The dynamic calibration of a hot wire by means of a sound wave

Hot wires of various design, operated by the constant resistance method, were calibrated by means of a plane sound wave of 2500 Hz. This is achieved by locating a small open resonance tube axially in a homogeneous incompressible airflow. Its third harmonic is excitated by superimposing a coaxial, damped, progressive, plane wave on the main airstream. The hot wire to be calibrated is situated in an anti-node of the particle velocity. The amplitude at this station is known from the pressure gradient of the standing wave measured at two small holes in the wall by a probe microphone. This device in turn is calibrated in a specially developed “pistonphone”. Numerical results of the diverse designs are communicated; an absolute accuracy of ±2% is achieved.