A compressed gas accumulator consists of a cylinder with two chambers that are separated by an elastic diaphragm, a totally enclosed bladder, or a floating piston. One chamber contains the fluid and is connected to the hydraulic line. The other chamber contains an inert gas (typically nitrogen), usually under pressure, that provides the compressive force on the hydraulic fluid. Inert gas is used because oxygen and oil can form an explosive mixture when combined under high pressure. As the volume of the compressed gas changes, the pressure of the gas (and the pressure on the fluid) changes inversely.
For low pressure water system use the water usually fills a rubber bladder within the tank (pictured), preventing contact with the tank which would otherwise need to be corrosion resistant. Units designed for high-pressure applications such as hydraulic systems are usually pre-charged to a very high pressure (approaching the system operating pressure) and are designed to prevent the bladder or membrane being damaged by this internal pressure when the system pressure is low. For bladder types this generally requires the bladder to be filled with the gas so that when system pressure is zero the bladder is fully expanded rather than being crushed by the gas charge. To prevent the bladder being forced out of the device when the system pressure is low there is typically either an anti-extrusion plate attached to the bladder that presses against and seals the entrance, or a spring-loaded plate on the entrance that closes when the bladder presses against it.
It is possible to increase the gas volume of the accumulator by coupling a gas bottle to the gas side of the accumulator. For the same swing in system pressure this will result in a larger portion of the accumulator volume being used. If the pressure does not vary over a very wide range this can be a cost effective way to reduce the size of the accumulator needed. If the accumulator is not of the piston type care must be taken that the bladder or membrane will not be damaged in any expected over-pressure situation, many bladder-type accumulators cannot tolerate the bladder being crushed under pressure.
A compressed gas accumulator was invented by Jean Mercier for use in variable-pitch propellers.