The present device is suitable for applications where the input pressure of a system varies with temperature. In the past a metering valve was controlled externally (either electromechanically or mechanically) to achieve a variation of pressure so as to maintain a constant Reynolds number. However, problems of mechanical failure or constant personnel attention make this approach unsuitable in many applications. Accordingly, it would be highly desirable for a passive control to be inserted in a flow path so that the pressure may be varied as a function of fluid temperature.