Replacement digital heart rate sensor/transmitter strap for all digital coded Timex heart rate monitors. Digital FM transmission: TIMEX's system protects your data from other heart rate monitors and most exercise equipment. Synchronized transmission prevents crosstalk from other heart rate monitors Continuous heart rate readings even during the most intense activities Chest strap: Comfortable, lightweight, & adjustable elastic Elastic strap fits chest sizes from 25 inches to 53 inches Owner-replaceable battery with 2-year battery life Water-resistant to 30m (Sensor will not provide accurate readings when under water). Can be used to add heart rate capability to Bodylink systems not originally equipped with heart rate functionality Compatible watch models (see watch back): M117, M516, M529, M565, M575, M584, M589, M597, M606, M607, M608, M818, M838, M846, or any model that originally included the "digital transmission" sensor