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A non-mechanical backflow prevention arrangement of water fittings with a horizontal injector and a physical air gap of 20 millimetres or twice the inlet diameter, whichever is the greater.
Maximum permissible fluid category against which a Type AD air gap is accepted as adequate backflow protection:
Backflow by back pressure: fluid category 5
Backflow by back siphonage: fluid category 5
Interpretation given in Regulators’ Backflow Specification - backflow prevention arrangements and devices: backflow prevention arrangements