Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 APPARATUS 29(c) Control and measurement of flow, and measurement of pressureAs seen in Figs. 15-17, flow rates may be controlled by capillaries of differentdiameter or by needle valves. By measuring the pressure at both ends of the capillary,and assuming streamline flow, the rate of flow may be calculated from Poiseuille'sequationwhere p1 and p2 are the pressures at entry and exit of the capillary, po is the pressureat which the rate is measured, I the length of the capillary (radius R) and q thecoefficient of viscosity of the gas or the gaseous mixture. Alternatively, and morereliably, the rate of flow is determined by measuring the volume of gas from anaspirator delivered by the capillary at a known pressure. This method may alsobe used for known, reproducible positions of the needle valve. The pressure in theRV will be a mean of that at entry and exit points. Conventional U-manometers,using mercury or oil for high or low pressures are suitable pressure-measuringdevices. These may often be shielded from reactants by strategically placed coldtraps. As an alternative for low pressures, the manometers can be replaced byMcLeod gauges of low compression ratios or oil-mercury magnifying manomete r (Fig. ~ 22). ~ The ~ magnification ~ ~ ~ factor ~ depends on the angle 8 and the diameterof the inclined and vertical tubes. For diameters of -7 and -25 mm and8 - 15", the magnification factor with respect to a mercury manometer is 14.- 1 Iv? u-References pp. 104-111Fig. 22. Oil-mercury magnifying manometer. From ref. 22c.