An apparatus for measuring the mass fractions of water and oil in a flowing mixture of oil and water through a pipe includes a sensor portion that measures sound velocity and temperature of the flowing oil water mixture at a first time and at a second time. The apparatus includes a temperature changer in thermal communication with the flowing fluid which changes the temperature of the flowing oil water mixture by a measurable amount between the first time and the second time. A method for measuring water mass fraction in a flowing mixture of oil and water through a pipe includes the steps of measuring sound velocity and temperature of the flowing oil water mixture at a first time with a sensor portion. There is the step of changing the temperature of the flowing oil water mixture by a measurable amount with a temperature changer in thermal communication with the flowing fluid. There is the step of measuring sound velocity and temperature of the flowing oil water mixture at a second time with the sensor portion.