Backflow Prevention Practice Exam

Water discharged through an air gap to an open tank surface has a pressure of ____?

1. 0 psi absolute

2. 0 psi atmospheric

3. 0 psi gauge

4. Static line pressure

The correct answer is: 0 psi atmospheric

The correct choice highlights the nature of pressure at the open surface of a tank where water is discharged through an air gap. When water is released into an open tank, it interacts with the atmosphere, causing the pressure at the surface to equal atmospheric pressure. This is because the tank is open to the surroundings, and atmospheric pressure is the force exerted by the weight of the atmosphere at a given location. Atmospheric pressure is typically referenced as 0 psi gauge; however, when discussing pressure in the context of an air gap, it is appropriate to refer to it as absolute pressure. In this context, the term used emphasizes that at the open tank surface, the pressure acting on the water is equivalent to the atmospheric pressure surrounding the tank. The other considerations, such as gauge pressure and static line pressure, do not accurately capture the condition at the open surface of the tank. Gauge pressure measures pressure relative to atmospheric pressure and would indicate a value other than zero in pressurized systems. Static line pressure is more relevant to fluids in closed pipe systems, which does not apply here. Thus, recognizing that the pressure at the open tank surface aligns with atmospheric conditions solidifies the reasoning for selecting this option.