pitot static system (pētōˈ) [key], device for measuring the rate at which a fluid flows. Among the principal applications of the device are an airspeed indicator for aircraft and a distance and speed indicator for ships. The device contains a short tube with one open end that faces directly toward the stream of air or other fluid. When no fluid is moving into the opening, a minimum pressure, called the static pressure, is exerted against it. When a stream is flowing into it, the pressure rises by an amount that depends on the velocity of the stream. Behind this tube is another tube with a number of small vents at right angles to the direction of the first tube. When a stream raises the pressure against the opening of the first tube, the pressure against the vents is still equal to the static pressure. A suitable gauge compares these pressures, using the static pressure as a reference, and gives a reading in units of velocity. For an aircraft this reading is called "indicated airspeed"; "true airspeed" requires a correction, made automatically by some airspeed indicators, for the local density of the air. Because of winds, neither reading accurately measures the speed in relation to a point on the surface of the earth. That value is known as groundspeed. Although airspeed is normally given in knots or miles per hour, for supersonic aircraft it may be given in Mach numbers, which give the ratio of the aircraft's speed to the speed of sound. For navigational purposes Mach numbers are converted to true airspeed.