Vacuum apparatus is widely used in research and industrial establishments for providing and monitoring the working environments required for the operation of many kinds of scientific instruments and process plant. The vacuum conditions needed range from the relatively coarse vacuum requirements in applications covering diverse fields such as food packaging, dentistry (investment casting), vacuum forming, vacuum metallur­ gical processes, vacuum impregnation, molecular distillation, vacuum drying and freeze drying etc. to the other extreme involving the highest possible vacuum as in particle accelerators, space technology -both in simulation and outer space, and research studies of atomically clean surfaces and pure condensed metal films. Vacua commence with the rough vacuum region, i.e. from atmosphere to 100 Pa * passing 6 through medium vacuum of 100 Pa to 0·1 Pa and high vacuum of 0·1 Pa to 1 J.lPa (10- Pa) until ultra high vacuum is reached below 1 J.lPa to the limit of measurable pressure about 12 I pPa (10- Pa).