use gas is pumped as shown by the arrows from inlet 60 at the top of the pump towards the right of the figure and exhausted through the outlet 62 at the bottom of the pump a method of retro fitting a vacuum pumping arrangement will now be described the cartridges 52 54 are removed rotated through 180 u00b0 and replaced as shown in fig3 components 64 66 68 70 are maintained in the same position in fig2 and 3 as can be seen in fig3 replacement of the cartridges relocates the inlet 60 and outlet 62 so that the inlet is at the bottom of the pump the outlet 62 is at the top of the pump and gas is pumped in a generally upwards direction from the inlet to the outlet since the cartridges have been replaced in a rotated position the motor is driven in an opposing second rotational direction to obtain the correct gas flow direction it will be apparent to those skilled in the art that further minor modifications are required to the existing pump assembly as shown by the horizontal arrows in fig3 the pumping mechanism pumps gas in a generally horizontal direction however the net flow of gas from the inlet to the outlet is in a generally upwards direction even though along some of the gas flow path through the pump may be horizontal or even in a downwardly direction it will be appreciated however that location of the inlet at the bottom of the pump resists the passage of contaminants into the pump from the vacuum line extending from a vacuum processing chamber the net flow of gas in the.