An air gap in a drainage system is the unobstructed vertical distance of free space between the outlet of the waste pipe and the flood-level rim of the receptacle into which the waste pipe is discharging. As an example in a commercial restaurant, the air gap would be between the drainpipe of a salad bar and the floor sink or tub drain.
An air gap in a water distribution system is the unobstructed vertical distance of free space between the lowest opening from any pipe supplying water to a receptacle (such as a sink or tub) and the flood-level rim of the receptacle. This type of air gap is commonly checked by residential property inspectors while examining the bathroom sink and the laundry tub. What you want to see is that the end of the supply fixture is above the flood-level rim of the sink or tub.
An air gap is not a device and has no moving parts. An air gap is the most effective and dependable method of preventing backflow. The air gap provides the potable water opening or outlet being terminated at an elevation above the level of the source of contamination.
The minimum air gap should be measured from the lowest end of a water supply outlet to the flood level rim of the fixture into which such potable water outlet discharges.
The minimum air gap should be twice the diameter of the effective opening of the outlet. For example, a pipe with a 2-inch diameter would need a minimum of a 4-inch air gap. www.nachi.org & www.apollohome.com