Wrist is anatomically problematic because of its mobility and bonyness. If you want something accurate and reliable, other than an electrically coupled chest strap, get an optical device such as the Scosche that fits in the forearm. You will, however, have to read your HR from a paired device, such as a cycle computer or a phone. I own all three types of devices and they all have their uses.
Also, I and a substantial number of people I know who have trained with chest straps, have developed contact dermatitis from them.