The GEL 2471, 1- or 2- channel speed sensor is used with electrically conductive measuring scales with an up to 4 mm air gap. 

It is a maintenance and wear-free operation due to non-contact measurement of rotation with, Strong reinforced walls in the sensor tube with a diameter of 20 mm provide increased protection against the impact of stones, grit and foreign objects.

If you are trying to build to a weight limit, the gel 2471 is perfect due to its construction with light-weight target wheels made of aluminium.

GEL 2471
Fields of applicationRail vehicles, traction control
Supply voltage10 to 20 V DC
Measuring range0 Hz to 20 kHz
SignalVoltage output
Temperature range-40 °C to +120 °C
Protection classIP 68
Number of channelsup to 2
Material of target wheelAluminium, steel (other on request)
Air gap0.7 mm; 0.8 mm depending on the module