What is the process to measure the speed of aeroplane

The airspeed indicator is the only instrument connected to both the pitot tube and the static system. Air from the staticsystem fills the case of the airspeed indicator, providing a"base" pressure against an expandable diaphragm. Air forced into the pitot tube as theairplane moves fills the diaphragm, which expands as ram air pressure (and speed) increase. A needle connected to the diaphragm rotates as the diaphragm expands. The needle's position on the instrument face indicates airspeed