Beurer PM 25 Gebrauchsanleitung Seite 29

Interference from other devices for measuring heart rates
The HR monitor as a receiver can react to interfering stray signals
such as other heart rate monitors within a vicinity of one metre. This
can result in incorrect measurements particularly when training in
groups. If you e.g. job next to one another, it can be beneficial to
wear the HR monitor on the other wrist or to increase the distance to
one's training partner.
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