Worn around the chest, the Polar T61 Coded Transmitter is an essential part of the Polar heart rate monitor. It monitors heart rate in real time which is the most reliable way to measure heart rate during exercise. Polar Transmitters are also 100% waterproof and ECG accurate. Research shows that if you stop to take your pulse, your heart rate will fall by 10-20 beats per minute. So don?????t guess your heart rate, know it exactly. Polar's new Transmitters are designed with comfort in mind. A compact, lightweight ergonomic design means you'll hardly notice it's presence. Unaffected by the elements, wet or dry. Polar T61 Coded Transmitters have been designed to exacting standards to ensure the highest quality. Each transmitter delivers up to 2500 hours of operation or five years of use exercising 10 hours per week.