On most days, the timer at a self-serve car wash shuts down the session after a set period. But during slow times, some car washes offer each driver an open-ended sudsing experience. The problem then becomes one of telling the timer that the customer has finished and left the bay, and that it should reset itself to keep the next patron from spiffing up free of charge.
Three technologies in common use don't solve the problem. Photoelectric sensors are easy to defeat. In-ground inductive loops can't be installed around the metal drain grate, and a loop at the exit would allow a new customer to slip in before the previous one has left. And observing the bay on camera costs too much.
Banner Engineering's M-GAGE magnetic sensor is a better way to go. It's a self-contained receiver about the size of a standard pack of gum and uses Earth's magnetic field as a baseline. The device incorporates magnetometers that detect magnetic flux changes along any of three axes. When a large ferromagnetic object such as a vehicle moves into the bay, the M-GAGE detects a distortion in the ambient magnetic field. In contrast to an inductive loop, the sensor continues to keep track of even stopped vehicles.
Because the goal is to detect a departing, rather than an arriving vehicle, the M-GAGE is tied to the controller in the payment box. When the patron parks in the bay and prepays for a wash, the M-GAGE is activated. The magnetic flux shift caused by the clean vehicle as it leaves the bay triggers the timer to end the washing session.