Fume hood with improved counterbalance system

摘要

A fume hood includes a housing with a work chamber, an access opening to the work chamber, and a sash that one may move between raised and lowered positions to open or close the access opening. It also includes a counterbalance system that maintains the sash at a desired position between two predetermined limits. An adjusting weight system reduces the weight provided by the counterbalance system or increases the weight of the sash to allow the sash to drop to a predetermined position after it has moved above that predetermined position.