A _______ backflow preventer is required for a car wash where chemicals are introduced into the system.

A reduced pressure principle backflow preventer is specifically designed to protect potable water supplies from contamination due to backflow in high-risk applications, such as a car wash where chemicals are frequently introduced into the water system. This type of backflow preventer operates by maintaining a constant pressure differential, preventing any potential pollutants from entering the clean water supply.

The mechanism of a reduced pressure principle backflow preventer features two check valves with a pressure relief valve in between them, which allows for drainage if there is a drop in pressure. This configuration is essential in applications involving toxic or non-potable substances, as it offers a higher level of protection compared to other types of backflow preventers.

In contrast, an air gap may be used in some applications but does not provide the same level of pressure regulation required when chemicals are involved. A double check valve is generally suitable for low hazard applications and may not adequately protect against backflow in situations involving harmful substances. A standard valve doesn't offer protection from backflow, making it unsuitable for scenarios that have a risk of contamination. Thus, the reduced pressure principle backflow preventer is the appropriate choice for car washes that use chemicals.