Type AF – Air gap with circular overflow

A non-mechanical backflow prevention arrangement of water fittings with an air gap measured downwards from the lowest point of the discharge orifice, which discharges into the receptacle, to the critical level. 

Maximum permissible fluid category against which a Type AF air gap is accepted as adequate backflow protection:

Backflow by back pressure: fluid category 4

Backflow by back siphonage: fluid category 4

Note: overflow and warning pipes shall discharge through or terminate with an air break, the dimensions of which should satisfy a Type AA air gap.  

Interpretation given in Regulators’ Backflow Specification - backflow prevention arrangements and devices: backflow prevention arrangements