Performance of critical flow venturis under transient conditions

Critical flow venturis (CFVs) can be used to measure flow under transient pressure and flow conditions with uncertainties of 0.13% or less (95% confidence level). Blow-down tests transferred 630 g of nitrogen during a 100 s interval from an unregulated gas cylinder (initially at 10 MPa) through a heat exchanger and a CFV into a known collection volume. Fast pressure and temperature sensors were used to calculate the CFV flow. The integrated CFV mass flows, integral (m) over dot dt, averaged 0.04% smaller than the mass Delta rho V of the collected gas. The heat exchanger was essential to the excellent agreement; without it, the percentage difference between integral (m) over dot dt and Delta rho V was -0.38%. We conclude that even under transient conditions, a properly instrumented CFV can be used as a reference flowmeter to evaluate other flowmeter types or to test gaseous fuel dispensers in the field.