Water at atmospheric pressure will absorb 4 per cent, its bulk of air. If the pressure of water be increased it will absorb 4 per cent, its bulk for each additional atmosphere of pressure. Hence, if the pressure in a system is 150 pounds, water will absorb 11X4 per cent. =44 per cent. its bulk of air, and the air will be soon absorbed from the air chambers; provision should therefore be made to recharge them when the air is exhausted. A pet cock in small air chambers and a stop cock in large ones very satisfactorily serve this purpose.

In arranging air chambers they should be so located that the energy or momentum of the column of water can be expended directly upon the air confined in them. By so locating them they receive the initial shock of the moving column of water and absorb most of its energy, thereby minimizing the intensity and reducing the number of high pressure waves. Air chambers should also be placed on the house side of water meters or other delicate apparatus that might be injuriously affected by water hammer. Under all conditions air chambers should be placed in a vertical position and never at the side or bottom of a pipe. If so placed, they will shortly fill with water and become useless. Air chambers placed above faucets are not so liable to lose their air by absorption, because when passing the inlet to an air chamber so placed, the pressure is greatly reduced and if the water at static pressure is saturated with air, the air will be released by the reduction in pressure and some of it will rise and recharge the air chambers.

Air chambers should be provided on all distributing drums, on the suction and discharge pipes to power pumps, on the house side of delicate apparatus, like water meters, and on the supplies to fixtures. A very satisfactory type of air chamber for basin or other fixture supply is shown in Fig. 79. The enlarged chamber on this supply provides ample capacity for small pipes of moderate length and being placed directly on the supply pipe it receives the initial impact of the moving column of water.