What absolute pressure could cause backsiphonage to occur in a hydraulic system at sea level?

In a hydraulic system, backsiphonage occurs when there is a negative pressure below atmospheric pressure in a water supply system. At sea level, the standard atmospheric pressure is approximately 14.7 psi (pounds per square inch). When the pressure in a system drops below this level, it can create a vacuum that allows contaminants to be drawn into the potable water supply.

The key to understanding backsiphonage is recognizing that the absolute pressure needs to fall below the normal atmospheric pressure for the risk to occur. In this context, an absolute pressure of 10.5 psi signifies a condition where the pressure in the system is significantly lower than atmospheric pressure (14.7 psi), thus creating the potential for backsiphonage.

Therefore, the correct answer—10.5 psi—demonstrates a situation conducive to backsiphonage because it represents a pressure that is below standard atmospheric levels, enabling the critical condition for unwanted flow reversal to happen.