The Model VM181 Vacuum Gauge is used as a troubleshooting aid to help to identify variances in the central vacuum system. The Model VM181 gauge has a rubber adapter that fits 2″ pipe and wall inlet valves. As a vacuum measurement, the gauge can measure 0-160 inches-of-water. AirVac power units normally sustain a vacuum level of 107-146 inches-of-water. Using a vacuum gauge helps to pinpoint leaks and blocks in the vacuum tubing, connections, and wall inlet valves.