Polar T41 Heart Rate Monitor. Rinse the elastic strap with water after every use and wash it regularly in a washing machine at 40ºc/104ºf. Thus your polar heart rate sensor gets activated, and now it will start recording the data from your heartbeat.
With a new cr2032 battery and an eyeglasses screwdriver, you should be able to replace the battery in just a few. Transmits heart rate data to polar heart rate monitors using 5khz transmission and compatible gym equipment. For what it can do, that is an outstanding price.