How Pressure Tanks Work
As water enters the holding area of a pressure tank, the diaphragm floats upward, which increases pressure and provides the connected pump and plumbing system with greater pressure. Just about every pump features an on/off switch that's activated by certain levels of pressure. Once the pressure limit is reached, the pump stops moving. Water can then be drawn out from the pressure tank without the pump being activated. Once the pump cut-in level is reached (by enough water being removed), the pump then turns on and adds more water to the pressure tank.
Sizes of Pressure Tanks
Pressure tanks come in several sizes. According to Home Institute, the standard size used to be 40 gallons. But because that tank could only hold approximately seven to eight gallons of water, larger tanks that include diaphragms have largely replaced it as a standard. Larger pressure tanks may be more expensive than smaller ones, but keep in mind that the more water a pressure tank can hold, the less the pump will need to turn on and pump more water in--which can extend its life. Home Institute suggests choosing a pressure tank that has at least twice the pumping capacity of the pump, particularly for well water systems.
Galvanized Pressure Tanks
Galvanized pressure tanks have been around the longest. This type of tank uses a cushion of compressed air to provide the necessary amount of water to the distribution system without turning on the pump. With this type of pressure tank, water and air are in direct contact with one another, so there's a possibility for the tank to become waterlogged--which can trigger the pump to run more than necessary. Adding or releasing air as necessary can help to keep a proper ratio of air and water.
Precharged Pressure Tanks
Pre-charged pressure tanks feature a glass lining and a rigid float. The rigid float completely covers the surface of the water, which slows the rate of air absorption. This type of pressure tank comes pre-charged, though it must be recharged on a yearly basis to prevent waterlogging. Recharges can be done using an air compressor.
Sealed Diaphragm Pressure Tanks
Sealed diaphragm pressure tanks are the newest ones on the market. This type of tank is made of metal or even fiberglass, and it contains an air cell or diaphragm that's made out of plastic or rubber. This kind of tank is also pre-charged, and it prevents air and water from coming into contact with one another. And unless the diaphragm ruptures, a sealed diaphragm pressure tank should never need to be recharged.