An instrument for measuring on-line the effective partial pressure of dissolved gases in a liquid includes a housing having a flow chamber for connecting in flow communication with a flowing stream of liquid to be measured, and a streamlined V-shaped wing rotatably mounted to the housing and extending across the flow chamber for presenting different angles of attack to the stream of liquid flowing through the housing chamber. The wing has an apex which remains located at a center of the stream of liquid flowing through the housing chamber as the angle-of-attack of the wing is changed relative to the stream of liquid. An actuating mechanism is coupled to the wing for rotating and thereby changing the angle of attack of the wing relative to the stream of liquid flow. A device, such as a viewing window or an acoustical detector, is coupled to the housing for use in identifying the occurrence of incipient cavitation bubbles at the apex of the wing.