This is the author pre-publication version. This paper does not include the changes arising from the revision, formatting and publishing process. The final paper that should be used for referencing is: M. Marino, A. Fisher and R. Sabatini, “The effects of tube deformities on the dynamic calibration of a tubing system”, in proceedings of IEEE International Workshop on Metrology for Aerospace (MetroAeroSpace 2015), Benevento, Italy, 2015. The effects of tube deformities on the dynamic calibration of a tubing system Matthew Marino, Alex Fisher, Roberto Sabatini RMIT University School of Aerospace, Mechanical and Manufacturing Engineering Melbourne, Australia Abstract - Using the Berge and Tijdemen method for tube calibration is powerful as it allows for tubes of various dimensions to be used in a dynamic pressure data acquisition system by using post-processing methods to calibrate for the tubes natural dynamic response. Knowing the tubes response and using the inverse Fourier transform to calibrate the tube system is accepted however knowing how tube deformities influence this calibration is not known. Small singular deformities caused by pinch, twist and bending, which corresponded to a pinch and internal area ratios less than approximately 5 and 3.57 respectively, do not affect the tubing response of a system. Significant effects on the tubes response only occur at pinch and area ratios above these values. Furthermore, pinching ratios above 5 are extreme and represent a tube that is pinched locally to the point where it is almost blocked. This is testament to the tubes resilience to local and internal diameter changes. It can be safely assumed that unwanted and unexpected dampening of a tubing system could be due to a local tube deformity. Keywords – tube deformation; dynamic tube response; dynamic calibration; pressure sensing; pressure measurement. I. INTRODUCTION Measuring fluctuating pressures requires the adoption of multidisciplinary sensing and data analysis technique. The method of measuring surface pressures, by pressure tapping a surface and connecting it via sealed tubes to a remote transducer, is a technique which is extensively used in wind and aerospace engineering [1, 2]. Recent developments have been achieved in the area of Micro Air Vehicles (MAVs) where pressure variations are used to sense turbulence and to reduce, correct or mitigate its effects on MAV flight [3-7]. In common applications of wing pressure tapping, the location of the pressure transducer bank is significantly displaced from the location of the sensing taps. Thus, long tube lengths to connect taps to transducers are typically needed. The dimensions of the interconnecting tubes skew the dynamic response of fluctuating pressure and cause either resonance or viscous dampening. Therefore, we cannot assume that the pressure measured at the desired location is unaltered before it is read by the remote pressure transducer [8]. Where static pressures are measured at a very low acquisition rate (f < 5Hz), a calibration of the tubing system may not be required. However, if higher data acquisition rates are needed, the dynamic calibration of the tubing systems is essential to correct for the errors introduced. Dynamic tube calibration is not a new topic with a number of well proven calibration methods used throughout the engineering disciplines. Additionally, improved methods of dynamically calibrating tube systems are currently emerging due to technology advancements. Tube manufacturing tolerances have shown to affect the dynamic calibration of a tubing system in amplitude and phase response. The manufacturing tolerance on the internal diameter is one of the key parameters that can significantly affect the amplitude response of a tubing system. Fig. 1 displays the amplitude variation using tubes with varying diameter and constant tube length. The tubes commonly used for dynamic pressure measurements are typically made of rubber or plastic materials. As they are commonly placed in relatively small spaces/enclosures, they sometimes require to be woven, bent and guided through the aircraft structures in order to reach the bank of pressure transducers. As such the tubes are susceptible to bending and pinching, which can affect the dynamic calibration of the tubing system. In this research, tube pinching and associated internal diameter changes are shown to influence the tubes natural frequency response. This is done experimentally using a device that can supply a dynamic pressure signal to the tubes and then sensed by the pressure transducers.