ISO 9635-4:2006 specifies construction and performance requirements and test methods for air valves, intended for operation in irrigation systems with water at temperatures not exceeding 60 C, which can contain fertilizers and other chemicals of the types and concentrations used in agriculture. It is applicable to hydraulically operated air irrigation valves of DN 15 (1/2 inch) diameter or greater, designed to be directly operated, i.e. the force is applied to the obturator by the float, either directly or via a mechanical linkage. The valves can be operated by a force applied through an adjustable pilot valve.