A filter apparatus includes a gas-impermeable outer conduit having an open end and a closed end. The cross formed by the radial arms is made up of a first diametral bar, a pin passing through the first bar at its center and protruding each side of this bar, and two radial bars which have an axial hole via which each of these radial bars is engaged on the pin. It is also relatively small and can be produced inexpensively while remaining relatively durable and robust.