Mechanics of 2-Liter Dispensers
The bottled soda dispenser with a mounting stand and lever is a style similar to commercial soda dispensers (some types of this dispenser have a spigot, keg style, rather than a lever). It offers one-handed pouring and is designed to keep the beverage carbonated.
This style of dispenser relies on a mounting stand as a stable support base for the two-liter bottle, which is suspended upside down in a bottle holder. The threaded mouth of the soda bottle is screwed into a nozzle attached to the base. An air tube extends between a stopper valve and a check valve. Soda is dispensed through spring-loaded actuating levers, which turn the dispenser on and off as pressed. Because of the vacuum created by the check valve, as the level of soda dwindles, the plastic bottle doesn't collapse or crunch but stays as taut as the unopened bottles on the shelves at the grocery store.
The stopper valve is mounted on the top of the actuating levers. When the lever is pressed to the "on" position, the stopper valve opens and soda is served from the apparatus. As soda begins dispensing, the check valve opens to allow air in, and also allows a steady flow of soda. During dispensing, this end of the air tube creates a vacuum inside the plastic bottle. This vacuum helps keep the bottle from depressurizing and therefore keeps the dissolved carbon-dioxide gas from escaping. When the dispenser is not in use, these valves seal both fluid and carbonation from escaping.
The second type of two-liter soda dispenser, the pressurized beverage dispenser, looks a lot like an old-fashioned seltzer siphon bottle, with a straw that extends into the soda bottle and a spray bottle style cap. A siphon tube (the straw) constructed with flow-obstructing devices to reduce the pressure of fluid as it dispenses is inserted into the soda bottle -- those special features mean less foam and less carbon dioxide gas are released as you pour a glass of soda. The tube is attached to an actuator, which is the spray head of the mechanism, and a plug in the actuator seals the liquid and gas from leaking.