Polar CS100, CS200, CS300 and CS400 cycling computers are not compatible with S-series cadence sensor. To get the right cadence readings, they need to be used with Polar CS cadence sensor™.
S-series cadence sensor
CS cadence sensor
If you use S-series cadence sensor with the CS cycling computers, the cadence readings will be double. The transmission technique between S-series cadence sensor and CS cadence sensor is a bit different which causes the compatibility difficulties and erratic readings.
Polar S-series speed sensor may work with the CS100/CS200/CS300/CS400 if the distance between the sensor and the cycling computer is short enough.