Analysis of a dynamic pressure measuring system

Abstract

A dynamic pressure measuring system composed of long connecting tube, transducer, D-C amplifier and galvanometer oscillograph were used in a wind study conducted by the Department of Agricultural Engineering, Virginia Polytechnic Institute. The instrumentation was used to measure and record dynamic pressures due to wind velocity. Data recorded in that study indicated velocity head pressure oscillations on some surfaces of the test structure in excess of 100 Hz. This investigation was undertaken to see if the recorded oscillations could have been generated in the measuring system or if they were truly windpressure variations.

Components of the pressure measuring system were modeled by transfer functions. From these transfer functions, a system operational transfer function was determined and used to define system frequency response.

The frequency analysis indicated the system was severely limited in response by the oscillating air column constrained within the interconnecting tube. The usable frequency range of the system (+ 10% tolerable amplitude error allowable) was found to go from zero to 34.2 Hz. By eliminating the air column this frequency range could have been extended to 99.4 Hz. Phase shifts in these frequency ranges were found to be negligible.

Transducers were mounted rigidly to exterior walls of the test structure. The measured fundamental natural frequencies of these walls (transducer mountings) were found to be well within the usable frequency range of the instrumentation; and, therefore, a source of vibration pickup.

To improve the measuring system reliability, recommendations were made to eliminate the air column and stiffen the transducer mountings.