An Experimental and Theoretical Investigation of Wave Propagation in Teflon and Nylon Tubing With Methods to Prevent Aliasing in Pressure Scanners

Abstract

Accurate static and dynamic pressure measurements provide the feedback needed to advance gas turbine efficiency and reliability as well as improve aircraft design and flight control. During turbine testing and aircraft flight testing, flush mounting pressure transducers at the desired pressure measurement location is not always feasible and recess mounting with connective tubing is often used as an alternative. Resonances in the connective tubing can result in aliasing within pressure scanners even within a narrow bandwidth and especially when higher frequency content DC to ∼125Hz is desired. We present experimental results that investigate tube resonances and attenuation in 1.35mm inner diameter (ID) (used on 0.063in tubulations) and 2.69mm ID (used on 0.125in tubulations) Teflon and Nylon tubing at various lengths. We utilize a novel dynamic pressure generator, capable of creating large changes in air pressure (<1psi to 10psi, <6.8kPa to 68.9kPa), to determine the frequency response of such tubing from ∼1Hz to 2,800Hz. We further compare these experimental results to established analytical models for propagation of pressure disturbances in narrow tubes. While significant theoretical and experimental work relating to the frequency response of connective tubing or transmission lines has been published, there is limited literature presenting experimental frequency response data with air as the media in elastic tubing. In addition, little progress has been made in addressing the issue of tubing-related aliasing within pressure scanners, as the low sampling rate in scanners often makes post-processing antialiasing filters ineffective. The experimental results and analytical models presented herein can be used as a guideline to prevent aliasing and signal distortion by guiding the proper design of pressure transmission systems, resulting in accurate static and dynamic pressure measurements with pressure scanners. The data presented here should serve as a reference to instrumentation engineers so that they can make higher frequency measurements (up to ∼125Hz, currently) and are able to quantify the expected pressure transmission line (tube) attenuation and know if aliasing will be a concern. This information will give engineers greater measurement capability when using pressure scanners to make static and dynamic pressure measurements.