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Vacuum pumps remove gases or vapour from closed systems

Vacuum pumps remove gases or vapour from closed systems. Purists who specialize in low pressure systems dislike the word 'vacuum' because, in the physical world, there is no such thing as a true vacuum or absence of gas. In this article we will continue to refer to any device designed to provide low relative pressure as a "Vacuum Pump".
In practice, low pressures are necessary in order to remove vapours and some suspended particles in laboratory applications. Vacuum pump can be as simple as one used to displace fluids (think of a syringe) or as complex as those required to simulate deep space.
In a laboratory, the means to move the vapour can be a simple venturi system driven by water flowing from a tap. Such pumps are used in aspirating liquid from surfaces where an intermediate container traps the liquid but prevents the liquid from entering the pump mechanism. Other systems can move quite large volumes of gases like a household water ring vacuum pumps cleaner which moves large amounts of air (2,000 litres per minute) but at relatively low differential pressure.
High vacuum pumps are used in pulling high differential pressures (ie. "deep vacuum") such as those used for freeze-drying or evacuating moisture from refrigeration circuits. Generally these pumps displace very small amounts of gases - less than 30 litres per minute of free air. The low pressures that can be achieved are about 1 micron of mercury column - like pressure in deep space.