Originally Posted by Hoohoodillyo
This is very simple... A common chamber is when the hose port and the ball barring "blow off valve" share the same area. A non common chamber is a hookah that has individual piping from the hose port and blow off port going into the vase. Kinda like their own stem going down. Common chambers are good for the fact that you can drill more holes to allow more smoke, but they get rusty inside of that are because there is no way for the moisture to completely leave that area.
I hope this helps... if you need I will do a diagram for you.
Basically this description is right. However, I will say that non-common smoke chambers tend to purge better although the difference is not great. I will say that I firmly believe that common chambers allow for greater smoke density and that the 4 inlet holes common to that design allow for higher volumetric efficiency which in effect means that you move more smoke with less effort (i.e. the draw is better).
A third, inferior sort of smoke chamber exists that is the Chinese style open chamber where the smoke enters directly into the hose. This style doesn't allow for the smoke to cool as much as either the non-common or common chamber allowing less condensation and a lousy purge.