Zenith EL PRIMERO 21 9004 Mode D'emploi page 13

With the watch in your hand, turn the crown clockwise until the
hand of the chronograph power reserve is fully wound (around
25 turns). The barrel that contains the mainspring has now been
wound.
Fully wound, the chronograph power reserve is around
50 minutes. This is displayed by the hand located under the
Zenith logo.
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