Leak Rate Uncertainty Parametric Study

Abstract

The desire of humans to explore beyond low-Earth orbit has provided motivation for technology development to reliably confine gases. The new technologies have driven the need to accurately quantify the mass flow rate past gas pressure seals. While the classic method of helium leak detection is widely accepted and reliable, the method has weaknesses when applied to determining air leak rates. The newest advance in air leak rate quantification acts as an extension of the pressure decay method. In this enhanced method, the downstream pressure is controlled and a constant pressure differential is maintained across the seal under test. The effect of the enhancement is improved measurement uncertainty of the leak rate over the basic pressure decay method which may chase a valid measurement as the differential pressure changes with uncontrolled downstream pressure. Theoretically, each measurement system should produce the same value for a given leak rate; however, the measurement uncertainty is dependent upon the instruments used. As this enhanced method can be accomplished using off-the-shelf equipment and instrumentation, the measurement uncertainty strongly depends upon each unique test set-up. A parametric study of the leak rate uncertainty equation was completed, and the effect of changing the quality and quantity of the pressure and temperature measurement instruments was observed. The study quantified the differences in measurement uncertainty at different test conditions depending upon the instrument choices, as well as, the differences made in uncertainty by modifying the test conditions alone. The calculated measurement uncertainty was beneficially lower when using a single high-quality pressure measurement device when compared to utilizing multiple low-quality pressure transducers. Similar results were shown for temperature measurement devices. The data acquisition sampling rate, initial pressure, and mass flow rate also affected the uncertainty values in some cases, but may not be variable parameters.