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.