Garmin Vivoactive 3 Faq page 9

For higher Fitzpatrick classifications, more melanin is present in the skin. The sensor may have
to work harder to find pulse which can require slightly more battery power
What about tattoos?
Tattoos absorb much of the light from the optical heart rate sensor which can cause major
problems with accurately detecting heart rate. The sensor needs to be on an area free of tattoos.