SRM's Professional Powermeter provides the ultimate objective measurement of cycling effort. Used by over 80% of the world's top pros, it gauges joules and watts with a mere +/-2% margin of error. Plus, this amazing tool also measures heart rate, speed, cadence, temperature, altitude and much more. Cannondale has teamed up with SRM to offer power measurement on Cannondale's super-light, ultra-stiff Si crankarms. It's compatible with windows software.