Conventional (Constant Volume): 

This is a proven model operated on Constant Volume principle. In this model, majority air enters through the opening of sash. Exhaust volume remains constant. This is very popular and economical model used in general low hazard applications.

Constant Volume by Pass Air: 

These types of lab Fume Hoods have the same design as conventional fume hoods but will have slots or louvers on the top front panel. Hence, the air bypasses through the slots as the sash is lowered down or closed. There is a sure intake of air either in closed or open sash position. This redistribution of air prevents high velocity streams inside the hood, which may cause turbulence and compromise chemical fume containment.