Vacuum studies with a VELO module

LHCb VELO modules will be operated in low vacuum with voltages as high as 500~V on both hybrids of the module. This note provides the first study on the breakdown voltages of a pre-production VELO module with varying pressures and venting with air. The breakdown voltages are measured by studying the leakage currents of the sensors on the module and the measured breakdown curve is compared to the theoretical breakdown for a parallel plate capacitor. The effect of breakdown on the noise of the sensors is also studied. It is concluded that the modules should not be operated at voltages above 500~V over a pressure range of 0.1-760~Torr.