Xiaomi’s Mi Band2, seems like a bargain for about $20 retail. And it is; unless you’re black.
Black users across the web have complained that the device can’t pick up accurate heart rates with its sensor unless it’s placed on lighter parts of their skin — like the palm of the hand — or you place a white sheet of paper under it (between the band and your skin). There’s an entire multi-page forum post of black fitness buffs complaining about this very problem, and it seems to have been an issue since at least October 2016 (with its previous model).
http://thenextweb.com/gadgets/2017/01/24/this-xiaomi-fitness-tracker-doesnt-seem-to-work-on-black-people/