Methods to calibrate a critical nozzle and flowmeter using reference critical nozzles

Abstract

Methods to calibrate a critical nozzle and a flowmeter against reference critical nozzles are developed to replace the time-consuming conventional procedures. The discharge coefficient of a critical nozzle at a low Reynolds number was measured in a series connection with a reference nozzle in the upstream position, and its Reynolds number dependence was obtained by changing the reference nozzle. The dependence of similar critical nozzles with negligible machining error measured at low pressures using the series connections and at atmospheric pressure using a constant volume tank system coincide within ±0.04%. The same configuration was employed to measure the stability of the choking flow rate, which revealed premature unchoking phenomenon. The discharge coefficient of a critical nozzle under a reference condition was measured by a combination of three series connections with two reference critical nozzles at the upstream positions. Reynolds number dependence of a critical nozzle was measured using a combination of three series connections with four reference critical nozzles. These two methods require only one pressure gauge whose sensitivity is constant in a narrow range. An air flowmeter was calibrated at various volumetric flow rates against only one critical nozzle by controlling the upstream pressure of the nozzle.