In conventional air data measurement, all parameters are determined on the basis of three independent measurements: static pressure, total (Pitot) pressure and outside air temperature. The most critical flight parameters, speed and pressure altitude, rely solely on these pressure measurements, which must be accurate and reliable as they contribute directly to flight safety. For this reason, measurements are provided by at least three separate systems. As regards altitude, precision is in the order of 0.25 hectopascals (hPa) on a scale of 100 to around 1,500 hPa – the equivalent to the pressure difference on the ground between the top and bottom of a door (2 meters) – within a temperature range of over 70°C.
Measurement stability must be guaranteed over several years, since this type of sensor must provide reliable data for hundreds of thousands of flight hours.
The need to reduce weight has led to the use of single-crystal silicon to produce these sensors by bulk micro-machining (over 100 sensors per wafer). The sensor core comprises an assembly of three layers. A vibrating bar is placed under tension by the movement of a diaphragm, and the frequency of vibration (in a vacuum) of this element provides a measure of pressure.
The sensitive elements are then assembled and modelled in terms of pressure and temperature, with the aid of primary pressure references (columns of mercury held in position by control systems and mounted on seismic masses to eliminate vibration). These cycles are repeated until the sensors are completely stabilised.