The ASI consists of an open-ended tube, called the Pitot tube, facing the relative airflow which transmits the total pressure to a diaphragm situated in the instrument case. Another pressure sensor, usually situated on the side of the fuselage and called the static port, measures the static atmospheric pressure. The static pressure is fed to the ASI case and will act on the diaphragm ‘filled’ with total pressure. The static pressure acting on the outside of the diaphragm will cancel the static pressure from inside the diaphragm. The resultant of this differential pressure will be the dynamic pressure (Pt – Ps = Pd) which will be mechanically transmitted to a pointer moving over a dial calibrated in knots (kn) or km/h. CAS/IAS is always the dynamic pressure, so the pilot is always reading on the ASI the IAS or CAS (if corrected for pressure and instrument errors, or if no errors at all).