If you are serious about your training, you may want to invest in a sport watch. It has a weight of only 93 g, which makes it easy to wear.
From Amazon
Garmin 010-02157-00 Fenix 6X Pro, Premium Multisport GPS Watch, features Mapping, Music, Grade-Adjusted Pace Guidance and Pulse Ox Sensors, Black
365rider
Garmin Fenix 6X PRO black with black band
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