The principle of the capacitance variable area flowmeter or rotameter for gas measurements is developed. This method utilised the change of the annular gap between the float and the tapered bore at different positions to measure the corresponding change in capacitance in the form of voltage. From performance tests conducted for air on both a simulated rotameter and an actual rotameter constructed based on the principle described, good agreement was obtained between analytical and experimental results. It was found that the output voltage from this rotameter depended upon the relative sizes of the diameters of the tapered bore and the float, and the angle of the tapered tube.
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