Walton Plastics Engineering offers an energy saving external bypass damper unlike any of its kind! The bypass damper is an offset damper externally located to supplement air flow into the fan external to the air conditioned lab instead of an overhead grill. This saves cost in removing less air through the fume cupboard upon closing the sash. The bypass damper means the fan speed is not altered maintaining the correct stack discharge flow rates while removing less air from the lab. The bypass damper is coupled with an airflow sensor that detects airflow pressure at the face velocity providing an accurate airflow detection alarm.