Abstract: A pressure regulating device for use with a breathing assistance apparatus which conveys gases to a patient requiring breathing assistance. The device includes a valve member disposed within a valve housing which when gases flowing through the regulator reach a predetermined level at least a portion of the gases are permitted to flow through a vent aperture externally, similar to known pressure relief valves. The present invention includes an adjustment mechanism engaged with the valve member and for adjusting the predetermined level, and a locking mechanism for permanently preventing access to the adjustment mechanism once the predetermined level has been factory set.