around the housing of the HR sensor can be a lot of dirt. Clean it with warm water and liquid soap, you can also use a soft tooth brush. Do the same with the straps. Then dry it. Dry your skin after shower and wait a bit until putting the watch on again. And don't wear it too tight. If you sweat a lot you should clean your watch every few days or after every run or workout. As the skin is damaged now you shouldn't wear the (cleaned) watch until everything is healed.