@sartoric did what you say and tightend and cleaned the band and went running for 5 km which delivered the complete same (!) average hr both on the S5 and the AW 5
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Then after a pause went walking with the dog and voila same problem: after a while the hr raises magically on the S5 to over 150 bpm, while it never surpasses 119 bpm on the AW5
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Will give it another try tomorrow… but it’s weird