The Impact Tube

Abstract

A simple impact tube having a hole facing upstream will give direct and accurate readings of the total pressure or energy of a moving fluid, except in special cases. It is not delicate in setting or design. If the static pressure is known, the kinetic energy or velocity head can be obtained by subtracting the static from the total pressure. If, further, the density is known, then the velocity can be derived. An application is given. The exceptional cases are velocity above acoustic and extremely low velocities. At supersonic velocity there occurs, just in front of the mouth of the tube, a very steep pressure rise, often described as a standing wave or shock plane. The tube now reads less than the total pressure, but this can be calculated by a formula which Rayleigh derived from a study of intense waves. The author, not having seen a direct demonstration separate from wave theory, attempts in the paper to give this, and to sketch what occurs. Test experience confirms Rayleigh's formula. For very low velocities, viscosity has influence; and the tube gives readings which exceed the static pressure by an amount approximately equal to the velocity head × [1 + (3/Reynolds number)].