The Polar H10 heart rate sensor provides ultimate accuracy. Compatible with Go Pro Hero, Polar Beat, gym equipment and 3rd party apps and has memory to record data. Unstretched length can be adjusted between 71.9 and 91cm. Provides advanced heart monitoring and features an entirely new measuring algorithm. Heart rate is measured accurately and without interference. Compatible with go pro hero 4 & 5; view your heart rate on your go pro recordings. The h10 has an hours worth of memory to record data without the need of a wrist unit. Works with gym equipment and polar club/team app & in water. Weight 0.6kg. CR2025 (not included). Manufacturers 2 year guarantee.