Air entrainment in secondary (brook) intake shafts creates two types of problems in the waterways of hydro electric power plants; blowouts and supersaturation of dissolved air. Blowouts may damage structures and supersaturation may cause gas-bubble disease in fish. Several preventive measures have been suggested to avoid supersaturation. At Driva hydroelectric plant a so-called vacuum gate was chosen. The present article describes prototype measurement of air entrainment and formation of supersaturation both under atmospheric conditions and under reduced air pressure in the shaft.