Abstract: The pressure of a heated fluid is reduced in a test chamber. A small vent in the upper portion of the test chamber receives a stream of liquid from the chamber and yields a steady hiss when the fluid in the test chamber is under sufficient pressure to maintain the fluid solely in the liquid phase and it yields a sputtering hiss sound when the pressure in the test chamber is reduced sufficiently to permit steam and/or gases to coexist with the liquid in the chamber thus permitting liquid plus vapor to enter the vent. The change in sound may be used to alert an operator and/or pressure fluctuations in the test chamber may be automatically monitored to produce an output signal or a feedback signal for automatic control.