The Gas Expansion Chamber helps to filter gases exiting the generator. It helps to condense water vapors that may contain molecules of the electrolyte solution as well as filter out other impurities. It makes it easy to catch cases where impurities my create foam so you can stop and address the situation before impurities get to any bubblers. Transparent mid section helps you see what is going on. 2 exit holes and seals on top included. A connecting non-kink 2 foot piece of tubing is included as well as a short tub that can be used for refilling. This assembly replaces the black cap on the base generator and screws onto it. The unit may not look exactly as in the photo deepening on available connections and parts but all are equally functional with the transparent mid section and gas exits on top. Read to connect to the base with tubing included that connects to the first bubbler.