Which method of backsiphonage prevention is considered least effective?

The method of backsiphonage prevention that is considered least effective is the barometric loop. Barometric loops create a physical elevation of the water supply line that can help prevent backsiphonage by using gravity to keep water from flowing back into the supply line. However, this method can be less reliable in certain situations, such as if there are changes in elevation or if the loop is not adequately maintained or if there are other operational pressures.

In contrast, vacuum breakers and air gap separation provide more reliable and effective protections. Vacuum breakers are designed to allow air into the system to break a vacuum and stop backsiphonage, while air gap separation is a physical distance that completely separates the potable water supply from potential contaminants, offering the highest level of protection by eliminating the risk of backflow. Backpressure units also provide reliable protection against backpressure conditions, making them more effective than barometric loops.

Overall, the barometric loop's reliance on gravity and potential susceptibility to mechanical failures makes it a less effective method for preventing backsiphonage compared to the other options listed.