A Velocity air meter measures the force of air rushing into an engine against a spring-loaded door in an air tunnel. A linear potentiometer rotated along with the hinge of the air door measures the angle of the opening and sends an electrical signal to the engine control unit (ECU) that changes as the door opens more or less. The ECU can deduce airflow into the engine based on this value after making certain corrections. The problem is that the force against the door is a function of two variables - weight of the air and speed of the air - and the ECU cannot distinguish, for a given door angle, whether the air is less dense but traveling faster or whether the air is denser but traveling more slowly. Therefore air velociy must be corrected for air density (temperature and barometric pressure) in order to reflect air mass (actual number of air molecules entering the engine per unit of time). Velocity air meters from Bosch have been commonly used on systems such as older Bosch L-Jetronic and as a part of many factory systems, though they now have mostly been superseded by mass airflow sensors which have many advantages and are less damaging to engine volumetric efficiency (VE).