A method for assessing the uncertainty of a secondary dynamic pressure standard using shock tube

Abstract

Pressures are often measured in fast transient regimes, even if the transducers are not calibrated in dynamic regimes. If the solutions proposed for primary calibration of the dynamic response of pressure sensors must be fully recognised, secondary methods are also needed to disseminate the standard to final users. A method for assessing measurement uncertainty, carried out by comparison with a reference transducer, traceable to primary standard, is proposed here. A typical application is gas pressure measurement. This paper follows and supplements the work done on the Mach number method in 2018. For this work the reference transducer is calibrated by the ‘collective standard method’ currently used in the ‘Laboratoire de Métrologie Dynamique’( LNE/ENSAM). This primary standard uses steps of pressure as inputs for dynamic pressure calibration that are generated by shock tubes (STs) and fast-opening devices (FODs). The uncertainty on dynamic sensitivity is evaluated from the quasi-static to the low, medium and high-frequency range (up to 10 kHz) using bandwidth comparisons. To calibrate a secondary standard transducer in gas, the method also requires one or two step generators: an ST for high-frequency range calibration and a FOD for the low-frequency range. Concerning the main results of this paper, the transducer to be calibrated and the reference transducer are placed symmetrically on the endplate of an ST. The amplitude of the pressure step generated by the tube is used to excite the transducers. Finally, the uncertainty on the sensitivity in dynamic conditions is determined by comparison with a model expected to be exhaustive. The results are provided as an uncertainty budget assigned frequency by frequency. The question asked in this work concerns the measurand used in secondary calibration: can a pressure step generated by a non-ideal ST be used in the process of calibration by comparison, i.e. for a secondary dynamic calibration? A method is proposed. Since the secondary method is derived directly from the primary one, this paper recaps the primary dynamic calibration method in gas and the budget of the associated uncertainty. Then the paper presents a secondary method and options to overcome the principal default identified in the ST frequency range, namelythe non-uniform pressure fields existing in the tube.