Meet the 3Dmagnetic Smart Sensor: a tiny gadget that uses Earth’s magnetic field to monitor your activity accurately. It can be used to measure swimming, rowing, paddling, tennis, and other activities. It can measure speed, cadence, kicks, strokes, and swings, depending on what you do.
The 3Dmagnetic sensor has a battery that lasts over 300 hours. It is compatible with iOS and Android devices. Costs $29 at this time.