@Damien1989,
Tell (remind) your father that the pitot tube doesn't - and isn't there to - measure airspeed. It's there to measure the dynamic pressure, (1/2.rho.v^2) which governs the aerodynamic behaviour of the wing and tailplane.
We're all living in a fantasy land where we "invent" a thing called the "Calibrated Airspeed" as if it's a real quantity and then pretend that the ASI measures it, or something close to it (i.e. IAS).
It would be much more honest to calibrate the ASI in Pa (or lbs/sq.in.) and note the stalling "pressure" of the aircraft, the never exceed pressure, etc. - then note (with only passing interest as it's really not that important) that the true airspeed at which each of these pressures is achieved depends on altitude, temperature and humidity.
That's too complicated though so we calibrate it in terms of the airspeed to achieve a given pressure at sea level.
But it's a fiction to call it an "airspeed" indicator.