Comparison of experimental mass inflow measurement methods with error analysis

A compressible flow experiment was developed to demonstrate engineering fundamentals and allow students to perform meaningful calculations on student-collected experimental data. Student teams employed four methods to determine the mass flow rate of air flowing into an initially evacuated tank under choked flow conditions. The four methods were (1) theoretical calculation of mass flow through a sonic nozzle, (2) direct measurement with the mass flow meter, (3) the mass point method (e.g., the difference in calculated initial and final masses inside the tank), and (4) the mass slope method (e.g., calculating the mass inside the tank via pressure and temperature data at each time step). These methods are presented along with their measurement uncertainty calculations. Example results demonstrated mass flow rate measurements and overlapping uncertainty bands using choked flow through a 0.096-inch diameter critical flow venturi resulting in flow rates of 1.051 ± 0.012 g/s, 1.045 ± 0.066 g/s, 1.030 ± 0.010 g/s, and 1.042 ± 0.005 g/s for Methods 1–4, respectively. The different methods allow instructors to have flexibility in varying the activity between student cohorts. The experiment demonstrated fundamentals of ideal gas behavior, choked flow, and polytropic compression processes. The experiment gave student teams the opportunity to conduct an experiment, utilize common measurement instruments, and apply classroom theory and concepts to solve a relevant engineering problem: quantifying the flow rate of a compressible gas. The associated Accreditation Board for Engineering and Technology (ABET) outcomes, applicable engineering standards, and general safety issues are discussed.