Air tracks use small streams of air to hold up carts so that they move with very little friction. They used to be very popular in physics classrooms. Using Motion Detectors on air tracks can be problematic because the air leaving the small holes in the track can create ultrasound that is picked up (and confuses) the Motion Detector. It may also be that the air currents created by an air track cause spurious reflections of the ultrasound. Here are some tips for using air track and Motion Detectors together:

• Test the apparatus with the air source turned off first. Confirm you get correct position readings with the cart position at various points along the track.
• Next try turning on the air and make sure this still works.
• Try adjusting the air pressure you deliver to the track. Use as little pressure as needed to float the cart with little friction.
• If possible, mount the detector several inches above the track.
• The air track carts must have a “flag” on them. That is, they need a object which can effectively reflect the ultrasound from the Motion Detector back to it. The flag only needs to be a few cm tall and a couple of cm wide.
• The flag needs to be very reflective (for ultrasound). Card stock may not work well, light-weight aluminum such as roof flashing, or other metal objects will.
• Our newer green motion detectors have a switch to choose between a person and a basketball or a cart on wheels. Normally the “cart” position would be best for short distance use, but some users say that with air tracks the other position (with person and basketball) works best. Experiment with this.

See also:
Why should I switch from an air track to a dynamics track?