Xiaomi Mi Smart Band 5 Manuel D'utilisation page 5

2. To achieve the optimal performance of the heart
rate sensor, make sure its back to contact with your
skin. When wearing your wristband, keep it neither
too tight nor too loose but leaving some space for
the skin to be able to breathe. Tighten the wristband
before you start to exercise and properly loosen it
a erward.
If the band can easily
move up and down the
wrist, or the heart rate
sensor cannot collect the
data, try tightening the
wristband.
Too loose
The band can
comfortably fit
around the wrist.
4
Just right