Influence of water vapor on the performance of an ultra-high vacuum system with getter-ion pump is described. An attempt is also made to avoid this influence by use of an internal heater provided in the system.
The ultimate pressure attained by evacuation after the system is exposed to dry air is one twentyth of that exposed to room air. When the dew point of the introduced air lies between -15°C and 30°C as practical performance, the ultimate pressure changes only by a factor of two. The heat of adsorption is estimated at about 11 Kcal/mol from the variation of the ultimate pressure with room temperature. This suggests that the gas evolved under the ultimate pressure is predominantly water vapor adsorbed on the wall. When room temperature varies from 5°C to 40°C, the ultimate pressure will be increased by a factor of ten.
The influence of water vapor can be avoided by an internal heater provided in the system. The ultimate pressure of this system after exposing to room air is as low as that of dry air.