Abstract: Method and apparatus for measuring the surface tension of a liquid inside a vessel (2), reactor, or inside a section of flow-through process pipe that is pressurized above normal ambient pressure, up to but not limited, to 100 psig (7000 kPa), includes a pair of tubes (2,3) having a small and large orifice in a modular probe assembly that allows the probes to be positioned at selected and variable distances below the surface of the liquid. A high pressure source (4) provides an inert nitrogen or process gas through a pressure regulator (5) to the input of two or three mechanical or electronic mass flow controllers (6,7,8), powered by an external power supply (9), which control the bubble rate at each orifice through manual adjustments, or electronic set points determined by a computer software program, independent of the pressure in the vessel, reactor, or flow-through process pipe.