sentences:  This in turn required periodic draining of the expansion tank, as well as periodic bleeding of the system, to maintain its effectiveness.The rubber diaphragm in modern expansion tanks prevents this undesired transfer of air, and helps maintain low levels of oxygen within the pipes, reducing corrosion in the system.When expansion tanks are used in domestic hot water (DHW) systems, the tank and the diaphragm must conform to drinking water regulations and must be capable of accommodating the required volume of water.