What does a water main supply pressure below atmospheric indicate?

A water main supply pressure below atmospheric pressure indicates a potential vacuum situation. When the pressure in the water main drops below atmospheric pressure, it can create a vacuum that may lead to a variety of issues, including the risk of contaminants being drawn into the water system. This can happen due to unforeseen circumstances, such as a sudden change in demand or a break in the system.

In practical terms, a vacuum can compromise the integrity of the water supply, as it suggests that the system is not able to maintain an adequate pressure, which is crucial for safe and efficient operation. It's essential to monitor this situation closely, as allowing a vacuum condition to persist can lead to contamination or even structural damage to the pipes.

Normal operation would suggest that the water pressure is adequately maintained and does not dip below atmospheric levels, while a failing valve typically indicates issues with flow or control, but not necessarily a vacuum situation. Siphoning is a process that involves the movement of water from one location to another and is not directly related to overall supply pressure in the main. Thus, the identification of pressure below atmospheric levels as a potential vacuum situation is vital for maintaining the safety and functionality of the water system.