Which device is intended to prevent back siphonage but should not be used under continuous pressure?

The anti-siphon vacuum breaker (non-pressure type) is specifically designed to prevent back siphonage, which occurs when there is a negative pressure in a plumbing system, allowing contaminants to enter the potable water supply. This device operates by breaking the siphon effect and thus protecting the water supply from potential contamination.

This type of vacuum breaker is not intended for continuous pressure applications because it relies on the presence of atmospheric pressure to function effectively. When water is flowing under continuous pressure, the vacuum breaker can be rendered ineffective, potentially allowing backflow if a drop in pressure occurs.

In contrast, other devices mentioned, such as backflow preventers and pressure compensating valves, are designed for different applications and can handle continuous pressure. Vacuum relief valves function in specific scenarios to prevent negative pressure but do not prevent back siphonage in the same way. Therefore, the characteristics and operational limits of the anti-siphon vacuum breaker make it the correct choice for this question.