@andreas-scheucher Hi, I am using a Wahoo Tickr Fit, which I can recommend highly, due to the consistent and reliable measurements - even during medical examinations the values where rather identical with the ones from the professional / medical equipment, due to the very comfortable way I can use it in all conditions and for a rather unlimited time, and last but not least because of the good operating time with approx 30 hrs. A friend of mine is using the Polar OH-1+ (new version), which is very good, too, and better if you want to use such a device while swimming.
As these sensors are not that much more expensive than the one you consider, I would recommend the Wahoo or Polar.