[kaimok]

T6 manual states often: "If you adjust the settings after activating the stopwatch, Suunto t6 will still use the old settings." That was the problem. I activated 1st the stopwatch, then paused it (pressed START/STOP), connected the chest belt and then activated the stopwatch (pressed START/STOP).

T6 manual, page 59:

4.2. TRAINING EXAMPLE
Example: Rita Runner
[...]
She puts on her HR transmitter belt, making sure the strap fits snugly, and checks that the electrodes are moistened. By long pressing ALT/BACK in the Training or SDP/DST mode, she connects the heart rate transmitter belt with her Suunto t6. In the Training or SPD/DST mode, she checks the stopwatch and resets it by long pressing the UP/LAP button. After that, she presses the START/STOP button which activates the stopwatch and interval timer, at which point she begins her training. The heart rate limits alert her during her training, whenever her heart rate is too slow
---

So:
1. connect the chest belt with T6
2. reset the stopwatch
3. activate the stopwatch

What I did was: 2-3-1

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