I had frequent problems with my series 9 watch jumping to erroneously high heart rates during workouts until I switched to an elastic watch band that I keep fairly tight. Years ago it seemed to like 205 or 210 for the false reading. Lately it likes to jump to around 190, though much less often. I've found I can make it revert to a plausible reading by briefly taking the watch off and putting it on again, which is an annoying interruption. What I'm wondering is whether the new series 12 is better about this since Apple made changes in how they do the heart rate monitoring. Maybe it's too soon to know since it was just released recently.
The software is fully up to date. I don't have any tattoos, scars, or other markings that would interfere with the sensor.
