Calibrating capillary gas flow meters by means of the transient method

The present article discusses the transient method of calibrating capillary gas flow meters which involves the application of a type of manometric flow meters, which measure the release of a substance by registering changes in the differential pressure on a measuring element. The measurement methodology presented in the article is but a concept proposing actions and calculations aimed at determining the calibration coefficient of a section of a measurement capillary in an outstandingly simple manner. This method is dedicated to capillary flow meters with very small gas flow rates. Calibrating flow meters by means of the transient method involves registering changes in the differential pressure Delta p(t) on both ends of a capillary tube while gas flows out of a measuring burette, through a calibrated capillary tube, to be released into the atmosphere. On the basis of a combination of the Hagen-Poiseuille equation and the ideal gas law, a relationship describing the phenomenon of a gas flow through a capillary tube was formulated, which, in turn, served as a basis for determining the calibration coefficients of the two capillary sections measuring 10 cm and 40 cm, with an internal radius of 0.015 cm. The calibration processes of both capillary sections were conducted for 21 different temperature values, using methane. The obtained values of calibration coefficients were juxtaposed with their model values obtained via calculation. The relative differences between the calibration-obtained values and model values did not exceed 7.65% and 4.83% in the case of the 10 cm capillary tube and the 40 cm capillary tube, respectively.