Air switches operated by ventilator pressure are used in combination with an electric circuit to cause an indicating light to be energized as long as a selected volume of air is moving through the ventilator per unit of time. A red light and buzzer are employed to indicate a low pressure, and a meter...http://www.google.es/patents/US4121199?utm_source=gb-gplus-sharePatente US4121199 - Sentinel system for grease extracting ventilators
