A device for the indication of dirt accumulation on an air intake filter by measuring the negative pressure in the clean air space of the filter, the device having a spring-biased diaphragm piston driving a pointer shaft by means of a bell crank with a gear segment, the pointer shaft carrying two diametrically...http://www.google.de/patents/US4688511?utm_source=gb-gplus-sharePatent US4688511 - Dirt accumulation indicator for air intake filters
