This is the more accurate cadence sensor we sell. It has 24 magnets, while the other ones have 12.
From the distributor:
"CA3 Compatible Mini PAS Sensor with 24 Poles for Immediate Engagement. For Square Taper Spindles with at Least 5mm of Exposed Spindle.
Mini 24 Pole PAS sensor for bikes with square taper cranks. This integrated sensor is fully contained and more compact than a conventional exposed magnet ring device, and the 24 PAS poles allows for almost instant power engagement the moment you start pedaling. Internal electronics have been customized to be compatible with the CA3's PAS plug with 10V power and quadrature PAS signals for installation on either the left or right side.
Installation requires that you have at least 5mm of exposed spindle between the face of the bottom bracket and cranks. Comes with plastic cable guide for bikes that have exposed brake or shifter cables under the bottom bracket. Available in standard 90cm cable length, or optional 120cm for unusual bike frames.
Note two critical measurements: square taper spindle diameter and bottom bracket shell diameter - if your spindle is larger than 16.5mm you may need to file the adapter hole. If your BB shell is larger than 43mm OD (common with some alloy frames), the sensor may sit at an angle, or need to be modified to install.