The CycleOps PowerTap 2.4 Speed/Cadence Sensor can be used as a standalone speed sensor in case you're riding with your Cervo 2.4 or Joule CPU, but not your PowerTap hub. Beneath the battery of this unit is a small switch that toggles between speed and cadence. By switching it to "speed", it turns your cpu into a wireless bike computer.
It's your choice: You can use this sensor to read wireless speed and distance or actual cadence if you'd like to get a more accurate, real-time measure of how many times you turn the pedals over per minute as opposed to the virtual cadence that a PowerTap Hub gives on its own. In other words, a PowerTap Hub calculates your cadence based on the positive torque applied to hub. It's accurate over the course of the entire ride, but there might be instances when it might seem slightly "off", such as in extremely high cadence situations, or when your pedal stroke is especially smooth.
The PowerTap 2.4 Speed/Cadence Sensor is wireless, communicating to your CPU via ANT+ technology. It only takes about a minute to install. One sensor will do either speed or cadence. Buy two if you want both measurements.