Models with static diving have the ability to alter their displacement by taking on or pumping out water. This can be accomplished through the use of a piston, inflatable bladder, or through a ballast tank. Boats that use a ballast tank generally fill the tank by opening a vent at the top, and force the water out by using compressed gas. There are variants that use water pumps for both processes. A liquid gas is dosed into the ballast tank to push water out. Gas-Snort Liquid gas is used to surface the boat in an emergency, otherwise the ballast tank is blown by the use of a snorkel tube at periscope depth and the boat is trimmed to surface to periscope depth with a full ballast tank. RCABS -Recirculated Compressed Air Ballast System. Originally developed by Darnell (UK) in the 1950s, this system uses a rubber bladder as the ballast tank and this is filled with compressed air supplied by a small compressor. The air is taken from the water-tight container (WTC) aft dry space to inflate the bladder.