How is an air gap defined in terms of measurement?

An air gap is defined as the vertical distance between the highest point of a fixture’s discharge pipe and the flood rim of the receptacle into which it discharges. This measurement is significant as it helps prevent backflow contamination by ensuring that no wastewater can siphon back into the potable water supply.

When an air gap is established, it creates physical separation, making it impossible for backflow to occur under normal conditions. This is critical in applications such as sink drains and dishwashers, where there is a risk of contaminated water mixing with clean water supplies.

The other options do not accurately reflect the definition or importance of an air gap in plumbing systems. For example, measuring from the discharge pipe to the ground or to the wall does not address the critical factor of maintaining a safe distance from potential contamination points, which is central to safeguarding drinking water.