My hear 2 DO NOT WORK I have exchanged 2 of them. I'm not even that dark. I'm a good brown and it works 0 percent of the time... I see a class action lawsuit in samsungs future.. for a item that does not work as advertised.
It is far from an exact science. Shine light into skin, measure the time between changes of color, calculate.
It also will not be the only heart rate option out there. It's stock software used to enhance the product. The real power comes with the installed apps.
I'm sure it's more of a software thing than a hardware thing, especially if you've already taken two of them back. Either wait for a new heart rate app or try and roll your own.
If you purchased the watch specifically for the heart rate capabilities, you may want to return it and wait until the product has had time to mature.
Good luck to you.