Patent ID: 8287250

Claim:
A method of operating a multistage vacuum pump unit comprising a plurality of vacuum pumps connected in series and in a multistage arrangement, so that the space inside of a to-be-vacuumized container connected to a gas inlet of a first stage vacuum pump becomes under a high vacuum condition, the multistage vacuum pump unit having a low vacuum operation zone and a high vacuum operation zone, the method comprising the steps of: branching part of the gas discharged from a gas-outlet of a lower number stage vacuum pump while the pump unit is operating under a low vacuum condition; and discharging the branched gas, toward an air atmosphere, through a branch passage including a low conductance line on which at least one check valve is provided for preventing the branched gas from flowing back; wherein a bore and a length of the low conductance line are configured to enable a gas flow that is branched from a vacuum pump of a lower number stage, in the low vacuum operation zone, to pass through the low conductance line, and to enable a gas flow in the high vacuum operation zone, to be prevented from flowing back from the outlet side of the low conductance line.