Vacuum pumps are used to reduce the gas pressure in a certain volume and thus the gas density. Consequently consider the gas particles need to be removed from the volume. They via one or several compression stages - the gas particles are removed from the volume which is to be pumped and ejected into the atmosphere. The gas particles are pumped by means of displacement or pulse transfer. Vacuum pumps are used where gas particles are to be removed condense on or are bonded by other means to a solid surface, which often is part of the boundary forming volume itself.