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A Pitot tube measures flow velocity by converting dynamic pressure into a height difference in a manometer. The stagnation pressure (total pressure where flow stops) minus static pressure equals the dynamic pressure: q = ½ρV². Pitot tubes are widely used for measuring airspeed in aircraft and water velocity in open channels because they cause minimal flow disturbance.
Bernoulli's equation, which you know from prerequisites, states that along a streamline in steady, inviscid, incompressible flow: P + ½ρV² + ρgz = constant. This is an energy statement — the sum of pressure energy, kinetic energy, and potential energy per unit volume is conserved. The dynamic pressure ½ρV² represents the kinetic energy of the moving fluid. A Pitot tube exploits this relationship by creating a controlled stagnation point: a small hole facing directly into the flow brings the fluid momentarily to rest, converting all of its kinetic energy into a measurable pressure increase.
The device in practice consists of two pressure ports. The stagnation port faces upstream and measures total pressure P_total = P_static + ½ρV². The static port is flush with the tube wall and measures P_static, the ambient pressure at that location. The difference is exactly the dynamic pressure: P_total − P_static = ½ρV². Solving for velocity gives V = √(2(P_total − P_static)/ρ). You only need to measure a pressure difference and know the fluid density — no other information about the flow is required.
From your understanding of pressure measurement, you know that pressure differences can be read directly with a manometer or differential pressure transducer. In a water-filled manometer connected to the two ports, the height difference Δh satisfies ΔP = ρ_fluid · g · Δh. Substituting back gives V = √(2gΔh) for the special case where the manometer fluid is the same as the flowing fluid. This clean result is why Pitot tubes are popular in introductory lab courses — the velocity comes directly from a ruler measurement of liquid height.
On aircraft, the Pitot-static system connects one probe facing the airstream (for stagnation pressure) and one set of flush ports on the fuselage (for static pressure). The difference drives the airspeed indicator. One critical limitation: the classical Bernoulli derivation assumes incompressible flow. At low aircraft speeds this is fine, but above roughly Mach 0.3 the compressibility correction becomes important. For subsonic aircraft the correction is a modest factor; for supersonic flight, a normal shock forms ahead of the probe and the analysis must account for the entropy rise across the shock — the Rayleigh Pitot tube formula applies instead. But the core physical idea — measure the pressure rise from stagnating a moving fluid — remains unchanged across all these regimes.