moreover connected on the one hand by a conduit 47 with a valve 48 to the inlet 44 of the second stage 55 on the other hand by a conduit 49 with a valve 50 to the silencer 40 the conduit 42 is itself connected between valve 43 and inlet 44 of the second stage 55 to a conduit 51 with a valve 52 communicating with the silencer 40 leading to the air the operation of this pumping sub assembly is as follows at the beginning of pumping and until a pressure of the order of 0 5 u00d7 10 5 pa is achieved in the residue suction conduit 53 the valves 39 and 48 are open the valves 50 43 and 52 are closed such that the two stages 54 and 55 flow in parallel into the silencer 40 one 54 via the conduit 38 the other 55 being supplied by the conduit 47 and emptying into the conduit 46 a maximum flow rate can therefore be pumped to a depression of the order of 0 5 u00d7 10 5 pa absolute when a depression of the order of 0 5 u00d7 10 5 pa absolute is reached the valves 39 and 48 close the valves 50 and 52 remain closed while the valve 43 opens the residual gases thus reach only the first stage 54 which supplies them via 38 43 42 to the inlet 44 of the second stage 55 which supplies them to the silencer 40 for exhaust into the air the first and second stages 54 55 then operate in series and can then easily reach the.