The garmin tri heart rate transmitter for 920xt and the fenix 3 has been specifically designed with triathletes in mind enhance your triathlon experience by pairing hrm tri with your forerunner 920xt or fenix 3 the hrm tri stores heart rate data when underwater then forwards it to you compatible watch garmins smallest and lightest module fits within the width of the strap its soft rounded edges and covered seams make the strap super comfortable out of the water too the transmitter reads