Airspeed Indicators measure the difference between total pressure and static pressure. The total (or measured) pressure is sensed by a pitot tube positioned at the stagnation point of the airstream and is carried to the airspeed indicator by a hose.
A static pressure sensor can also be connected.The measuring element is an open diaphragm unit with excellent elastic properties; hysteresis (differences between advance and return movement) and secondary elastic effects are negligible. The effect total pressure and static pressure acts on the diaphragm, causing a deformation, that is transferred to the pointer by a linkage and gear mechanism. The different operating ranges of the aircraft can be identified by colour markings on the airspeed indicator dial. The instruments are insensitive to shock and vibration.