Can I also hyjack teh thread.
How does the power kit for a polar HRM work?
how does a powertap and SRM crank work in measuring power?
The Polar system measures the tension and speed of the chain. Chain tension in Newtons multiplied by chain speed in meters per second gives Watts of power. The chain tension is measured by the chain's frequency of vibration, hence the need for chain weight and length between the gears. The sensor functions like an electric guitar pick up. An application specific integrated circuit or PIC derives tension from the frequency of vibration and filters out all but the fundamental harmonics. Depending on functions requested, processiing takes between 2 and 4 seconds.
PT and SRM work by measuring torque. Torque in Newton Meters multiplied by rotational speed in radians per second (2 pi radians per revolution) gives Watts of power. In each, there is an element which all torque is transmitted through. On this element is a strain gauge which measures the elastic response of the element to torque and represents it as a change in resistance.