Pressure tank

[1] Referring to the figure on the left, a submersible water pump is installed in a well.

Modern systems isolate the water from the pressurized air using a flexible rubber or plastic diaphragm or bladder, because otherwise the air will dissolve in the water and be removed from the tank by usage.

Eventually there will be little or no air and the tank will become "waterlogged" causing short-cycling, and will need to be drained to restore operation.

The diaphragm or bladder may itself exert a pressure on the water, but it is usually small and will be neglected in the following discussion.

Assuming that the temperature is equal for both cases, the above equations can be solved for the water pressure/volume relationship in the tank: Tanks are generally specified by their total volume Vt and the "drawdown" (ΔV), which is the amount of water the tank will eject as the tank pressure goes from Phi to Plo, which are established by the pressure switch:[2][3] The reason for the charging pressure can now be seen: The larger the charging pressure, the larger the drawdown.