A pressure regulating device includes a housing having an inlet, a main outlet, an J an external venting outlet. An internal venting aperture is between the inlet and the external venting outlet. A valve is between the external venting outlet and the internal venting aperture. When the pressure of gases in the flow path is below a predetermined level, the valve at least partially blocks the internal venting aperture. When the pressure is above the predetermined level, the valve opens to allow at least a portion of the gases to flow from the inlet through the internal venting aperture and the external venting outlet. The valve is shaped to open in such a manner that the portion of gases flowing varies with the flow rate, such that the pressure at the main outlet is substantially constant over the flow rate range.