Abstract: Steam turbine hydraulic control system utilizing hydraulic fluid more dense than water accumulates any moisture present as vapor over the fluid reservoir. A detector for such moisture vapor which indicates unacceptable moisture admixed with the hydraulic fluid is connected in gas flow relationship with fluid reservoir at a level above the fluid level. A detector element, preferably a polished ferrous metal plate which readily rusts in the presence of water vapor, is maintained in a detector chamber having transparent portions, allowing for visual inspection to detect rusting and, hence, the presence of moisture in the hydraulic fluid.