This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
The presence of atmospheric air in water prevents a thorough impregnation with carbonic acid gas. Air is a great rival of carbonic acid, in fact reduces the absorption of gas by water. According to Liebig, one volume of atmospheric air displaces nearly 20 volumes of carbonic acid gas, and this figure demonstrates the great importance of removing the air from water. Carbonic acid containing more than 3 per cent, of atmospheric air is absolutely unfit for use. Also the atmospheric air which the apparatus contains should be removed to prevent its being mixed with the beverage.
If the gas is generated from bad materials the carbonic acid will be loaded with bad odors and thus impregnate the beverage. The purity of carbonic acid gas is, therefore, an important point, to be by no means overlooked. To the subject of purification of carbonic acid gas and the removal of atmospheric air we must give particular attention. Carbonic acid gas is one and a half times heavier than atmospheric air, its specific gravity being 1.5245, and it therefore sinks to the bottom, while the air remains on top and can be removed by means of a blow-off cock or loosening a cap on top of a cylinder.
The principle of this "blowing off," or "removing of atmospheric air," is explained thus:
The atmospheric air displaces at the usual temperature and the usual atmospheric pressure up to twenty volumes of carbonic acid gas, depending on time; but the atmospheric air is on the other side displaced by carbonic acid of more than 4 atmospheres pressure (60 lbs.). When water is impregnated with carbonic acid, under pressure, in a closed vessel, when the liquid is at rest, the atmospheric air, being lighter than the carbonic acid and displaced, will collect above the surface of the liquid. When a blow-off cock is opened, or a cap loosened, this atmospheric air will escape violently along with the uncombined carbonic acid. As soon as the pressure in the cylinder gets diminished by letting escape its surface gaseous contents, a certain amount of the gas already absorbed by the liquid is eliminated in consequence of the decreased pressure, and supports the displacement of that combined gaseous contents above its surface, viz., atmospheric air and uncombined gas. But this displacing process is not a sudden one, it needs time; therefore the removing or blowing-off operation should be several times repeated, and a pressure of more than 60 lbs. maintained in order to secure the thorough removal of atmospheric air. A patented system of removing the atmospheric air from water by suction we give especial consideration to on another page.