What is the function of a captive air (expansion/bladder) tank?

The primary function of a captive air (expansion/bladder) tank is to maintain minimum pressure in the system when a booster pump cycles. This is achieved by using a flexible bladder within the tank that is filled with air. When water is drawn from the system and the pressure drops, the bladder compresses, allowing water to continue flowing. This helps prevent the pump from cycling too frequently, thereby stabilizing the pressure in the system, reducing wear on the pump, and ensuring a consistent water supply.

The other options do not accurately represent the role of a captive air tank. For instance, while storing excess water refers to a tank's ability to hold a volume of water, it is not the primary purpose of a captive air tank. Similarly, filtering water is a function associated with filtration systems and not the expansion or pressure stabilization aspect. Mixing chemicals relates more to chemical dosing equipment rather than the tank’s function in pressure maintenance. Thus, the choice focusing on maintaining minimum pressure aligns perfectly with the designed operation of captive air tanks in water systems.