A method and apparatus for measuring the deposit forming propensity of a continuously flowing fluid having a bulk pH of about 1 to about 12 comprising measuring the rate at which deposit forms on a quartz crystal microbalance having a top side comprising a working electrode in contact with the fluid and a bottom side isolated from the fluid, wherein the pH of the fluid proximate to the microbalance is controlled electrochemically at about 1 to about 14 by applying to the working electrode a cathodic current of about -0.001 to about -100 mA/cm² or an anodic current of from about 0.001 to about 100 mA/cm² and wherein the working electrode is coated with or made of a conductive material on which the rate of hydrogen gas evolution is at least 10 times lower than on a gold cathode in acidic solution.