A mainline device is installed in the ‘horizontal’ main sewer line permitting all sewage to flow through it as it exits the dwelling. The moveable valve needs periodic cleaning to ensure trouble free operation, to ensure that it is functioning properly and not being hampered with contaminants. Even a piece of dental floss might get snagged and prevent valve closure at a critical time. It’s said that every mainline device has the potential to fail. The Sentinel is installed in the ‘vertical’ drain pipe, up and clear from the main sewage discharge so that normal sewage outflow never touches the Sentinel to compromise its operation.