KB004804: Can an Oil Free Rough Pump be Used With a Diffusion Pump MS? - General GC/MS 

Oil Free Rough pump is not recommended with diffusion pump GC/MS systems. The  Oil Free Rough pump brings the vacuum to about 1 Torr. But the sensor of the diffusion pump will switch ON the diffusion pump at 300 mTorr. So if the Oil-Free pump is installed on a diffusion pump system then the diffusion pump will never turn on.