The water vapour contained in the atmosphere contaminates a vacuum systems, when it is exposed to air. The degree of contamination will depend on the surface characteristics, treatment of the system walls, the relative humidity and the turbulence of air, the amount of time the system is opened to the atmosphere, etc. If the temperature of the exposed surface is low, possibility of water condensing on the surface exists. In order to study the effect and optimise procedure for achieving high vacuum in a vacuum chamber initially containing water, a vacuum chamber has been fabricated. Pump down time has been measured for different amount of water placed in the vacuum chamber with varying pumping speeds. The paper presents a review of similar activities and results of the experimental study. Introduction:The beam chamber of VEC superconducting cyclotron [1] has a number of penetrations in the median plane in between the upper and lower superconducting coils to accommodate extraction components and beam diagnostic components. Because of the compact structure, adequate space was not available at the penetrations for placing a thermal shield. The temperature at these penetrations is quite low. It is observed that moisture from air condenses there and considerable time is taken to achieve good vacuum, specially, in monsoon. In the case of water leakage in the system, considerable amount of water gets accumulated in the beam chamber. Because of the complex geometry of the cyclotron, it is quite difficult to remove water from the restricted areas. It is also not possible to bake the acceleration chamber to enhance the rate of moisture removal.An experimental set-up has been fabricated to study the pump down characteristics in the presence of water in a vacuum enclosure. The paper presents a literature survey on work carried out on similar lines and results of experiments carried out. Review of similar activitiesChan 2010 [2] examined the effect of environment humidity on the rate of thermal outgassing from an aluminium chamber. An aluminium glove box and an air shower, which provided dehumidified environments with water vapor concentrations of 0.1 ppm and 5 ppm respectively, were utilized to assess the effect of environmental humidity on the rate of thermal outgassing. Chan observed that only 4 h of evacuation was required to yield a small rate of the order of 1 x 10 -13 mbar L s -1 cm 2 of thermal outgassing upon filling with super-dry nitrogen without baking. A smaller rate of thermal outgassing was obtained using a drier venting gas, or by exposure to a drier environment. A thermal outgassing rate of the order of 1 x 10 -11 mbar L s -1 cm 2 was achieved by venting the glove box with super-dry nitrogen inside and controlling of environmental humidity at 0.1 ppm.Yamazaki 2010 [3] carried out control of water vapor in a nitrogen gas purge line in addition to surface treatments of chambers using buff polishing and electrolytic polishing, followed by

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