A Simple Device for the Preparation of Feed Gas-Air Mixtures of Very Low Gas Concentrations
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In process studies it at times becomes necessary to feed to a system a constant compnsition of gas-air mixtures of very low concentrations of gas up to parts per million levels. Usually, the gases are available in cylinders under high pressure, and it becomes difficult to control the gas flow due to high fluctuations in the gas flow rate controlled by ordinary laboratory devices, thus necessitating the use of highly sophisticated controllers. This problem can be overcome by a simple laboratory setup described below. A 2000-mL glass flask with a side opening is provided with a standard glass joint through which pass (1)a tube with a sparger a t one end and a stopcock (1in the figure) at the other end and 12) the dischareeend of a eonvenientlvsized buret. Thedischarm end of the flask is connected to one end of a three-way stopcock (2 in the figure) that Ean he opened to atmosphere and to the air duct. At the start the gas cylinder is connected to the flask through stopcock 1, and stopcock 2 is kept open to the atmosphere. The gas from the cylinder flushes all the air in the flask. After all the air has been displaced from the flask, stopcocks 2 and 1are closed. The volume of gas in the flask can he calculated from the gas law at that particular temperature. An inert liquid such ssparaffin oil, which will not react with the gas, is run at a required rate from the buret to give the desired gas-air composition. The gas-air sample can he collected and analyzed for the gas content. The analysis gave good agreement with the required composition. This simple device can be easily used provided that the gas is denser than air and that displacing liquid does not react with the gas. V. Ramaswamy National Chemical Laboratory Pune 41 1 008, lndla
Volume 64
Number 4
April 1987
375
