there is disclosed a device adapted to accurately deliver a selected concentration of a primary gas in a diluent gas , such as ambient air . the device of the invention includes a hollow diluter body having generally opposed ports for the ingress of a primary stream of gas and the egress of a dilution stream of gas . the interior of the body defines a dilution chamber and a vent chamber , both of which are open to the egress port . the top wall of the body is provided with a port for communication between the dilution chamber and the exterior of the body . a vent port in the housing provides communication between the vent chamber and the exterior of the device . pressure sensitive sealing means normally closes the vent chamber to prevent communication between the chamber and the exterior of the device and is operative responsive to pressure in the vent chamber to open and allow communication between the vent chamber and the exterior of the device through the vent port .