These Edwards vacuum pump exhaust check valves are typically used to help prevent air from rushing backward through the pump during powered down. They are simply a one-way valve, they are design to inhibit the backwards airflow. Even with one of these check valves install the pump will eventually increase to atmospheric pressure. These check valves are helpful to efforts to prevent process powder from being sweep backwards with a large air rush. They are typically used on Edwards dry semiconductor pumps, iH, QDP, & iQDP dry pumps.