The SDMN6 dual-port manometer measures gas and static pressure and also accurately tests pressure switches by simulating a draft with an internal pump. A pressure switch is a safety device that prevents the furnace from running in an unsafe manner. The SDMN6 tests the pressure switch to determine if it is working properly, or if it is starting to fail which can cause the furnace to run in an unsafe manner. Plus, with the SDMN6 you can easily calibrate any adjustable pressure switch to manufacturers' specification, so there's no need to carry a large variety of pressure switches on your truck.