The ASI measures the airspeed in terms of the difference between the pitot and static pressures detected by either a combined pitot-static probe, or a pitot probe and static vent, as appropriate. The indication needle will be set in motion by the expansion and compression of the membrane. Due to the air pressure differences in higher and lower altitude a static port is needed to generate a reliable indication anytime in the flight. The static port(s) provides the static pressure which equals the pressure inside the housing to the pressure outside aircraft. The dynamic pressure, directly from the pitot tube, is connected to the membrane. The higher the airspeed the higher the dynamic pressure while the static pressure only changes when the flightlevel or the atmosphere changes. This difference in dynamic and static pressure results in a movement of the membrane. The movement of the membrane sets a few gears in motion, which are connected to the indication needles.
The compression of air brings a downside with it. Compressed air, due to high velocity, has an higher pressure ratio than non compressed with normal air velocity. The pressure varies with the square of the airspeed. With higher speeds the dynamic pressure builds up and the membrane expands linear to the pressure. The response characteristic shows a greater deflection acceleration at lower speeds then the high speed deflection. This results in a non-linear scale on the indicator. To prevent the unwanted side of the pressure build up, the use of a non-linear scale is compensated by a square-law compensation. The most commonly used method is the use of en lever attached to the membrane that is progressively altered. By high deflections of the membrane the pointer increases for small deflections and vice versa.
Another method is the use of a tuning spring. The spring bears against the membrane and applies a controlled retarding force to membrane expansion. The force is governed by sets of ranging screws which are pre-adjusted to contact the spring at the appropriate points of increasement. As speed and differential pressure increases, the spring rate increases and its effective length is shorted. This compensates the use of a variable magnifying lever or a non-linear scale. The airspeed after compensation is the Calibrated Airspeed (CAS)
The use of digital air data computers takes over the mechanically measured indicated airspeed. In this case the dynamic and static pressure are connected to the Air Data Computer (ADC). The signal from the ADC goes to the instruments which display the airspeed by the use of servo's or by displaying the airspeed on LCD or TFT screens. The computer compensates also for position errors which is called the Computed airspeed. The Computed airspeed is used the calculate the CAS. The photo to the right displays both the primary and navigation display of the Boeing 747-400 making use of the above mentioned systems.
The CAS is then used for the calculation of the Equivalent Airspeed (EAS). This airspeed is calculated from the measured pressure difference p when using the constant sea level of density. The ADC automatically compensates for the compressibility of air at the pitot tube.
The True Airspeed (TAS) in the EAS compensated for the changes in temperature and density variations on the different flight levels.