During a main drain test under flow conditions, the pressure observed is the residual pressure.

• A. Ambient pressure
• B. Residual pressure
• C. Static pressure
• D. Dynamic pressure

Answer: (B)

When water is flowing in the main during a drain test, the reading you see on the gauge reflects the pressure that remains in the pipe after the resistance of the system (friction in the pipe, fittings, and any elevation differences) has reduced the supply pressure. This remaining pressure is called the residual pressure. It’s measured under flow, so it isn’t the ambient atmospheric pressure and it isn’t the static pressure you’d read with no flow. It also isn’t the velocity-related dynamic pressure, which isn’t what a standard pressure gauge is measuring in this context. The residual pressure tells you how much pressure the system can maintain while water is moving, which is why it’s the correct concept for a main drain test under flow conditions.