The impurities from sweat will start to build up to the strap if it isn't washed often enough. The impurities will block the connection from your skin to the strap causing errors in heart rate and eventually causing connection loss (HRM shuts down). Impurities can also cause irritation to the skin.
It is recommended to rinse the strap with water after every use and machine wash the strap after every fifth use. Using only water is not enough in the long run. The HRM strap stands approximately 100 washes at 40ºC/warm with the most common laundry detergents. Detergent should not contain optical brighteners or bleaching agents. Use of soap or fabric softeners is not recommended. Use of laundry pouch is recommended. Sports Tracker Heart Rate Monitor strap does not resist soaking, dry-cleaning or bleaching. Air dry on a flat, non-heated surface after washing.
Store Sports Tracker Heart Rate Monitor strap and the transmitter separately. Do not fold the sensor area in the belt. Do not expose the strap to direct sunlight or cold conditions for extended periods.