The crash sensors must be able to detect a collision and convert it into corresponding signals within a matter of milliseconds. The accelerating forces acting on the sensors immediately after a collision can be as high as 100 g (100 times the earth’s gravitational force). The measurement principle employed by the crash sensors is usually based on the effect of inertia. When a vehicle is stopped abruptly by an impact, all bodies or objects which are not firmly fixed to the vehicle will continue to move at the impact speed. The sensors’ design enables them to measure this acceleration and relay it to the control unit as usable data. Many Volkswagen models are fitted with ultra-fast pressure sensors in the front doors to detect a collision from the side. When the vehicle is hit from the side, the outer door panel is pushed inwards, creating excess pressure inside the door. If the change in air pressure exceeds a certain limit, the sensor will relay a corresponding signal to the airbag control unit. Acceleration sensors are furthermore fitted in the vicinity of the C-posts so that collisions from the side which do not cause deformation of the front doors can also be detected in time.