An accurate flowmeter based on the reduced constant-pressure method was devised for admitting the sample gas into the gauge calibration system. This flowmeter consists of a U-tube manometer, an accurate micrometer for measuring the gas pressure, and a diaphragm gauge for checking the deviation of the gas pressure. The gas pressure was kept constant by raising the mercury level in the U-tube manometer. The flow rate could be obtained from the gas pressure, the cross-sectional area of the U-tube manometer, and the rise-up rate of mercury level. With the U-tube manometer of 34. 0±0.1 mmφ in diameter, the flow rate of 10
-6 to 10
-2 Torr·
l/s could be easily measured with an accuracy better than 1%. An actual example in which this flowmeter was used for the gauge calibration was presented.
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