The primary function of a pressure tank in a pumped water system is to store water under pressure so the pump does not have to come on every time there is a small, intermittent demand for water. The amount of usable water in a pressure tank is called drawdown, and is the amount of water drawn from the tank between the time the pressure switch cuts out turning off the pump, and cuts back in, turning the pump on. Tanks are sized with enough drawdown to allow the pump to run a minimum of one minute between cycles, letting motor cool, as recommended by motor manufacturers. A 10 gpm pump would require a pressure tank with 10 gallons of drawdown, etc.
There are two types of pressure tanks, captive-air tanks (also called pre-charged, diaphragm or bladder tanks), and conventional tanks, (also known as hydro-pneumatic, galvanized, ASME, and epoxy-lined tanks). Using the term hydro-pneumatic to describe a conventional tank is a misnomer. All pressure tanks used in the groundwater industry are hydro-pneumatic meaning they contain water (hydro) and air (pneumatic). In this article, we will use the terms "captive air" and "conventional" to differentiate the two types of tanks.