An opening is formed in a bottom wall of a developer reservoir, where a U-shaped diaphragm is secured. The diaphragm is bent downward when developer powder is present in the developer reservoir more than a predetermined amount. When the quantity of developer powder decreases below the predetermined amount...http://www.google.com.au/patents/US4174902?utm_source=gb-gplus-sharePatent US4174902 - Detection of developer powder amount contained in a developer reservoir
