Adjustable pressure-limiting valve

It allows excess fresh gas flow and exhaled gases to leave the system while preventing ambient air from entering.

[1] Such valves were first described by the American dentist Jay Heidbrink, who used a thin disc that was held in place by a spring.

[2] The valve is adjustable and spring-loaded, allowing the opening pressure of the valve to be controlled by screwing the valve top which modifies the pressure on the spring.

[1] A very light spring is used, so that at its minimum setting the valve can be opened by the patient's breathing alone using low pressures.

[3] In contemporary APL valves, three orifices or "ports" are present: one for intake of gas, one for return of gas to the patient, and an exhaust port for waste gas which can be connected to a scavenging system.