https://patents.google.com/patent/US6470997
The air gun is further designed to release the compressed gas so that the release time or gun fire time is monitored with sufficient accuracy. The fire time is measured by a timing coil that determines the location of the air gun shuttle by sensing a magnet disposed on the shuttle. The timing coil is designed to eliminate interference caused by magnetic flux generated by the actuation of an air gun solenoid valve.

Several methods for monitoring air gun fire times are well known in the art. One method involves the sensing of movement by the air gun shuttle. This method employs a series of magnets that move with the shuttle and a coil of wire located in a stationary position on the air gun. When the shuttle opens, the magnets in the shuttle approach the coil causing the number of magnetic flux lines encircled by the coil to increase. This action induces a voltage in the coil. The voltage is then monitored to determine the precise firing time of the air gun.