A Versatile Pneumatic Instrument Based on Critical Flow

Abstract

Gas flows through two restrictions in series, the second one or both being a nozzle operating with sonic throat velocity so that discharge is independent of the downstream pressure. For fixed restrictions, the ratio of entrance pressures at the two restrictions depends on their relative sizes and the ratio of entrance temperatures. After calibration, the pressure ratio may be used to measure temperature with a sensitivity of 1°C or better over a range of 2000°C. With the same arrangement, the temperature being constant, the intermediate pressure is a direct measure of mass flow. A sensitivity of better than 0.1 percent is possible in this measurement. For given temperatures, the device can be used as a ``pressure divider,'' with the intermediate pressure controlled by adjusting the restrictions, or as a pneumatic micrometer. Gas analysis is accomplished by removing one component of the gas flow between nozzles. A vacuum pump is used, if necessary, to ensure critical discharge through one or both nozzles; divergent cones on nozzles may be used to minimize pressure drop.